Writing: Input Wanted Building a Generation Ship: The SFV Exodus

[Strato Incendus]

Thanks again for all of your answers! :)

@Melbourne Guy : ...in your case for the more detailed behind-the-scenes info on who wrote what for Star Trek! ;)

I can see a cool scenario where security teams are trained in AG marksmanship but rebels are not.

Referring to your spoiler: That is awesome! I really want to make this plot-relevant now. Indeed, the security officers are supposed to be more experienced, and though quite a few of the rebels have or obtain weapons (including some of the security officers joining their cause), so far I was lacking a way to show this difference in expertise.

The protagonist was taught how to shoot by her father, by she never had to use it, and did not really want to, either. Perhaps her training only consisted of shooting across the width of the ring in a practice room, rather than up or down the ring.
Although it would not really make sense to set up the practice shooting rooms this way if this is so relevant for the actual training of real security officers.

"Losses" are relative in this regard; both sides have an incentive not to deliver lethal shots, since they all know they might be the last survivors of humanity. (Contact to Earth and the other inhabited worlds of the Sol system has been interrupted at this point - so far, I blame that on a solar storm, but given that this would only hit Earth, for example, while leaving the rest of the planets in the same system unharmed, I might need a better justification within the story.) "Losing" in these battles means "being stunned and captured by the commander's forces, which might lead to the captives becoming unwilling participants in the continuation of the human species".Talking about spinward and antispinward in general, I have a few more quick questions regarding sports on the ship. Especially since @Filip Larsen said it might actually be more plausible if some parts of the ship do not have full 1G (or 1.05 G even, in my case, so that the crew adapt to the gravity of Teegarden b). So far, there is only 1.05 G (on the rings) or 0.21 G (in the central pipe). Of course I am aware that it gradually shifts from the outside of the ring to the first deck of the ring, all the way up the elevator shaft on the spokes. But if there are sections that have something in between 1.05 G and 0.21 G, a lot more people would be affected by the need for a daily 2-2.5 hours exercise routine.
So far, someone spending all day on one of the rings could theoretically get by with just as much exercise as one would recommend on Earth, while someone working all day in the central pipe would have to do the 2-2.5 hours that ISS inhabitants do per day.

Here come the sports questions:
1) My initial mental image for the sports scene during the third chapter (to show how it is mandatory / necessary for survival on board) included swimming. So the first big question is: Would it be possible for the ship to have a swimming pool somewhere on the rings? Size-wise, this would not be a problem: Competitive swimming pools are 50 metres long, 25 metres wide (remember the inner corridor width on the rings is 32 m). But how would the water behave? The ring circumference, depending on the deck the pool would be placed on, is always around 1.6 kilometres.

2) The current sports scene has the characters run two laps around the ring instead, which amounts to about 3.2 kilometres. This is just for warming up, so it takes most of them 12-14 minutes to complete. Except for one particularly unsportive character, who would need twice the time (I actually have some of the other characters lap him close to the end of his first round). The rest of the 90 minutes (slightly less than on the ISS, since most characters do spend quite some time on the rings with normal gravity each day) is spent on bodyweight exercises. The question here is: Would they run spinward, or anti-spinward? Would this make a difference, and if so, what would it look like?

3) While they are running around the ring, I describe what the rest of the gym floor looks like, including fields for various kinds of ball sports - basketball, among others. I imagined the field to be perpendicular to the ring circumference, i.e. across the 32 metres of the corridor width. I can already tell this way wise, since otherwise, the rules (of physics) would not be the same for the team playing spinward vs. the team playing antispinward. But now it might make a tactical difference whether you attack on the left vs. the right side of the field - and of course, simply during ever pass that occurs in the game. An actual basketball game is not described in the first book, but there will be one at the beginning of book 2.

4) At one point later in book one, the main character runs away from someone else on the lab ring - so she has to choose a direction: spinward or antispinward. What difference does this make (now talking about sprinting rather than long-distance running, i.e. potentially higher speeds)?
If she ran anti-spinward and vaulted forward (=jumped) at one point, would she partly fall back into the direction she came from?
If she ran spinward and jumped, would she smash into the floor coming up to meet her?

 

[DaveC426913]

If she ran anti-spinward and vaulted forward (=jumped) at one point, would she partly fall back into the direction she came from?

Definitely would not fall back, no. She'd float higher and longer than normal. If she gets enoguh speed, she could conceivably go quite high and long (unless the ceiling is too low.

The thing is that can be a curse as well as a blessing.

While you're floating you have no traction, and cannot speed up.
You need traction to get up to full speed.
It would be tricky the find the right balance of straying on the deck long enough to get speed but not waiting so long that you don't weigh enough to get that traction.

If she ran spinward and jumped, would she smash into the floor coming up to meet her?

She wouldn't smash into the floor but it would come come faster than expected, she is danger of tripping unless she knows what she's doing. But, if she doesn't trip, she would get excellent traction for sprinting.

 

[Filip Larsen]

Would it be possible for the ship to have a swimming pool somewhere on the rings?

That should be feasible. A little care has to be taken in the design regarding the mass it represents, but nothing that is not also the case with areas of heavy machinery and such.

Overall, a good design for the mass distributions on the rings will probably try to ensure that mass in general is distributed fairly evenly so each ring has a center of mass is very near the geometric center allowing for a pure rotation without too much torques between transferred between the rings. Exactly what what type of dynamics one would see if for example one of the rings becomes very unbalanced is not really clear to me, but I would be surprised if such unbalance would not in some way result in troublesome loading of the structure at some point. Luckily, if the center of mass from the ring structure is designed to be near the geometric center from start it is fairly easy to include some water tanks (which is needed anyway) at difference sections and pump water around to fine-tune that center of mass.

Would they run spinward, or anti-spinward? Would this make a difference, and if so, what would it look like?

If they run spinward they would feel a little bit heavier, and if they run anti-spinward they would feel a little bit lighter. This was covered in another thread with some detail: https://www.physicsforums.com/threads/walking-around-the-ring-on-a-spinning-space-station.1008798/

So if running on a <1G ring level to train muscle mass it will probably make sense to run spinward to get that extra load. On the other hand if you run for "fun", then running anti-spinward may be more entertaining in some sense.

There is an effect here that may play into a low-G competition scenario. If you need to accelerate hard (including changing direction fast) on the flat ground you need good friction between the floor and your shoes, so, with everything else being equal, being heavier allows you to accelerate faster for situations where this friction dictates your acceleration limit. I assume normal athletes in 1G do not usually hit this limit, but if you have muscles for 1G and then try to accelerate at lower and lower G's then at some point you will likely experience that you start to slip. Around that G-level there will probably be noticeable difference on how to change running direction depending on if you are running spinward or anti-spinward to begin with. Coupled with the Coriolis forces this could make for some pretty interesting patterns of movement the athletes would have to adapt to.

If she ran anti-spinward and vaulted forward (=jumped) at one point, would she partly fall back into the direction she came from?

For a ring that rotates with a tangential speed much larger than anyone's running speed you can with good approximation understand it from the rotating system simply as running in a normal gravity with but with gravity that slightly depends on your speed and direction. Running spinward (anti-spinward) will increase (decrease) the effective gravity and the faster you run the more this "effective" gravity will vary from standing still. So if she runs anti-spinward and jumps, she would be able jump a bit higher and further than if she did the same jump in spinward direction.

 

[Filip Larsen]

if the center of mass from the ring structure is designed to be near the geometric center from start it is fairly easy to include some water tanks (which is needed anyway) at difference sections and pump water around to fine-tune that center of mass.

Let me add that in addition to adjusting the position of the center of mass such "balancing tanks" or trim tanks would, if also placed at different depths, allow for adjusting the direction of the principal momentum axis of each ring, which then allow for minimizing the dynamic loads at various points in the ring structure, which again allow designing for an overall lighter structure without having it flexing too much during normal use. Constructing an extended rotating structure that rigidly can absorb some degree of mass imbalances without flexing (i.e. without giving the crew the feeling they are on a rolling ship at sea) is not a trivial task.

The above is mostly hand-waving for saying there is a design tradeoff between comfort of the crew and the bulkiness of the ring, and a mechanism such at trim tanks can be one way to ease up on this trade off. Depending on the exact geometry and placement the pools you mention could also be part of such a trim system, perhaps only used in extreme situations.

 

[Strato Incendus]

Alright, let's get back to this thread again. I want to revisit a question that was brought up earlier, namely the duration of the journey:

To recap the parameters so far:
Target destination: Teegarden b (12.5 light years from Earth)
Travel speed: 10% light speed
Travel time: 125 years

Somebody pointed out that this of course does not sound like constant acceleration, but like a short burst of acceleration at the beginning, and a similarly sudden stop at the end.

If the ship were to use constant acceleration, though, how long would it take to get it up to 10% light speed?

I wouldn't actually mind if the journey took a little longer than 125 years, since that way I could comfortably get five generations to live on the ship before the plot starts (the main characters are all members of Generation Five, where Generation One is the first one born on the ship, meaning the original crew was Generation Zero).

I won't change the starting time of the story to a later date (that will remain on 1st January 2475, with 25 years left to go until arrival). But I could set the ship's departure from Earth to an earlier time than exactly 100 years earlier. Right now, the ship is set to have left on 1st January 2375. (Granted, currently there's a minor Star Trek reference hinging on this fact, since this is the year in which the story of Deep Space 9 ends. But there would be ways to reference that elsewhere, I'm sure.)

 

[jedishrfu]

You can use a classical computation:

$v = a * t$ with $v = 0.1 * c$ and you get to pick "a" to find the time in seconds "t". I would choose a = g so that the passengers experience Earth like gravity.

$a = g = 9.8 m/s^2$
$c = 3.0 * 10^8 m/s$
$v = 0.1 * c = 3.0 * 10^7 m/s$
$t = v / a = (3.0 * 10^7) / (9.8) = 3.06 * 10^6 secs = 354 days$

 

[Melbourne Guy]

You can use a classical computation:

Or, you can do what I do and plug it into an online calculator!

I find https://math-physics-calc.com/acceleration-speed-distance-calculator is useful, as it includes a number of acceleration, distance, and speed options, which helps when you're musing over questions like the duration of the journey.

 

[jedishrfu]

You forget that I am a robot with no need for such online tools. ( psst I used the Google calculator)

 

[DaveC426913]

$t = v / a = (3.0 * 10^7) / (9.8) = 3.06 * 10^6 secs = 354 days$

This is one-way acceleration? So just shy of one year to get up to .1c?This raises a question: how much time is saved via dilation at .1c?** Is there really any motivation for trying to take advantage of relativity if it only shaves 5/1000ths of the trip off?

**answer: .1c = a dilation factor of 1.005 - that's 8 months saved over a 125 123 year coasting journey.

So, it sounds like the only factor driving coasting velocity and trip time is total fuel (divided by 2) (i.e. they can only carry enough for 2 years of burn).

Knowing that in the design phase, is it possible the designers might have made different decisions?

Is it possible this results in a plot manipulation so obvious as to jeopardize story plausibility?

 

[Filip Larsen]

it sounds like the only factor driving coasting velocity and trip time is total fuel

Usually propulsive mass and energy is indeed the limiting factor for any kind of transport, also in space

And for interstellar travel this is no exception, but here with the added bonus challenge provided by the rocket equation, i.e. the insanely high fuel-to-payload mass ratios required if you want to go there in a timescale that makes human sense. I have yet to hear about a generation ship concept that both is feasible in engineering and sensible for its passengers.

 

[DaveC426913]

Usually propulsive mass and energy is indeed the limiting factor for any kind of transport, also in space

Yes but does it take the same or similar amount of fuel to get there in 200 years as 125 years?

Correct me if I'm wrong but a slower transit would save fuel. So a fuel constraint, if considered in isolation, would drive the duration of the trip up.

This means the upper limit duration of the journey might be determined by a secondary factor, such as only room for 125 years of supplies and resources, or failure lifetime of critical parts.

 

[Filip Larsen]

Correct me if I'm wrong but a slower transit would save fuel.

Yes, that is what is what I meant by fuel in general being the limiting factor on how quickly you can travel anywhere really. To me you just sounded somewhat surprised to discover this holds for interestellar travel, so I didn't want to miss the oppertunity to make a teasing remark about its generality

 

[John Strickland]

I have no idea if this discussion is ongoing or not.
You seem to need to know some more basic physics so that any description of an accident on the ship would make sense. This does not need to affect the characters as much unless it is in the story design. I used 3 colonization ship designs in a non-fiction book published in 2021. Here are some basic rules.

For nearby stars, the majority of an interstellar voyage using fusion power or similar method would be under thrust, with a shorter period of cruise (no thrust). For distant stars, the cruise phase would be the majority of the trip. Most past starship designs seem to use only a small amount of fusion fuel, while in reality the mass of fuel should be 25 to 100 times the dry mass of the ship, such as a 10 million ton ship with a 1 billion ton fusion fuel mass. As you accelerate, (probably at 1-1000 of a G), for year after year, there is little stress on the ship's structure.

The main issue your design seems to be ignoring is protection from cosmic radiation: CGR and "pseudo-radiation "sleet" caused by the ships high speed hitting atoms in interstellar space. If the fuel is in front of the passengers and cargo, it can protect them from the sleet, and the cargo can protect them from the CGR which comes from all directions. Thus the rotating habitat rings need to be INSIDE the ship. You do need pairs of habitat rings which rotate in the opposite direction to prevent ship spin.

A buffer mass of water ice can be in front of the ship to protect it from moderate size impacts, but some impacts will wipe the ship out without a laser system to vaporize the particles. Yes the bearings need to be able to be repaired while in use, so they also must be in pairs. A narrow ship is better than a wide one to reduce chances of impacts. Hitting a sand grain at that speed would be catastrophic.

The better the fusion process, the faster the ship can reach cruising speed. We do NOT know how efficient any given fusion engine design will be! There are multiple engine designs and multiple fusion processes which could be used. Good progress is now being made in fusion.

After the cruise phase which could be very short, the ship must decelerate to get rid of the huge speed and kinetic energy. Depending on the ship's mass and thrust, the deceleration phase might take 1/6 as long as the acceleration phase, but the amount of deceleration would be significantly higher, perhaps starting at 1/100 G, as the bulk of the fuel is no longer on board. You do need to get to zero velocity when you reach the target star system or your voyage will never end! You need lots of cargo mass, and volume to grow food.

A reasonable colonization ship would carry 1000 or more passengers, but some of these could be in true hibernation, once we figure out how to do it. You will also probably need equipment to terraform the destination planet, or the equipment to build it.[Moderator's note: promotion removed]

 

[Strato Incendus]

You can use a classical computation:

Okay, so according to this calculation, the acceleration time would just add about a single year to the overall journey, am I right?
In fact, it's already established as of now that the crew spent an entire year on board the ship while it was still in the orbit of Earth, to test how well they could handle that lifestyle.

**answer: .1c = a dilation factor of 1.005 - that's 8 months saved over a 125 123 year coasting journey.

I'm not sure if I understand this correctly: Does this mean the estimated journey would be two years shorter overall? Or 8 months shorter?

I am aware of time dilation, but I kind of blurred the lines on that a little bit. In the inciting-incident scene, the crew receives some messages from the Sol system. The commander says "So let's hear what (person X) had to say to us about 10 years ago... give or take half a year due to time dilation." So it's not actually clarified whether this message is 10-and-a-half or 9-and-a-half years old. ;) Not that it particularly matters to the crew anyway, given that they have no way to physically interact with Earth.

Knowing that in the design phase, is it possible the designers might have made different decisions?

Is it possible this results in a plot manipulation so obvious as to jeopardize story plausibility?

That would of course be a major plothole. Can you elaborate on what you mean by that?

So far, the plans were just "we launch the ship in 2375, the journey takes about 125 years at a constant 10% of light speed, so by 2500, the crew should arrive at Teegarden b".

This means the upper limit duration of the journey might be determined by a secondary factor, such as only room for 125 years of supplies and resources, or failure lifetime of critical parts.

One of the reasons I currently cite in the story for why the ship can't afford to go faster, even if it had more energy resources, is the response speed of the deflector systems to get rid of space debris in the ship's path. Loosely referencing an Isaac-Arthur video here where one of the fictional officers says "We can go as fast as we want, but if we go e.g. twice as fast, we'll encounter larger space debris twice as often / twice as quickly" etc.

In terms of failure lifetime of critical parts, that's indeed another problem I've been thinking about: As far as I am aware, current concepts for nuclear-fusion reactors would only last a few decades, not over a century. So if they went with a 21st-century nuclear-fusion reactor, chances are it would fail after less than half of the journey. They might already be coasting at 10% light speed at that point, but they still need the reactor for electricity, air- and water recycling, and pretty much everything else.

Then again, I've already established that I need some sort of disaster to occur in the reactor (in a separate thread). So far this is connected to a minor collision (also partly involving human failure on part of one weapons officer), but I could just chalk it up to regular wear and tear instead. This would seem more "mundane", but at the same time, perhaps also more realistic for precisely that reason.

 

[DaveC426913]

Okay, so according to this calculation, the acceleration time would just add about a single year to the overall journey, am I right?

One year at each end. You need another year to decel.

I'm not sure if I understand this correctly: Does this mean the estimated journey would be two years shorter overall? Or 8 months shorter?

Sorry. Out of 125 years total, The ship spends a year acceling and another deceling. The intervening 123 years are shortened subjectively by a total of about 8 months - due to the time/length dilation factor of 1.005.

Technically, the accel and decel years are shortened too, but the effect on journey duration is miniscule. I think it's less than 1 day each.By the way, I am using no complex math to figure any of this out. This calculator tells me the dilation factor at .1c is 1.005. Everything else is just simple arithmetic.

The "one year accel" is entirely premised on the excellent @jedishrfu's post #56 that concludes 1g accel will get you to .1c in 354 days.

I am aware of time dilation, but I kind of blurred the lines on that a little bit. In the inciting-incident scene, the crew receives some messages from the Sol system. The commander says "So let's hear what (person X) had to say to us about 10 years ago... give or take half a year due to time dilation." So it's not actually clarified whether this message is 10-and-a-half or 9-and-a-half years old. ;) Not that it particularly matters to the crew anyway, given that they have no way to physically interact with Earth.

If Earth set up a clock that sent a laser pulse New Year's Eve, the pulse would appear to arrive late. Every year that passes, it would be later by 1.825 days. By the half-way mark of the journey (that's after 75 years), the pulses and messages would be about 4 months out-of-date. It would not be until the 112 year of ship-time that messages would be 6 months late.

That would of course be a major plothole. Can you elaborate on what you mean by that?

No need. You plugged it:

One of the reasons I currently cite in the story for why the ship can't afford to go faster, even if it had more energy resources, is the response speed of the deflector systems to get rid of space debris in the ship's path. Loosely referencing an Isaac-Arthur video here where one of the fictional officers says "We can go as fast as we want, but if we go e.g. twice as fast, we'll encounter larger space debris twice as often / twice as quickly" etc.

Thats a perfectly valid reason to limit the top speed.

 

[John Strickland]

Maybe you have a table somewhere in the past posts that give the ship's dry mass. For the ship sizes I am seeing, I assume the ship is over 1 million tons dry. If you want to reach a cruising speed of 0.1 C (30,000 km/sec) in 1 year, that means you must gain over 820 km/sec every day (300,000 meters per second div by 365 days = 821 m/sec. with 9.5 meters per second average at a constant acceleration of about 1/3 G (821,000 meters/sec div by 86,400 seconds per day). This would be an Expanse-sized engine system. Figure out the total continuous energy output requirement for such an engine system - it would be scary. Do you have an anti-matter generator powered by the engine itself, to use the anti-matter up as fast as it is created? That is one method that has been suggested to avoid going boom.

With the same thrust, the time to decelerate would be a fraction of the acceleration mass since all of the acceleration fuel mass, the majority of the mass, is no longer on the ship. Note that for all such interstellar ships under thrust, the turnover point is normally much closer to the target star system.

If you go twice as fast along exactly the same path, you would hit exactly the same number of interstellar particles, but you would hit each one with twice the velocity, which means something like 4 times more damage per hit. I assume there will be extensive trials with large, thin cross section ships to test the number of such particles before humans try such a voyage.

John Strickland

 

[Strato Incendus]

You seem to need to know some more basic physics so that any description of an accident on the ship would make sense. This does not need to affect the characters as much unless it is in the story design. I used 3 colonization ship designs in a non-fiction book published in 2021. Here are some basic rules.

Thanks for your input! Some of these things I have indeed addressed in past posts, others I have not. I will get to them one by one.

For nearby stars, the majority of an interstellar voyage using fusion power or similar method would be under thrust, with a shorter period of cruise (no thrust). For distant stars, the cruise phase would be the majority of the trip. Most past starship designs seem to use only a small amount of fusion fuel, while in reality the mass of fuel should be 25 to 100 times the dry mass of the ship, such as a 10 million ton ship with a 1 billion ton fusion fuel mass. As you accelerate, (probably at 1-1000 of a G), for year after year, there is little stress on the ship's structure.

The target destination has been established: Teegarden b, 12.5 light years away from Earth. Now it depends on what you mean by "nearby" or "distant", since that is relative.

The main issue your design seems to be ignoring is protection from cosmic radiation: CGR and "pseudo-radiation "sleet" caused by the ships high speed hitting atoms in interstellar space. If the fuel is in front of the passengers and cargo, it can protect them from the sleet, and the cargo can protect them from the CGR which comes from all directions.

I was indeed working with Isaac Arthur's suggestion of having the water on board act as the main radiation shield. This is why I widened the rings: Not just to be able to hide the security doors in these 16-metre-thick walls, but also to have the water run through them. As far as the trunk of the ship is concerned, at times I indeed imagined the central pipe surrounded by water, like in a tunnel for visitors at an aquarium. I thought that might look kind of cool. Though it would probably look better from the outside, through a window.

And precisely because of radiation, it has already been established that the ship does not have any windows - only screens pretending to be windows. Meaning, their default mode is to depict whatever the cameras on the outside are showing. But you can switch them to show landscapes, or other things that might make life on the ship resemble life on Earth more closely. For example, in a passenger's quarter, a common use is to get the "window" screen to imitate a sunrise in the morning.

The passengers are also receiving regular anti-radiation treatments in medical form (this is established in the second chapter). But I fully admit even more protection might be in order, especially at the front of the ship.

Thus the rotating habitat rings need to be INSIDE the ship.

That takes us back to the cylinder vs. rings debate that we had at the very beginning. Hiding the rings inside the ship would make it look more sleek from the outside. However, it would also blow up the "central pipe" to the same diameter at the rings themselves (500 m). Or rather, there would be an "inner central pipe" around which the rings rotate, and the outer hull. The main problem with this approach is that it increases the ship mass drastically, because its surface would be much larger with that, too.

You do need pairs of habitat rings which rotate in the opposite direction to prevent ship spin.

This is something we have already discussed above. My suggested solution was to have some of the five rings rotate in one, others in the other direction. And since five is an odd number, the farm ring, which extends further inwards and therefore is greater in mass, could amount to the mass of two rings.

We have also discussed at length the distance between the rings, and ended up bringing them down to 60 metres - which is less than the outer thickness of a ring itself (64 metres). So any two of them could be construed as a pair of rings, the distance between them is equal.

Can we now use this to specify which ring turns into which direction?
Reminder: From front to back, it's public ring, lab ring, habitat ring, factory ring, farm ring

If the farm ring rotates clockwise, would the next two rings (factory and habitat ring) have to turn counter clockwise? Then lab ring clockwise again, public ring counter-clockwise?
Currently, on the page it says that the first and last ring rotate clockwise (public ring and farm ring), while the middle three (lab, habitat, factory) rotate counter-clockwise.

A buffer mass of water ice can be in front of the ship to protect it from moderate size impacts, but some impacts will wipe the ship out without a laser system to vaporize the particles.

Yes, the weapons are being used to vaporise particles, which is aided by the ship's AI. In the first act, one officer in this position (of course there are several, working in rotating shifts) is being set up as a potential source of disaster, since he's a less-than-ideal fit for his job. (Obviously, on a generation ship, you can't select people as thoroughly; you only have those to work with who are born on the ship.)

Normally, the AI is smarter and makes fewer errors than the humans anyway. But if, at the mid-point plot twist, the AI fails for some reason, and it is suddenly up to the human to compensate for that, this guy could be to blame for the disaster occurring.

Yes the bearings need to be able to be repaired while in use, so they also must be in pairs.

Do you mean the bearings for the protective ice mass in front of the ship here? Or the bearings for the lasers?

A narrow ship is better than a wide one to reduce chances of impacts. Hitting a sand grain at that speed would be catastrophic.

The Exodus is longer than wide (860 metres long, 100 metres in diameter for the central pipe). The rings of course have a diameter of 500 metres, but anything that passes in between the outer ring and the central pipe has a reduced chance of hitting the ship. (Of course, the elevator shafts in the spokes of the rings could still be hit.)

If the entire ship, including the rings, is surrounded by the hull, though, this wold make the ship almost as thick as it is long (500 metres), thereby increasing the chances of impact of something, in the off-chance case that some particle goes under the radar of the deflector system.

The better the fusion process, the faster the ship can reach cruising speed. We do NOT know how efficient any given fusion engine design will be! There are multiple engine designs and multiple fusion processes which could be used. Good progress is now being made in fusion.

Given that the ship is launched in 2375, I think we can safely assume that their fusion process will be far more efficient than anything we might be able to develop during the 21st century.

After the cruise phase which could be very short, the ship must decelerate to get rid of the huge speed and kinetic energy. Depending on the ship's mass and thrust, the deceleration phase might take 1/6 as long as the acceleration phase, but the amount of deceleration would be significantly higher, perhaps starting at 1/100 G, as the bulk of the fuel is no longer on board. You do need to get to zero velocity when you reach the target star system or your voyage will never end!

Yes, I am aware they need to slow down that much at the end. I've actually considered using the "ship spin" to their advantage: Making all the rings turn into one direction, causing the ship to flip over, then accelerate in the opposite direction to slow down. =D

You need lots of cargo mass, and volume to grow food.

The surface required to grow food is drastically reduced by a) the use of hydroponics for all the crops and b) in-vitro meat grown in the lab. There are no live animals on board, nor does the ship need to carry a lot of soil to create fields on the farming ring. If I remember the stats I've read correctly, hydroponics could bring down the required soil and water usage for farming by up to 90%, compared to traditional farming.

A reasonable colonization ship would carry 1000 or more passengers, but some of these could be in true hibernation, once we figure out how to do it.

1000 is indeed the average crew size: The ship started with 500 people (250 couples, at least one from each country on Earth), and they had children just a few years into the mission, thereby quickly bringing the population up to 1000. Potentially, there could be up to 1500 people living on the ship if all members of three generations live out their entire lives on it. But, as I mentioned earlier, there is the cultural component of the "exit pill" (like in Star Trek "Half a Life", as well as a similar Vulcan custom).

You will also probably need equipment to terraform the destination planet, or the equipment to build it.

Teegarden b is similar in mass as Earth (slightly heavier), and in the habitable zone of its star. The main issue is that it's most likely tidally locked, so people will have to live in the fertile "twilight zone" between the desert side facing the sun, and the ice side facing space. However, that isn't something terraforming can do a lot about.

Maybe you have a table somewhere in the past posts that give the ship's dry mass.

Since the measurements of the ship aren't finalised yet (the length might be, but right now we're back to discussing the diameter, because of rings vs. cylinder), we haven't tried to estimate the overall mass yet.

Do you have an anti-matter generator powered by the engine itself, to use the anti-matter up as fast as it is created? That is one method that has been suggested to avoid going boom.

So far, I've only established nuclear fusion as a power source. One way to show even to laypeople that this ship is much less ambitious than those in Star Trek: No anti-matter, no warp drives, not even light speed.

If you go twice as fast along exactly the same path, you would hit exactly the same number of interstellar particles, but you would hit each one with twice the velocity, which means something like 4 times more damage per hit. I assume there will be extensive trials with large, thin cross section ships to test the number of such particles before humans try such a voyage.

In the backstory, there has been a Breakthrough Starshot 3 mission sent to Teegarden b - an unmanned probe launched towards it with a giant laser (like in the actual plans for missions to Proxima Centauri right now), at close to the speed of light, to create images of the surface and examine the make-up of the atmosphere.

How these unmanned probes slow down at their destination is still being discussed by the Breakthrough Starshot researchers at the moment, as far as I know. They could just fly by and create some images, then vanish back into the interstellar medium. If they're supposed to drop a landing rover or something similar, slowing down and getting into the orbit of the planet is probably a must, though.

 

[John Strickland]

I am glad to see other people interested in what I refer to as "slowboats", realistic interstellar starships that are not FTL ships as in Star Trek, etc, but rely on the physics we know. I am not a physicist, engineer or mathematician, but merely a generalist who can ask the right questions and track down the answers with algebra and Excel! I can also think like an engineer. There is a good reason for my focus, since I know that a fusion powered vehicle can actually slow down to zero velocity at the target star system unlike most of the other concepts!

As I understand we can only communicate via text in this forum (no images apparently) and we cannot post contact info. I would like to be able to send you some examples of what I have done, and would like to see even crude sketches of what your vehicle looks like if we can get permission to communicate directly. A picture really can be worth many thousands of words!

Even after reading much of the text I find it hard to visualize what your ship looks like. I am imagining a set of tuna can shaped sections with the "rings" or habitat tori inside them, strung along the ships axis like fat beads. I strongly suggest that you create a baseline design, which is not the final design, but would then allow us to discuss the ship with actual numbers, such as dry mass, fuel mass, thrust, mass fraction, etc. Improving the baseline design leads to a final design as you can see how design changes improve it.

If I know these basic physical parameters, I can then cover additional issues which depend on them. Say you have a ship that has a dry mass of 4 million tons and a wet mass of 1 billion tons (fusion fuel). This puts the dry mass ratio into the same general range as chemical rockets. You want a high mass ratio and a low dry mass compared to the fuel mass.

As an example, my 3rd ship design takes over half of its voyage of 80 years to the Centauri system to accelerate, and the fusion engines are very large, so they do not melt from the fusion reaction (photons and particles both). (Fusion thrust is very good due to the high temperature of the exhaust, but the reaction also radiates heat and particles in all directions). Thus the engines are a significant part of the ships dry mass. It is hard for me to visualize a realistic slowboat that can accelerate to 0.1 C in 1 year without the whole ship being a giant engine.

The nice thing about the low acceleration on the ship is that there is little stress on the ship's structure. With a ship that has an initial mass of 1 billion tons, and a final mass at destination of about 4 million tons, the total fusion thrust of 7 huge fusion engines is only about 660,00 metric tons or to satisfy the physics purists, about 6.5 billion Newtons. Of this fuel mass, only about 6 percent is used to decelerate! Thus it is clear that the deceleration will be much stronger than the acceleration and will take much less time.

So with a baseline design, you can then calculate the optimal trip time and cruise period for your vehicle. Based on what I calculated for my designs, you might travel 40 years during acceleration, and 5 years during deceleration, so the cruise phase would be much longer than the acceleration. At 12 years distance with 0.1 C velocity, the trip spent at cruise phase (as a comparison value) would be 120 years, so the whole trip would take longer than that.

Shipshape:

Obviously narrow is better to reduce chances of collision. The chances of something hitting the sides of the ship at that speed are very low, so the best idea is to have all of the ships structure relatively close together behind the ice shield in front. The mass of the ice shield is much more than the side shielding so all of the tori would get a low dose from the “velocity sleet”. You only need the equivalent of 10 meters of water as side and rear shielding, so the tori rings should be relative close together to save mass and to act as shielding mass for each other. A lot of the dry mass of my design is in the form of supplies for use at the destination planet, and this dry mass is also used as shielding.

My ship design is very boring looking, it is just a giant cylinder, but it is very efficient in mass usage. Note that the habitat tori (what I believe are your rings), must be in a vacuum to rotate without drag, so the only drag is the magnetic bearing pairs for each torus hub. My tori are only 200 meters across with 20 meter wide torus tubes (the doughnut part) as there is only a fraction of the crew and passengers awake at any given time. I would put all of the tori inside a thin Whipple shield hull cylinder around all of them to protect from possible side hits (still unlikely due to the speed of the ship vs the speed of any local object).The shield could be 50 meters thick in layers to absorb any hits, with the ability to replace any spalled portions of the shield. The bearings which allow the tori to rotate can still be attached to the central tube which would not rotate. Some structure along the sides of the ship would be needed to support the edges of the ice shield. Note that the ice shield should be slightly wider than the widest diameter component such as the rings to prevent side collisions.

Which direction the rings or torus turn in is not significant, but the mass spinning clockwise must match the mass spinning counterclockwise. It is better to have all tori the same mass. The cargo does NOT need to be inside the tori. A small amount of energy would be used to counter any magnetic friction in the bearings.

Humans would not be able to react in time to vaporize any object fast enough. Let the computer do it. Human pilots could not land a Falcon 9 rocket, it takes a computer.

If you do not assume some fusion process has been developed and set an efficiency value for the engines, you simply cannot calculate the trip time accurately as you do not know the acceleration and thrust available.

Animals would be stored in hibernation or as embryos or DNA data that can be reconstituted back into DNA when there is a place for the animals. If you have artificial gravity, you can have live cats as cat boxes will still work even in low gravity! You would need a better cat litter recycling system.

It is unlikely that we will discover any truly Earth size planets with oxygen atmospheres, so atmospheric terraforming operations will probably be needed.

My anti-matter comment was slightly snide, but the reason I call these ships slowboats is also ironic, in that an FTL ship goes vastly faster (if the speed is measurable at all), then a realistic slowboat. See if you can decide on a realistic fusion reaction to use. I like the Boron-hydrogen reaction as the fuel can be Hydrogen boride liquid, which is not fully cryogenic and is denser than water (less tank mass).

Scout missions can be done without slowing down as the cameras and instruments can take images and measurements and then feed them back to Earth after the flyby just like the Pluto mission. Thus vehicles that cannot slow down are fine for this use.

 

[Strato Incendus]

In fact, we can use images here - just not anything that has been used for promotion already, as far as I understand it?

My ship is rather "boring-looking" from the outside, too: Just a long pencil shape with the five rings wrapping around it. So here is a very crude depiction of it (using the same public domain background as on the previous page of this discussion).

Since I made this image, the rings became wider and the distance between the rings shorter. So we'd probably have to imagine the ring width and the sections between the rings to be almost equal in length (64 metres ring width, 60 metres between-ring distance).

On the previous page of this thread, I have already uploaded a cross-section of one of the rings, with the central pipe in the middle.

This is not to scale yet, since we haven't decided on final measurements so far.
However, as it currently stands, the diameter of the trunk (central pipe) is 100 m, the inner diameter of the rings 500 m (given the height of the five decks etc. discussed previously, the outer diameter of the rings is 526 m).

Regarding the bridge section, I wanted it to remain in a fix position relative to the direction of movement. Hence, I could not place it on one of the rotating rings. So the question is how to ensure something that resembles regular walking on the bridge. In The Expanse, they have the magnetic-boots solution. This might work well for the crew working on the bridge normally. But for the rebels storming it at the end of the mutiny, it seems unlikely that they would all have access to such magnetic boots.

My tori are only 200 meters across with 20 meter wide torus tubes (the doughnut part) as there is only a fraction of the crew and passengers awake at any given time.

You mean a diameter of 200 metres? Or a radius of 200 metres?
For all I know, both seem to small. Meaning, they would have to rotate too fast to create Earth-like gravity, and then you get into troubles with the Coriolis effect.
A radius of 225 metres, or a diameter of 450 metres, for all I know, is the minimum required to have the ring rotate slowly enough while still producing 1 G.

Which direction the rings or torus turn in is not significant, but the mass spinning clockwise must match the mass spinning counterclockwise. It is better to have all tori the same mass.

Okay, so it is not crucial that each ring is directly followed by another ring rotating in the opposite direction, as long as the total mass of rings rotating clockwise vs. anti-clockwise evens out?
In my case, if mass (public ring) + mass (farm ring ) = mass (lab ring) + mass (habitat ring) + mass (factory ring), then everything should be fine.

Ensuring the same mass on all rings is difficult, though, especially when they serve different purposes. The habitat ring will have most of the people on it; the factory ring and lab ring will have fewer people, but a lot more gear etc.

The shield could be 50 meters thick in layers to absorb any hits, with the ability to replace any spalled portions of the shield. The bearings which allow the tori to rotate can still be attached to the central tube which would not rotate. Some structure along the sides of the ship would be needed to support the edges of the ice shield. Note that the ice shield should be slightly wider than the widest diameter component such as the rings to prevent side collisions.

Having an ice shield of over 526 metres in width in front of the ship sounds quite ironic: In the chapter that currently discusses the danger of collisions, some obvious comparisons between the Exodus and the Titanic are being made. So far, it's just that a collision with something much smaller than an iceberg would be enough to destroy the ship completely. But if an iceberg is actually the thing protecting the ship from just that, that would make the analogy even more curious.

Of course, collecting ice would have a different purpose, too: Restocking water supplies on board in case some of it gets lost. At the start of the plot, water consumption on board is quite strictly regulated (mainly when it comes to showers, though).

This is another question I'm not sure about: Isaac Arthur argues the main constraining factor for water recycling on board a spaceship would be energy, and the amount required for water recycling would be negligible, compared to the amount of energy required to accelerate the ship.

In either case: One officer detects a nearby ice cluster and suggests to take the ship on a detour to pick it up. This would allow the crew to loosen the restrictions on water consumption quite a bit - which would make the crew happier and improve the commander's standing with them, too.
However, it would also make the ship heavier, and they'd have to slow down to pick up the ice (the question is: How much and how fast?). The commander rejects the request, stating she won't take the ship on a detour of over half a year, just so that her crew could live a more decadent lifestyle.

The point of the scene is to show that the commander places the success of the mission above the well-being of her crew members.

If the ice could also be used to repair the protective sheet in front of the ship, though, this would give her more of an incentive to go on the detour and pick up the ice reserves, as suggested. It would no longer make sense for her to regard this as a mere "luxury problem". In other words: It would either destroy the scene - or make her seem irresponsible for letting the opportunity slip.

Then again, she could be arguing that the present ice shield works just fine. This hubris would then be analogous to the Titanic leaving the harbour with only half the suggested number of life boats.

Humans would not be able to react in time to vaporize any object fast enough. Let the computer do it. Human pilots could not land a Falcon 9 rocket, it takes a computer.

Yes, that's what I mean by "AI assistance": Normally, the computer does the job. The question is what room there is for human failure to lead to a disaster, then. Perhaps the "inattentive officer" is responsible for keeping an eye on the computer, rather than the deflection of particles itself - and he overlooks an impending malfunction.

The main catastrophe happens at the reactor, though. So I could just have the reactor fail for other reasons, without any collision or partial collision happening (because most likely, such collisions would be strong enough to annihilate the entire ship anyway). This moves over into the parallel thread I created on which disasters to use. The main reason I need the catastrophe to affect the reactor is because the plot requires the ship to lose speed for some reason, extending the journey as a result.

Potentially, the following sequence of events could happen:
- The inattentive officer overlooks the impending malfunction of the deflector system. The system would keep working, but catch fewer objects in time if the ship kept going at 10% light speed.
- To save the crew, the commander orders the ship to slow down - to a quarter of the speed (2.5% light speed), where the deflector can keep up despite performing at lower efficiency now.
- The sudden stop then in turn is what causes damage to the reactor, because of the high amounts of energy needed to slow down this quickly.
- A bunch of damage would occur inside the ship, too. The question is how fast the ship could reasonably slow down from 10% to 2.5% light speed (=i.e. not to 0) with humans still being able to withstand the forces at play here (even if only for a short while).

It is unlikely that we will discover any truly Earth size planets with oxygen atmospheres, so atmospheric terraforming operations will probably be needed.

Yes, I know the chances of the atmospheric composition matching our requirements is low. I've been to Teegarden b in Space Engine, and there it also shows the atmospheric composition, which isn't suitable. However, I don't quite understand yet to what extent the atmospheric composition of exoplanets can be known at this point?

I have a vague layman understanding of how researchers infer the number of planets around a star, their respective masses, and their distance to the star (habitable zone or not). But as long as we don't know anything specific about any given exoplanet's atmosphere, sci-fi authors are still free to simply "postulate" these factors. =D For example, I also postulate that Teegarden b has a magnetosphere - which isn't necessarily a given, either. Especially tidally locked planets seem to be less likely to have magnetospheres, but tidal heating can still create one - taking Ganymede as an example (tidally locked with Jupiter).

Advanced terraforming abilities would open up a bunch of plotholes, since the obvious question is: If the crew has the means to terraform a planet with inadequate atmospheric composition, potentially also one without a magnetic field, if they could basically turn any given piece of rock out there which lies in the habitable zone of its star into a second Earth - why not pick a destination much closer than Teegarden b? They could give Proxima b a shot instead. (In fact, originally the plan was to send the ship there, and it would have traveled much more slowly. But Teegarden's star is a much calmer red dwarf than Proxima Centauri. The frequent eruptions of Proxima Centauri were a knockout argument against it.)

See if you can decide on a realistic fusion reaction to use. I like the Boron-hydrogen reaction as the fuel can be Hydrogen boride liquid, which is not fully cryogenic and is denser than water (less tank mass).

Thanks for this specific suggestion! ;) I'll look further into that specific reaction!

Scout missions can be done without slowing down as the cameras and instruments can take images and measurements and then feed them back to Earth after the flyby just like the Pluto mission. Thus vehicles that cannot slow down are fine for this use.

For images of the surface from space, sure. What about dropping landing rovers to actually explore the surface? If they're launched from a probe zipping by the planet at close to light speed, the direction and velocity of the dropship containing the rover would also be influenced by that, wouldn't it? ;)

I need to plausibly demonstrate in the story that Teegarden b has been sufficiently explored by unmanned missions, so that people on Earth are absolutely certain it will be habitable for humans, before they invest the enormous material effort to actually send a generation ship there.

 

[John Strickland]

This physical design means that all of the habitable space is outside the protection of the ice shield (on the front of the ship) and is also unprotected from the "velocity sleet" radiation by the mass of the fuel. Is the fuel for your ship stored in the central cylinder? What is the mass ratio for your ship. It should be at least 100 ( wet mass over dry mass). I assume that each habitation ring has a torus inside. If so, the Major diameter of your tori is about 500+ feet and the minor diameter is the thickness of each torus tube, Like a huge bicycle tire. I also assume that you have shielding around each torus. This almost doubles the shielding mass needed, making the mass ratio much less. You need a ring shaped ice shield in front of all of the rings and the rings should be close together so that one rings radiation shield mass can partly shield the adjoining ones. The fuel can also be in a ring of tanks between the ice shield and the habitat rings. The ice shield and fuel tanks to NOT need to rotate. This can reduce the core cylinder to a structural cylinder with a non-rotating navigation section also behind the ice shield.

The problem with this design is that the struts that attach the rings to the core are exposed to impacts at 30,000 km/sec. That is why it is much better to have a narrower habitat design with everything behind an ice shield, and preferably behind the fuel tank section. A 500 meter wide habitat ring is over 1500 feet in diameter, This is what you would build in the target system, not for use in an interstellar vehicle. That is more like a "Worldship" concept. Getting to the target star system sooner is more important than a huge habitat.

My first designs were wide like this but I shrank them down to get a huge mass ratio value. The tori for design 3 have a radius of about 100 meters, a Major diameter of 200 meters, and a tube cross section (minor diameter) of 20 meters. The coriolis effect would only be noticed in certain circumstances, such a jumping sports, etc.

Do not worry about microgravity in the ships bridge. The bridge crew would spend most of their time in the rotating sections. It would make the story more realistic and interesting.

The ice shield is made of water ice filled with anti-spall fibers, to prevent loss of larger chunks of ice if it is hit. Ice shields are a standard concept. If a particle ruptures a fuel tank, how do you slow down? The ice shield is there to act as a shield. At these velocities, ice is as good as metal and water is better as a radiation shield than an EQUAL mass of metal. you could make the ice shield in the form of a giant Whipple shield with layers separated by shock struts. Then a strike on the front layer would not wipe out the layers behind it and the shock would be mostly absorbed in the shields own structure. Some of the shielding water stored in other areas can be used to replace lost material from the ice shield.

Yes, the water ice in the shield can also be melted and used as water! You cannot replace any water or gases during the trip so you must have a compartmentalized method of storing backup amounts of all such materials, which can also be used as CGR radiation shielding, You need shielding between the fusion engines and the habitation areas also due to the radiation they would create.

Yes, you CANNOT take on any supplies DURING the voyage, as you would have to slow down to zero velocity to do so, using up your precious deceleration propellant and leaving you stranded in deep interstellar space! Your ship could be designed to shed duplicate sections of materials in case too much propellant was lost in an accident. Such a detour is impossible to do if you want to reach your destination.,

Humans could detect malfunctions but the system would have multiple hardware backups. For the detection system to fail would require "a major malfunction".

An interstellar vessel cannot slow down or stop "suddenly" any more than a train can stop suddenly. It has a huge mass and a huge amount of kinetic energy which has to be reduced gradually, over a period of YEARS. You need to investigate the amount of thrust a given fusion engine can produce, In my design, the fuel tanks are like metal balloons to save mass, and they would be crushed by any high deceleration even if that were possible. The typical acceleration - deceleration ranges for a slowboat are between 1/10,000 G and 1/100 G. So in that one respect a slowboat is actually "slow".

You need life with photosynthesis to have a stable, breathable oxygen atmosphere with enough buffer gas like nitrogen. There are just so many factors that weigh against a living world that there may be less than half a billion such worlds in our entire galaxy. Fortunately there should be enough rocky planets between 0.5 and 1.2 Earth diameters to provide terraformable planets.

A "habitable" planet also assumes a "rock" large enough to have gravity strong enough to hold its atmosphere. The lowest such size is about that of Mars or slightly larger.

I agree that Proxima does not look very favorable right now, but Alpha Centauri A or B could have planets that are not visible to us due to the tilt of their orbital planes, (all random).

A scout mission that has an orbiter and/or lander must obviously slow down to near zero velocity. However, as there is no crew, its payload could be much smaller and the trip faster. Orbiters are far more valuable than landers, but a set of landers should be used to try to get samples of surface materials to make sure there are no high levels of toxic elements. Elemental abundances can vary from star to star. So both orbiter and landers are needed.
A super-AI would be required to make decisions on targeted surface locations and to control radio transmissions back to Earth. The main vehicle could stay in a high orbit so its signal would only rarely be blocked by the planet of interest. By this time, we might even have aware AI's.

 

[John Strickland]

Here are some basic values and relationships for interstellar flight.

A light year is about 6 trillion miles or about 10 trillion kilometers. If the Centauri system is exactly 4 light years distant, it is then 40 trillion km distant. If you could travel at exactly 0,1 C for the whole trip, the trip would last 40 years, allowing no time to accelerate and decelerate. A destination 12 light years away would be 120 trillion kilometers distant and a trip would take 120 years.
In reality, most of the time for nearby stars (within 8 light years or so, most of the trip time is accelerating and decelerating. Any starship has some maximum delta-V value. This is determined by the ship's actual thrust and its mass ratio, (wet mass - with fuel divided by dry mass). The higher the mass ratio, the faster the ship can go as an average velocity and the larger the total delta-V capacity value is.
The Delta-V capacity must be divided among the acceleration phase, the deceleration phase, and the terminal maneuver phase, as well as some kind of reserve. The mass ratio for the whole voyage is a single value, but the mass ratio for each PHASE of the trip is a component of that value, as the acceleration for each phase is different. If there is a cruise phase, no fuel is used. For example, in a given design, the total delta-V capacity might be 61,000 km/sec or 0.203 C. The acceleration phase uses 30,000 km/sec, and the deceleration phase uses the same amount, while 1000 km/sec is reserved for maneuvers in the destination star system.

Note that the mass ratio for the deceleration phase would be very different than that for the acceleration phase, since most of the fuel is already gone by the start of the second phase. Part of the acceleration phase is accelerating the fuel that will be used to decelerate. If we had an arbitrary 10 million metric ton ship, that had 1 billion tons of propellant on board, its starting mass ratio would be 100: 1000 million / 10 million.

But then in the second phase, depending on the actual thrust, the ship will have probably used up say 800 million tons of fuel, so the deceleration mass ratio is now 200/10 or 20. The mass ratio for the acceleration phase mass ratio would be 1000 million over 210 million or 4.76. This means the starting deceleration will be about 5 times higher than the starting acceleration. The deceleration fuel is precious, since there is less of it but ton for ton, it is now 5 times more effective. The mass ratio for the second phase is now higher, and the ship has the SAME THRUST it had at the start, so the deceleration time will be much shorter than the acceleration time. The exact values depend on the ships mass, thrust and mass ratio during each phase.

Fusion engines do not produce a lot of thrust for the energy they use. For an engine using the He-D reaction, my 7-engine cluster uses a staggering 484.4 kilograms of fusion fuel per second. A significant part of this mass gets turned into energy, heat, and a release of X-rays, infrared radiation, neutrons and fusion products hopefully directed away from the rear of the ship, Unless the engine is large enough and cooled actively, the engine will be vaporized in short order.

Each engine puts out a staggering 2000 terawatts, with the cluster of 7 putting out 14,000 terawatts of power. Note that the sunlight hitting the Earth is 174,000 terawatts, so this one ship is generating 8% of all the power the sun gives the Earth. Yet for all this heat output, the total thrust is only about 400,000 metric tons (depending on the fusion reaction and the formula used to calculate the thrust!), and pushing against a 10 million ton starship with 1 billion tons of fuel on board produces a starting acceleration of (about) 0.4 million / 1000 million = or 4/10000 of a G or 1/2500th G. This shows why the acceleration phases for star voyages take a long time. As the fuel gets used up, the ship accelerates faster and faster.

When designing an interstellar vehicle, you do need to start with these realities. The scale of the challenge is huge, but the reward would also be world-changing - literally, as it would allow humans to spread among the stars.
.

 

[Melbourne Guy]

@John Strickland, your realistic treatment highlights why most generation ship stories have liberal amounts of handwaving going on. Also, they are stories, and too much detail can bog down the narrative flow, but a consistent set of physics at least makes the story telling easier to write.

 

[John Strickland]

Thanks for the support. I enjoy both realistic and less realistic SF, including that with FTL ships, etc. What counts is how good the story is. I promote interest in fusion powered slowboats as I think they are more realistic than the laser-driven light sails, they can slow down and stop, they can carry humans, fusion is needed for large scale terraforming, and fusion is not magic physics since stars and H-bombs do work. I wish the laser and light sail people well since if they can make it work, it would be a great technology for scout missions. Once you find a really promising planet, a fast, fusion powered scout mission with orbiters and landers would be in order.

 

[Filip Larsen]

There is no doubt that generation ships when viewed in isolation are an excellent setting for sci-fi stories, but when compared to other potentially more feasible slowboat designs where the passengers are not supposed to toil generation after generation in a self-contained human society, then the whole premise of launching a generation ship mission in the first place seems ridiculous, making it tricky to create a believable background story.

A generation ship is by definition a sort of mission where the motivation to join up is about saving humanity from extinction of life in our solar system or a similar dire situation since joining up carries the moral implication that all but the last few generations on that ship will live their entire life on that ship. Alternatively the life on the ship will have to promise a better life on board than by "staying behind" in the solar system. Granted, if told that in 10 generations time (assuming the ship mission takes that long) no life could exist in the solar system, then passengers might accept to condemn 8-9 generations of their off-spring to life on board the ship so that the 10th generation and forward can get a chance to rebuild humanity on that promising planet.

But take away the generation-thing and it all makes much more sense. For example, it is not too unreasonable to assume that long term hibernation can become a feasible thing, at least in the context of interstellar travel. If passengers can hibernate for how ever long it takes to get there, then this makes a huge difference both to the scale of the ship design (allowing them to be simpler) and the motivation to join up. Allowing the passengers that join up to also experience the end goal lines up very well with the existing human emigration drive where you can take your family with you and start again at a more promising place. Granted for sci-fi such a setup likely only makes for interesting stories when someone then accidentally wakes up (even my wife liked that recent movie, and she normally steers away from sci-fi), or ship has to "wake" the crew due to unforeseen circumstances.

Another "simplification" for the Hail Mary type of missions would be a seed or ark ship that only "needs" to bring the DNA of people and animals and would then, put short, "grow" them when conditions where right. This is of course only feasible if "growing" a human population back into thriving condition becomes possible and research into this would likely be consider unethical until the need for such a Hail Mary mission arises. But in that case one could argue that compared to a generation ship it is perhaps more humane to research into making this possible and spare the, say, 8 generation for a "meaningless" life and spare the ship design for having to support a sustainable environment for 50k+ humans for over hundreds of years.

This last one also touches with the inherent dilemma of designing extremely long term technology in a world where technology constantly evolves. As long you believe you can shave off 2X years of mission time by researching for X year more, or increase the likelihood of successful mission by a significant factor by doing that research, then you will end up doing what we are doing now, that is researching until something better comes along.

All this is not to detract from any well-thought out sci-fi story about generation ships, just a few observations that the concept in the grand scheme of things may be a bit unrealistic in the first place.

 

[Strato Incendus]

This physical design means that all of the habitable space is outside the protection of the ice shield (on the front of the ship) and is also unprotected from the "velocity sleet" radiation by the mass of the fuel.

Can't the size of the ice shield be larger than that of the central pipe? It could be like a disk in front of the ship's nose, with the same diameter (or better a slightly larger one) than the outer diameter of the rings further back.

Is the fuel for your ship stored in the central cylinder?

Yes.

I assume that each habitation ring has a torus inside.

The rings are rotating around the central pipe, so the rotating hubs are sections of the trunk of the ship. Meaning, the outside of the rings rotates with them. It's not like the torus is rotating inside the stationary hull of the ring. They're not "giant indoor ferris wheels" like the Alem in Ashgabat, Turkmenistan (though I do have one scene in a virtual-reality chamber where they replicate that place, and that comparison is indeed brought up).

I also assume that you have shielding around each torus.

Yes, since the rings have 16-meter-thick walls to contain, among many other things, the security doors that are wide enough to seal off the entire corridor, as well as the water supply for each given ring. Since the rings are at a 90° angle to the direction of movement of the ship, they have plenty of water shielding in the walls facing both forward and backward.

So the only thing that's left to shield are the inner circumference (facing the central pipe) and the outer circumference (facing space). Or, from the perspective of the crew members, the ceiling and the floor. We could lead water through there, too, or use some other material. In either case, this would merely extend the ring inwards (as it is already being done with the farm ring), and/or outwards, without making any changes to the usable living area for the people inside.

You need a ring shaped ice shield in front of all of the rings and the rings should be close together so that one rings radiation shield mass can partly shield the adjoining ones. The fuel can also be in a ring of tanks between the ice shield and the habitat rings. The ice shield and fuel tanks to NOT need to rotate. This can reduce the core cylinder to a structural cylinder with a non-rotating navigation section also behind the ice shield.

Now we're back to how close or far the rings can be together to begin with. The graph made by @Filip Larsen suggested that I should stay below 80 m distance between the rings. Right now, it's at 60 m between each two rings. We haven't established a minimum distance between the rings yet, as far as I can tell.

The problem with this design is that the struts that attach the rings to the core are exposed to impacts at 30,000 km/sec. That is why it is much better to have a narrower habitat design with everything behind an ice shield, and preferably behind the fuel tank section. A 500 meter wide habitat ring is over 1500 feet in diameter, This is what you would build in the target system, not for use in an interstellar vehicle. That is more like a "Worldship" concept. Getting to the target star system sooner is more important than a huge habitat.

Well, this wouldn't be a problem if the ice shield is just wide enough, would it?
As I've said in my previous post: The ring can't really be smaller than 450 m in diameter (=225 m radius), or it would have to rotate too quickly in order to produce Earth-like gravity.

That fast rotation would regularly result in people on the rings falling onto their noses, because the Coriolis effect affects the liquid in our vestibular organs, too. And as far as I am aware, this could already affect people while just normally walking down the corridor.

I fully agree with you that the design of the ship needs to start with physical realities - and this happens to be one of the first things I researched for my ship design: How large the rings have to be.

My first designs were wide like this but I shrank them down to get a huge mass ratio value. The tori for design 3 have a radius of about 100 meters, a Major diameter of 200 meters, and a tube cross section (minor diameter) of 20 meters. The coriolis effect would only be noticed in certain circumstances, such a jumping sports, etc.

Well, that's bad enough: My crew has to run at least two laps around the public ring every day, followed by various other types of physical activity. Sure, those who spend the majority of their days on the rings have to work out less. But anyone with a job in the central pipe has to put in 2 to 2.5 hours of exercise by day, just like current astronauts on the ISS, since they spend most of their time in zero gravity.

Do not worry about microgravity in the ships bridge. The bridge crew would spend most of their time in the rotating sections. It would make the story more realistic and interesting.

Indeed, they're mainly on the rings. But the final confrontation happens on the bridge, so if that happens in zero gravity, it will be quite a shift to the scenery.

Yes, you CANNOT take on any supplies DURING the voyage, as you would have to slow down to zero velocity to do so, using up your precious deceleration propellant and leaving you stranded in deep interstellar space! Your ship could be designed to shed duplicate sections of materials in case too much propellant was lost in an accident. Such a detour is impossible to do if you want to reach your destination.,

I was thinking about ships that suck up hydrogen while traveling through the interstellar medium. Isaac Arthur has outlined how this sucking-up process would slow the ship down more than the thrust the fusion engine could create with it. However, for the same reason, this might make for a great method of slowing the ship down at the end, without having to use additional fuel.

Humans could detect malfunctions but the system would have multiple hardware backups. For the detection system to fail would require "a major malfunction".

Yes, I'm thinking more of a chain of unfortunate events. I'm a psychologist, so one metaphor we like to use is that of a cheese: As long as the "holes", the weak points in every layer, don't overlap, a security measure at the next level behind the current one can stop a disaster. If you have several holes overlapping and forming a "tunnel", that's how a potential catastrophe can bypass all security measures.

An interstellar vessel cannot slow down or stop "suddenly" any more than a train can stop suddenly. It has a huge mass and a huge amount of kinetic energy which has to be reduced gradually, over a period of YEARS.

Given the hydrogen-pickup method I just described: How long would it take to slow down a ship that way, from 10% to 2.5% light speed?

You need life with photosynthesis to have a stable, breathable oxygen atmosphere with enough buffer gas like nitrogen. There are just so many factors that weigh against a living world that there may be less than half a billion such worlds in our entire galaxy. Fortunately there should be enough rocky planets between 0.5 and 1.2 Earth diameters to provide terraformable planets.

A "habitable" planet also assumes a "rock" large enough to have gravity strong enough to hold its atmosphere. The lowest such size is about that of Mars or slightly larger.

I agree that Proxima does not look very favorable right now, but Alpha Centauri A or B could have planets that are not visible to us due to the tilt of their orbital planes, (all random).

Currently, Teegarden b has one of the highest, if not even the highest Earth-Similarity Index. Its mass is only marginally larger than that of Earth (1.05). As long as we don't know more about its atmospheric composition, it makes more sense to "postulate habitability" for this particular planet than for any other one out there.

Sure, I could simply make up an entirely new world of its own. Much like I could invent a new Wolf-Rayet star that points its future gamma ray burst at Earth, rather than working with the already known WR 104. But I think it's part of the "science" of science fiction to start with what's already confirmed. Otherwise, I could simply postulate the existence of an Earth 2.0, rather than working with the most likely existing restriction that Teegarden b orbits a red dwarf star and is tidally locked.

A scout mission that has an orbiter and/or lander must obviously slow down to near zero velocity. However, as there is no crew, its payload could be much smaller and the trip faster. Orbiters are far more valuable than landers, but a set of landers should be used to try to get samples of surface materials to make sure there are no high levels of toxic elements. Elemental abundances can vary from star to star. So both orbiter and landers are needed.
A super-AI would be required to make decisions on targeted surface locations and to control radio transmissions back to Earth. The main vehicle could stay in a high orbit so its signal would only rarely be blocked by the planet of interest. By this time, we might even have aware AI's.

Interesting idea! So you mean a probe orbiting the planet would already be waiting for the generation ship once it arrives? The generation ship itself will not stay in orbit, since people will want to take as much of the existing technology (and all the convenience it provides) with them down onto the surface.

A generation ship is by definition a sort of mission where the motivation to join up is about saving humanity from extinction of life in our solar system or a similar dire situation since joining up carries the moral implication that all but the last few generations on that ship will live their entire life on that ship. Alternatively the life on the ship will have to promise a better life on board than by "staying behind" in the solar system.

Yes, as I've said in the opening post: The reason for why the Exodus left Earth is established in the very first chapter, in the commander's New Year's speech: The angle of the Wolf-Rayet star WR 104 has been recalculated, so that it's considered confirmed now that Earth and its entire solar system are in the line of fire of a future gamma ray burst. She explicitly refers to WR 104 as a "death star", and one of the main characters casually says "too bad we can't just blow this thing and go home". ;)

But take away the generation-thing and it all makes much more sense. For example, it is not too unreasonable to assume that long term hibernation can become a feasible thing, at least in the context of interstellar travel. If passengers can hibernate for how ever long it takes to get there, then this makes a huge difference both to the scale of the ship design (allowing them to be simpler) and the motivation to join up. Allowing the passengers that join up to also experience the end goal lines up very well with the existing human emigration drive where you can take your family with you and start again at a more promising place.

Sleeper ships are indeed the next thing people on Earth vow to attempt, once the mission of the generation ship Exodus is over. In fact, that's how the third part of the trilogy is supposed to end: With the landing of the crew on the surface of Teegarden b, while the people on Earth have picked up their messages from the end of book 1 in the meantime (22 years have passed in between). And given the footage they've received, showing what happened on the ship throughout book 1, the survivors in Earth's solar system vow to never build another generationship again.

In the epilogue of part III, then, another 25-30 years later or so, I might actually show the first such sleeper ship arriving on Teegarden b.

Another "simplification" for the Hail Mary type of missions would be a seed or ark ship that only "needs" to bring the DNA of people and animals and would then, put short, "grow" them when conditions where right. This is of course only feasible if "growing" a human population back into thriving condition becomes possible and research into this would likely be consider unethical until the need for such a Hail Mary mission arises.

Without going too much into the philosophical side of my story: Where exactly and in whom does this "need" arise? ;) The people on Earth, threatened by a gamma ray burst, don't personally profit from new humans being created elsewhere, using a seed ship.

The whole point of the generation ship is to export an already functioning civilisation, so that they can quickly colonise a new world while retaining as much of their knowledge as possible - and that new world can then serve as a destination to evacuate those who are left on Earth (=will be born in the meantime while the generation ship is travelling).

A seed ship could only start a new human civilisation either in the stone age, or, even though they would have all the technology of the ship at their disposal, they wouldn't know how to handle it.

But in that case one could argue that compared to a generation ship it is perhaps more humane to research into making this possible and spare the, say, 8 generation for a "meaningless" life and spare the ship design for having to support a sustainable environment for 50k+ humans for over hundreds of years.

The people of my crew would heavily object to you calling their existence "meaningless". ;) In fact, they'd probably argue they have the most meaningful existence of all, because it contributes to a purpose greater than themselves: The colonisation of a new planet and the survival of humanity as a whole.

Now, whether there is any meaning to that particular purpose, that can of course be questioned... and indeed, my story will do just that. =D

All this is not to detract from any well-thought out sci-fi story about generation ships, just a few observations that the concept in the grand scheme of things may be a bit unrealistic in the first place.

I think the main reason the concept seems tempting to a lot of astrophysicists (i.e. people who know much more about the technical limitations than I do) is because, at least in theory, a generation ship could already be built now. It would just take much more primitive forms of fuel, and thus an even longer amount of time, to get even just to the closest star, Proxima Centauri.

My main concern is not the physical feasibility, but the psychological and ethical issues linked to the very concept of a generation ship. And while some seem to agree with me on that instantly, wondering who in their right mind would ever think of going on board such a thing, others are happy to keep entertaining this notion as something we might just subject future generations to.

Fusion engines do not produce a lot of thrust for the energy they use.

What about black-hole drives / Kugelblitz drives? Meaning, the creation of a black hole out of light, rather than matter, using several lasers.

I just consulted Isaac Arthur's videos on that again yesterday. =D It seems like they would potentially even be able to get a ship close to light speed (which is kind of something I need for the second book, so that the people landing on the planet at the end of part III are actually still those the reader grew to care about since book 1 ^^).

The smaller the black hole, the higher the energy output and the shorter the acceleration time to reach a given percentage of light speed. However, the smaller black holes also have a shorter lifespan. So they'd have to find a sweet spot. According to Isaac Arthur's chart presented in that video, a black hole with a lifespan of 125-135 years (=the planned duration of the Exodus's journey) would only accelerate the ship at very low G forces, requiring about 8 years to get it to just 1% of the speed of light. (Is this a linear function? Meaning, would it take 80 years to get it to 10% light speed, then?)

Metaphorically though, the image of a black hole behind the ship would be fitting in many ways. (They're trying to outrun the thing that would destroy them, they hide it where they least want to look etc.)

(At least) two potential problems with this:

1) In his video, Isaac Arthur mentions the idea that the ship would "stand like a skyscraper on top of the black hole", thereby eliminating the need (or even usefulness) of rotating rings. Given how integral to the setting and plot these rings have become by now, though, of course they are something I wouldn't want to sacrifice.

2) A black-hole drive seems to require the technology to reflect gamma rays: Such panels would be placed around the black hole, either in form of a hemisphere or a 360° sphere. Could those same panels then be used to somehow protect humanity against an impending gamma ray burst? So that it would be more feasible to just do that, rather than sending a generation ship elsewhere in the first place?

 

[Filip Larsen]

The people on Earth, threatened by a gamma ray burst, don't personally profit from new humans being created elsewhere, using a seed ship.

If you are trying to argue that people left behind would feel they "profit" more with a generation ship than with a seed ship I don't follow why. If I had to vote for either option in that situation I think I would vote for the option that gave the highest chance for humanity to continue elsewhere. And it would probably be unlikely that both options would be estimated to have equal chance of success.

A seed ship could only start a new human civilisation either in the stone age, or, even though they would have all the technology of the ship at their disposal, they wouldn't know how to handle it.

The concept of a seed ship obviously also need includes technology to "raise" the initial batch of people to become a educated functioning society (or have a hibernating "first generation" that can be woken to do the same) otherwise we surely lost something important in the transit.

But I am not trying to argue that a generation ship in your specific story is unrealistic, just that generation ships as a concept appears to have a fairly narrow usage where it would make sense to use them, all things considered. And if you story provides for such a scenario, as I understand it does, then that is just super

And as you explain, if the initial ship is just there to "prep" for more humans to come from Earth later seems to establish the proper "excuse" of doing something "quick and dirty" and not spend 50 years researching a better option first.

 

[John Strickland]

@Filip Larsen offers a good discussion of the rationale to have slowboats / generation ships / colonization ships in the first place. Assuming that it is possible to maintain space settlements in our own solar system, there is no immediate need to launch such ships for humanities survival, However, it is the things you don't know that can get you. Suppose there was a dictator on Earth that wanted to control all people living anywhere in the solar system. (Such people are depicted in the latest books in the Expanse series.

The other rationale is one fundamental to LIFE itself - to survive, life must grow and expand. If all life is on one planet, eventually life itself (ALL life) is at some risk of extinction. An exoplanet is so far away that only in space operas are dictators able to control multiple star systems.

The name I often use for this type of ship is slowboat, (a very ironic name since they eventually do move incredibly fast). One of my designs can reach the Centauri system in only 80 years, (given the values I assign and that no fusion engines yet exist in the real world). So for "nearby" stars (say within 8-10 light years), there may only be a few generations on the ship and some of those who board may yet live to see it arrive at an exoplanet. Did the pioneers, who set off in Conestoga wagons for California, Oregon and other distant "promised lands" ask THEIR children if they knew how dangerous the trip would be. For the inhabitants, the ship would be comparable to a large village or small city, with a comparable area inside it, without even trying to build one the size of a "worldship".

 

[Filip Larsen]

So for "nearby" stars (say within 8-10 light years), there may only be a few generations on the ship and some of those who board may yet live to see it arrive at an exoplanet.

If the transit time is so short that most young couples signing up will know their children gets to walk on the new planet and they even might have a chance to get a glimpse themselves, then that increases the motivation to join up quite a bit I would say. But I still think the choice to join up is only going to be "easy" if there is a large (perceived) benefit in joining up, meaning either a serious disaster (end of life) for you or your children if staying behind, or a possibility for a much better life if joining up.

I must admit that when thinking about how likely people would be to want to sign up for a generation ship mission, I have thought the alternative to be to continue to live on a planet (e.g. Earth and Mars). If we allow for scenarios where generations of humans have already lived full or most of their their life in space, then for such "space-borne" people I would think it will probably be much easier to choose to go on a generation ship, even if its destination is many generations away. But then again, if you have lived your whole life in space and accept that way of life for good and bad, then you would probably at least need a good reason to join up other than the promise of a new planet for your off-spring.

 

[Strato Incendus]

If you are trying to argue that people left behind would feel they "profit" more with a generation ship than with a seed ship I don't follow why.

From the colonisation itself, the people on Earth don’t profit in either case, you’re right. What they will profit from is having a working civilisation on another planet to escape to. So my argument is based on the assumption that it will be easier for a society of living human beings to establish a new civilisation on another planet than for one created by a seed ship.

The concept of a seed ship obviously also need includes technology to "raise" the initial batch of people to become a educated functioning society (or have a hibernating "first generation" that can be woken to do the same) otherwise we surely lost something important in the transit.

This is of course an option. However, it would require a much more capable AI than anything we tend to imagine currently, and that can lead to all kinds of new plot holes. The AI might even be the main governing force in that case. And a civilisation created by an AI would have to take the AI’s word for everything they’re doing. To me this sounds reminiscent of the Star Trek: TNG episode “Conundrum”, where the crew lose their memory and have to figure out what their mission on the Enterprise even is, who holds which position etc.

The AI on board of my ship is more of a mixture between the Enterprise computer and Siri / Alexa. In fact, her name is IRIS (Intelligent Response-to-Inquiry System). Which is also partly inspired by Iron Man’s “Jarvis”.

And as you explain, if the initial ship is just there to "prep" for more humans to come from Earth later seems to establish the proper "excuse" of doing something "quick and dirty" and not spend 50 years researching a better option first

Indeed. Given the constant threat of the gamma ray burst, where you can never know when it will go off (and once you see it, it’s too late), the plan is “a) get some people to Teegarden b as quickly as possible, b) have them set up a somewhat functioning civilisation there, c) evacuate the rest”. The latter point means: If no faster drives can be developed in the meantime, nobody living on Earth will ever get to see the planet with their own eyes. But at least their children won’t live on a planet devastated by a GRB. If however sleeper ships etc. can be developed in the meantime, then exporting the current people from Earth would indeed become possible.

Assuming that it is possible to maintain space settlements in our own solar system, there is no immediate need to launch such ships for humanities survival,

The solar system is indeed colonised, but a) as far as I am aware, a gamma ray burst would / could affect the entire solar system, and b) none of the other places are exactly made for humans. Because much like on a generation ship, you basically have to always stay inside:

Spoiler

- On Mars, there are largely subterranean (technically: sub-martian) settlements, to better shield against the radiation, since Mars doesn’t have a magnetic field. A similar scramble for this planet occurred between different nations on Earth, much like it once happened with Anarctica.

- On Venus, NASA’s High-Altitude Venus Operational Concept (HAVOC) has been realized. At the start of the story, there have at least been 12 such cloud cities on Venus (although one of the earlier attempts ended in a massive disaster, where such a city crashed to the ground because of damage to the balloons). People who go to live on Venus have a reputation / stereotype of being adventurers, risk-seekers, living for the day etc. Yet, there are some who also have their children in one of these cloud cities. Is that riskier than having them on a generation ship? Or less risky?

- Mercury has a single outpost called FREDDIE-01, and it’s only for research purposes. I originally thought it could be located in the twilight zone between the hot day side and the cold night side - a parallel to where the generation-ship crew would have to live on the much more hospitable Teegarden b. However, as I’ve then found out, Mercury isn’t actually tidally locked - it just rotates very slowly. Meaning, there is no day- and night side, and whenever you’re on the day side, the surface heats up rapidly. This outpost would therefore have to be pretty far underground, I guess.

- There are further stations on Ganymede, Callisto, Io, Europa (they had a legal battle with Venus over who got to use “The Final Countdown” as their anthem), and Titan.

- Then, to bridge the gap between HAVOC and Bespin, the HAVOC design has been adapted to work on gas giants, too: The crew receive one message from an admiral stationed on HASSLE-03 (High-Altitude Saturn Solution of Levitating Elements). These cloud stations are very high up, where the pressure is equivalent to that on Earth. Sadly, in contrast to what Bespin made us believe, there can‘t be any oxygen in such layers of gas giants (meaning, even if a different gas giant had oxygen). So you can’t actually go outside without a spacesuit.
Cloud stations on gas giants of course also need to withstand much higher wind speeds than on Venus. The generation ship is facing different problems, like the barrage of particles and background radiation, as discussed before. But at least wind speeds of over 1000 km/h are not something they have to deal with. So again: Is having a child on a Saturn cloud station riskier than on a generation ship?

- The HASSLE design has been transferred onward from Saturn to Uranus. People who go there are of course the literal butt of every space joke in the society of the solar system in the 24th/25th century. The guy whose message the crew receive from there - a distant offspring of the common ancestors he has with the protagonist, i.e. a distant relative of hers - only went there because he had lost a bet.
Even some of the missions themselves were in on the jokes:
The reason for the settlements in the clouds of Uranus is the construction of a gas refinery (with the eventual construction of a ring with “chandelier cities” hanging down into the atmosphere). This is another concept proposed by Isaac Arthur. So the current initiative is called “Refinery Erection and Colonisation Trial Uranus Mission”. And since the chandelier cities are eventually supposed to replace the cloud settlements, meaning the latter are only temporary, those are called “Balloon Units for Temporary Testing”. I think you can infer the abbreviations for those missions yourselves. ;)
The hydrogen from the refineries is eventually supposed to be used a) for fusion reactors, of course, but also b) to attempt to terraform Venus. Since it’s much closer to Earth in mass than Mars is, this actually seems a lot more promising.

- Neptune has a similar mission going on as on Uranus, given the similarity between the two planets. However, since it’s further out, there’s only one cloud post there so far, called Nautilus.

- I‘m still contemplating whether planet 9 has been found or not in this setting; the degree of evidence for its potential existence seems to be varying constantly right now. If it is real, to stick with the Roman-Gods theme, I predicted it would either be called Juno or Minerva (Hera or Athena). Since it’s most likely a gas planet, if it exists, Juno as a smaller match for Jupiter seems to make more sense. In either case, planet 9 has not been colonised yet.

Suppose there was a dictator on Earth that wanted to control all people living anywhere in the solar system. (Such people are depicted in the latest books in the Expanse series.

My story is actually moving away from the “One-World Government” trope that seems to be omnipresent in most other sci-fi settings. ;) From an evolutionary perspective, humans are tribal creatures, who evolved in nomadic settings with a few dozen to a hundred people; my observations of our behaviours so far do not give me any reason to assume people at large would ever be fine with a global government (like the UN filling that role, in case of The Expanse).

I get how this concept is useful if you want to make a story about interplanetary political conflicts (hence, the same approach is used in Star Trek). However, in Deep Space Nine, they’ve also briefly mentioned some “inner” conflicts on Bajor itself, for example.

In my setting, though, the majority of people still live on Earth, and the colonies in the solar system, given the inhospitality of Mars, Venus, Titan etc., are still largely dependent on resupplies from Earth.

The other rationale is one fundamental to LIFE itself - to survive, life must grow and expand. If all life is on one planet, eventually life itself (ALL life) is at some risk of extinction. An exoplanet is so far away that only in space operas are dictators able to control multiple star systems.

Well, if a gamma ray burst doesn’t make the solar system uninhabitable: The sun going red giant definitely will do so one day. ;) Of course, that’s such a big time scale that the threat no longer seems as imminent. Thus, it would no longer work to explain any “quick and dirty” approach. If anything, this would show that the society on Earth has become very conscientious if they’re planning that long-term. ;)

No dictator needed; the universe itself is dangerous enough. And I prefer that: Most readers are aware of the human potential for evil, but they’re largely ignorant about the many things out there in the cosmos that could kill us just as easily. They might be aware of asteroids and nearby supernovae. Not of gamma ray bursts, or rogue brown dwarfs passing by our solar system and messing up all the gravity relations.

Did the pioneers, who set off in Conestoga wagons for California, Oregon and other distant "promised lands" ask THEIR children if they knew how dangerous the trip would be.

Here we get into the two possible perspectives on this:
If there’s no ethical problem with having kids on Earth, then there also isn’t one with having them on a generation ship, or elsewhere in the galaxy. This was e.g. Neil deGrasse Tyson’s line of argumentation.
However, of course he opens himself up to the inverse conclusion, too: If there is an ethical problem with having kids on a generation ship, what does this say about Earth? ;)

For the inhabitants, the ship would be comparable to a large village or small city, with a comparable area inside it, without even trying to build one the size of a "worldship".

The Exodus isn’t as large as it is (with rings 500 m in inner diameter) because it is supposed to be a worldship; it is as large as it is for technical constraints first (1 G add sufficiently slow rotation for humans to be able to handle the Coriolis effect). And second, so that the crew doesn’t feel completely “crammed into” this spaceship.

You could probably fit more people into my ship as it is, but aside from the food and water supply problems that might emerge, the setting itself would become so claustrophobic that it could drive a crew spending their entire life aboard insane. Even though the story is about how survival and well-being can be in opposition to each other: A certain level of well-being for the crew is required to ensure their survival, and thus the success of the mission.

If the transit time is so short that most young couples signing up will know their children gets to walk on the new planet and they even might have a chance to get a glimpse themselves, then that increases the motivation to join up quite a bit I would say.

My current ship would only take slightly longer than a slowboat, at 125 years for the whole journey. This will not be enough for Generations Zero and One to still walk on the surface. But given how the generations “overlap” (e.g. millennials are the children of boomers, but there’s another generation in between, Gen X), Generation Five are the children of Generation Three, who are the children of Generation One.

So the first generation born on the ship (Generation Zero are their parents who originally boarded the ship) know that their grandchildren will walk on the planet’s surface. Of course, Generation Five will be in their 40s and 50s by then, so Generation One will no longer be around to witness that, and Generation Three won’t necessarily do so either.

But I still think the choice to join up is only going to be "easy" if there is a large (perceived) benefit in joining up, meaning either a serious disaster (end of life) for you or your children if staying behind, or a possibility for a much better life if joining up.

The gamma ray burst of course raises the question of how to protect the people who remain back on Earth (the vast majority of humanity at this point). The main thing is the construction of underground settlements: In particularly deep Metros (like in Moscow or Pyongyang), in deep natural caves (the deepest one being the Veryovkina cave in Abkhazia / Georgia at 2 km depth), or in artifically-made new ones.

At latest once the burst hits Earth, everyone will be living underground. If your stuck in an enclosed space like that anyway, the difference to life on a generation ship might not longer feel as significant. And if you board such a ship, at least your different offspring will get to live on a planet’s surface again, as people on Earth used to.

I must admit that when thinking about how likely people would be to want to sign up for a generation ship mission, I have thought the alternative to be to continue to live on a planet (e.g. Earth and Mars). If we allow for scenarios where generations of humans have already lived full or most of their their life in space, then for such "space-borne" people I would think it will probably be much easier to choose to go on a generation ship, even if its destination is many generations away.

Yes, self-selection will be at work here. Just like only somewhat adventurous people would ever consider living in a cloud city on Venus, for example (though “city” is certainly an overstatement, they’re not remotely comparable in size to what would count as a city or even a town on Earth. ”Cloud Village” just doesn’t sound as good =D .)

 

[John Strickland]

to Strato Incendus
Yes, the ice disk should cover (be between) all ship structures and any particles in the vehicle's flight path. It should probably not be supported only by the core cylinder, but also by other struts placed around the circumference of the vessels "hull." This non-rotating hull can house the shielding that would otherwise need to be housed in the rotating sections, vastly reducing the stress on the bearings. In my design, multiple obstacle detector systems peek out from around the circumference of the shield, with significant redundancy. Lasers can pop out beyond the ice disks protection zone to zap a particle in the ships path. A gas cloud will definitely have less impact on a ship than a particle.

The fuel tanks should be inside the hull, which can be mostly just a thin Whipple shield. I still do not see a fuel mass to dry mass ratio for your vehicle. To reach significant interstellar velocity, the fuel mass must be much larger than the dry mass. The central cylinder does not seem to have enough volume to support a significant fuel mass.

"The rings are rotating around the central pipe, so the rotating hubs are sections of the trunk of the ship. Meaning, the outside of the rings rotates with them.

This makes me think that the rings are multi-level. The force of gas within the pressurized sections can be held in most efficiently with a torus shape. It would take more structure to create a torus that is higher than it is wide, like one with an oval cross section. If the habitation rings are close together, the shielding mass, which can be attached to the thin hull, would be much less.

It would be much safer and more practical to have the outside of a bearing for a ring mounted ON the core, rather than whole sections of the core rotating independently. Such a design would turn the whole ship into something like a cylindrical roulette wheel! The front and back of the ship should not rotate at all in respect to each other, and it would be a difficult system to design with all of those huge bearings. This would also impact the fuel storage issue.

"the rings have 16-meter-thick walls"
I use the huge fuel mass to block all of the velocity sleet radiation coming from the front direction, and shielding around the rotating tori to shield them from the sides. There also must be one radiation shield layer between the engines and the habitation sections. 10 meters of water is enough to block GCR coming from the sides. It is more important to have fuel volume than water volume, but you cannot replace any fluid during the trip, so you really do need very large redundant amounts in case of tank punctures, etc. The same is true of all gases you would need to replenish, oxygen, nitrogen, helium, etc. You can get more O2 and H2 from the water using power.

"to how close or far the rings can be together to begin with"

I am not sure why you think the rings should be separated this far. For rotating structures in a vacuum where no vehicle would operate, spacing of 5 meters should be fine. This also allows the sharing of side radiation shielding mass. Can you give me the reason you want such a large separation? The threat comes from ahead of the ship, not from the side. The rotating sections are not going to hit each other without a catastrophic impact on the core, another reason to have a hull with struts to support the ice disk in front.

"The ring can't really be smaller than 450 m in diameter (=225 m radius), or it would have to rotate too quickly in order to produce Earth-like gravity."

Where is the source that specified this radius? Most sources I have seen for over 40 years indicate that people tolerate up to 2 RPM without a problem. Some allow even faster rotation. NASA has steadfastly refused to do experiments with rotational gravity, which also has prevented low gravity experiments with mammals for gestation and normal growth.

"My crew has to run at least two laps around the public ring every day:

Do you remember the Skylab crew who ran around and around inside one level of the Skylab, using their velocity to create their own "gravity" against the wall/floor. They had no problem at all.

"I was thinking about ships that suck up hydrogen while traveling through the interstellar medium. This might make for a great method of slowing the ship down at the end, without having to use additional fuel."

The utility of this method has long been proven to be zip. If the interstellar medium were that dense, it would be hard to see the stars. It really is VERY EMPTY. Same for slowing down. A design that uses this method would be doomed to travel between the stars until they ran out of volatiles. With a slowboat, you can only stop one time! That one time MUST BE at the target star.

"As long as we don't know more about its atmospheric composition, it makes more sense to "postulate habitability" for this particular planet than for any other one out there."

This would be a very dangerous gamble with the lives of the crew and the success of the mission. I would assume that the world is sterile and does not have an oxygen atmosphere. If it did have, caution would need to be exercised against infection from any microorganisms on the planet. You do not want a "green slime" invention killing the crew before they even land. The first thing a ship would do would be to get into a near-polar orbit around the planet and do detailed mapping and mineralogical searches for the best landing site for several years. A cold world - equatorial, a hot world, polar. High air pressure, a high plateau, low pressure - a deep valley, seashore or canyon. etc.

In the typical old SF story, the ship lands at a random location as soon as it arrives. Like the Jamestown settlers, this may be a very bad location! For your story, the survey period can be condensed to a few months. Then you need a plan to allow survival on the planets surface, and a way to travel repeatedly between the ship and the planet. This means a LOT of equipment, and vehicle bays in the ship.

"The generation ship itself will not stay in orbit, since people will want to take as much of the existing technology (and all the convenience it provides) with them down onto the surface."

How will this huge interstellar ship possibly land on the planet? It is not aerodynamic for entry and does not have the structure for a landing. Would it land on the surface vertically or horizontally? Would the habitat rings continue to rotate with the floors becoming vertical? Sounds like suicide to me. That is what ferry ships are for. You have a bunch of them to carry people and equipment down. The first thing you need on the surface is shelter and the hardware to make rocket fuel so the ferry ships can take off again.

"Sleeper ships are indeed the next thing people on Earth vow to attempt."

With the medical technology advancing very fast, we should have true human hibernation (like squirrels, not bears) much sooner than starships. However, unlike the well-done movie "Passengers", there MUST BE an awake team on duty at all times to handle problems. The team members would rotate during the trip.

"The whole point of the generation ship is to export an already functioning civilisation, so that they can quickly colonize a new world while retaining as much of their knowledge as possible"

I strongly endorse this reasoning. I see no rationale for frozen human zygotes to be sent to the stars, even with aware AI robots to care for them. I am sure that such a rationale can be invented, but it is not a desirable situation.

"In fact, they'd probably argue they have the most meaningful existence of all, because it contributes to a purpose greater than themselves: The colonization of a new planet and the survival of humanity as a whole."

Another strong endorsement for the rationale of having slow boat colonization ships, gamma ray burst or not.

"What about black-hole drives / Kugelblitz drives? Meaning, the creation of a black hole out of light, rather than matter, using several lasers."

A lot of this I consider arm waving by various people. Nothing wrong with it for an SF story, but this makes the story somewhat less believable than one based on known physics. SF stories are as much about LIMITATIONS as they are about what people CAN do.

 

[Strato Incendus]

First of all, I just want to say that I appreciate how we've moved from the initial reactions being "Why do you care about the realism of your ship design so much? It's sci-fi, readers won't care!" to @John Strickland probably caring even more about all of this than I myself do.

This is ideal, since now you've figuratively become the barrage of particles and background radiation my ship will have to face - trying to poke holes into my design wherever you can. And of course, I prefer this to happen at the current stage, before I fill the ship with "more life". ;) Science is about trying (or at least being willing) to falsify your own claims, after all.

I still do not see a fuel mass to dry mass ratio for your vehicle.

That's because we still haven't agreed on the final measurements of any part of the ship. We have so many moving targets at the moment (ring width, ring-to-ring distance, overall ship length, diameter of the central pipe)... many of these factors seemed to have been settled on the first two pages ;) . But of course, given your valid criticisms, they are up for debate again now.

The central cylinder does not seem to have enough volume to support a significant fuel mass.

How would you propose to expand the cylinder then? By increasing its diameter? Or by increasing its length? The initial design of the ship was 3 kilometres long. I shortened it considerably as I shortened the distance between the rings.

Then again, have we even agreed on what type of fuel we're talking about here? I assume hydrogen for the fusion reactor. But if we're including black-hole drives, this might translate to "hydrogen for the fusion reactor to power the lasers which then create the black hole made of light".

This makes me think that the rings are multi-level.

Yes, they are. I've shown this in the image I've uploaded on a previous page of this thread. I'll upload it again here:

Spoiler

It would be much safer and more practical to have the outside of a bearing for a ring mounted ON the core, rather than whole sections of the core rotating independently.

You mean the rotating hub is outside the hull of the central pipe? That would work, too - and make for more epic zero-gravity fights within the central pipe itself.

One reason for why I wanted a certain distance between the rings was so that I could have a long enough tunnel in which there was no gravity. (This is relevant for another question further below.) If the hub is on the outside, there would be no gravity in the entire central pipe.

However: How do you get into the lifts going down the spokes, then? The entrances to the shafts would then be locked in place in the central pipe, and you could only use a lift whenever the hub rotating outside would pass by one of these entrances. More importantly, the ring hub couldn't simply "stop" to allow lift passengers to enter.

If the hub is part of the wall of the central pipe, as it is the case in the current design, then the entrances to the lift shafts rotate together with the lift shafts themselves, along with the rest of the ring. So you could walk along the rotating up (at about 0.2 g, given that the diameter of the hub is a fifth of the inner diameter of the ring) until you reach an entrance to a lift.

10 meters of water is enough to block GCR coming from the sides. It is more important to have fuel volume than water volume, but you cannot replace any fluid during the trip, so you really do need very large redundant amounts in case of tank punctures, etc. The same is true of all gases you would need to replenish, oxygen, nitrogen, helium, etc. You can get more O2 and H2 from the water using power.

If 10 metres are enough, that's good to know, so it seems like I'm in the right ball park with the thickness of the ring walls. Obviously, the walls will contain more than just water - electricity and heating, too, to start with. The additional 6 metres would even allow me to include "crawling corridors", like the Jefferies Tubes in Star Trek. These could come in handy when planning a mutiny and having to hide away from security officers patrolling the corridors etc.

I am not sure why you think the rings should be separated this far. For rotating structures in a vacuum where no vehicle would operate, spacing of 5 meters should be fine.

The ring distance is one of the first things I asked for in this thread, and @Filip Larsen made this nice little graph to indicate the maximum distance between the rings (80 m, although that was still based on the ship being 3 km long). As I've said before, we hadn't determined the minimum distance yet.

If 5 metres works, this would of course reduce the mass of the ship even further. But it might just mean that the ship has a longer back end for all the fuel, so the central pipe wouldn't actually get any shorter.

Where is the source that specified this radius? Most sources I have seen for over 40 years indicate that people tolerate up to 2 RPM without a problem.

From this discussion on Quora. Specifically, Robert Frost's post(s). 2 RPM is indeed what he works with, and then he provides a formula that results in a minimum radius of 223.49 m for 1G at 2 RPM (or 224 m in his second post).

I want my rings to be able to create Teegarden b's gravity, which is 1.05 G. So I quickly calculated that with the same formula, arriving at a minimum radius of 234.5 metres.

There are other people in that discussion suggesting smaller ring diameters, but they come at the cost of higher RPMs (like 4).

I've upscaled the radius of my rings to 250 metres, to have a little more leeway, i.e. allow for even less than 2 RPM. What trained adult pilots and astronauts can handle might differ from what the average citizen who just happens to be born on a starship can handle, especially thinking of the children on board.

As you've said, we can only estimate, since we don't have a lot of real-world testing to point to yet in this regard.

Do you remember the Skylab crew who ran around and around inside one level of the Skylab, using their velocity to create their own "gravity" against the wall/floor. They had no problem at all.

That was long before I was born. But as far as I can tell, this mission had a very short duration, especially compared to the journey of my generation ship, which takes over a century. Again, consider the simple fact that there are children on board - including infants who can't actually walk / run by themselves yet.

Even if everyone else could exercise perfectly fine in zero-gravity, using the same techniques as current astronauts on the ISS: The infants would need artificial 1G gravity created by a ring, so that their muscle and bone structure doesn't deteriorate before they've even begun to learn how to walk and run themselves. ;)

The utility of this method has long been proven to be zip. If the interstellar medium were that dense, it would be hard to see the stars. It really is VERY EMPTY.

I am aware how empty it is. But if at least some physicists (like Isaac Arthur) still keep proposing such methods, I as a layman won't object right away. Even though a lot of Isaac Arthur's proposals sound rather utopian to my ears.

Then again, a lot of astrophysicists also keep suggesting the idea of generation ships. And that's something where I as a psychologist have to object. That's kind of part of the reason why I'm writing this story.

"As long as we don't know more about its atmospheric composition, it makes more sense to "postulate habitability" for this particular planet than for any other one out there."

This would be a very dangerous gamble with the lives of the crew and the success of the mission. I would assume that the world is sterile and does not have an oxygen atmosphere.

I meant "postulating habitability" from the author's perspective - not from any character involved in the story. ;) Teegarden b, given it's high Earth-similarity index and our current inability to know the atmospheric composition, is my best "real" candidate planet, shy of simply inventing an entirely fictional one of my own.

Within the story, as I've outlined above, of course they've sent a bunch of unmanned probes to the system, at much higher velocities (Breakthrough Starshot method), to examine the candidate Teegarden b thoroughly and confirm its habitability for humans. Otherwise, they never would have sent a generation ship there in the first place.

If it did have, caution would need to be exercised against infection from any microorganisms on the planet. You do not want a "green slime" invention killing the crew before they even land.

This danger is established in the very first chapter, when the protagonist's partner is introduced: He's late to the anniversary celebration, because he's working at the biolab, where he had to get everything set up for the next virus exposure tomorrow. Responding to the complaints of his fellow crew members, he states that they wouldn't want to end like H. G. Wells's Martians when they land on Teegarden b, would they?
In other words: On board, they constantly develop artificial diseases, to keep the crew's immune systems as sharp as their physical fitness.

Of course, they can still be hit by some unforeseen new microorganism when landing on the planet. Which is why...

The first thing a ship would do would be to get into a near-polar orbit around the planet and do detailed mapping and mineralogical searches for the best landing site for several years. A cold world - equatorial, a hot world, polar. High air pressure, a high plateau, low pressure - a deep valley, seashore or canyon. etc.

...the first "person" they send to the surface is indeed an android. Examination of the surface, identification of good landing spots etc. has already been done in part by the previous unmanned missions (with the rovers continuing to transmit data in the meantime, as the ship was traveling towards its destination). But of course, once they're there, they still do quite a bit of such scans themselves, up-close. The landing spot they settle on in the end is indeed a vast plateau, with ample access to water from the mountains.

In the typical old SF story, the ship lands at a random location as soon as it arrives.

Nope. That's not happening in my story. If only for the fact that they have a long ethical debate first, about whether they should actually land the ship in the first place. Because guess what: Some people have gotten used to living out their lives on board. And as long as the ship is functional, why change anything?

How will this huge interstellar ship possibly land on the planet? It is not aerodynamic for entry and does not have the structure for a landing. Would it land on the surface vertically or horizontally? Would the habitat rings continue to rotate with the floors becoming vertical? Sounds like suicide to me. That is what ferry ships are for. You have a bunch of them to carry people and equipment down. The first thing you need on the surface is shelter and the hardware to make rocket fuel so the ferry ships can take off again.

Another thing I've explained on the previous pages of this thread ;) :

The rings unattach from the central pipe, then dismember further into their subsections. This is the reason why I need doors wide enough to block the entire corridor on the rings (and hence, walls thick enough to hide these doors): They have to seal off the corridor airtight once the sections disconnect from each other.

The sections of the rings then become the first "buildings" on the surface - thereby providing instant shelter to the entire crew. This means the outside of the rings will be facing to the ground, so they will have to be covered in the same type of coating as the underside of a space shuttle, so that they can enter the atmosphere.

Naturally, this will require engines on each section of the ring, so that they can steer during the landing process. But then again, somehow the rings also had to be set in motion originally, to kick off their rotation. Having thrusters on the rings might therefore be useful for this purpose, too.

The central pipe and the bridge section do not land on the planet - they no longer serve any purpose at this point. Especially if the majority of the central pipe, according to your suggestions, would have to be used for fuel anyway. They remain in orbit and eventually get blown up, once the entire crew has landed.

Spoiler

(Well, in fact, the autopilot for the dismantling process is damaged, so one person has to stay behind on the bridge to orchestrate the landing. This is the "bittersweet ending" for the third part.)

With the medical technology advancing very fast, we should have true human hibernation (like squirrels, not bears) much sooner than starships. However, unlike the well-done movie "Passengers", there MUST BE an awake team on duty at all times to handle problems. The team members would rotate during the trip.

I think this would clash with the idea of exporting a cohesive, functional society:

- If there's a rotating crew going in and out of hibernation, society could go through drastic changes during whatever number of years they are unconscious. So whoever wakes up might wake up into a world they no longer recognise, and they would require an update briefing every time. Here we are back with the "Conundrum" Star-Trek example again.
Second: The "living" society on the ship would also be shaped significantly by who is awake at a given time and who isn't. Releasing someone from their duty and sending them into their hibernation shift might be a great way to get rid off unwanted opposition.

- If there's a sleeper ship and a generation ship in one, meaning one part of the crew living out their lives and having children normally, the rest of them being in hibernation until their arrival, this would create a split society on the new planet: Those who have slept the entire time would still remember their lives on Earth as their most recent memories, while those who were born on the generation ship wouldn't have any memories of Earth at all.
Added to that, the people from Earth who boarded the ship originally stem from all countries on the globe; so if they all stick to their former home's customs - given that, as far as they recall, they just left them - there would be greater conflict among the hibernation passengers than among those who've already "grown together as a crew" in the generation-ship subgroup.

A lot of this I consider arm waving by various people. Nothing wrong with it for an SF story, but this makes the story somewhat less believable than one based on known physics.

I've heard Kugelblitz drives discussed by a variety of channels, including such that don't have anything to do with sci-fi writing per se. As I've said above, Isaac Arthur seems to pretty much jump on any potential idea with just a slight chance that it might work. But the first time I heard about Kugelblitz drives was actually from PBS Space Time (a physics channel, not a writing channel). As far as I can tell, they tend to stay more in the realm of what might be possible - at least compared to Isaac Arthur.

SF stories are as much about LIMITATIONS as they are about what people CAN do.

Yes, but those limitations need not be technical. In Star Trek, most of the time, the plot-relevant limitations are ethical, political, or psychological ones. Technical stuff in Star Trek is usually just some made-up gobbledygook, to give Geordi and Data something to say. What actually matters is how the characters then interact with each other, who proves their ingenuity in what ways etc.

And despite my story striving for a greater deal of realism, or at least an "update" according to the current state of the science, it's still very much in the spirit of Star Trek.

In contrast, a story shaped largely by the technical limitations of space travel is Amazon's "Mars" series (by Brian Grazer and Ron Howard). This show demonstrates pretty drastically what a Herculean task it's going to be to "just" successfully land and set up a first colony on Mars. So when I watched that series, the technical limitations and realism was all I cared about. The characters, meanwhile, couldn't have been more dull and boring (especially the captain).

There is a certain danger of falling into the old "Titanic-Door debate" here.

Spoiler

I already figured out as a kid that obviously, there would have been enough space on top of that door for Jack to climb up, so that both him and Rose could have survived. My father gave me the "not enough space" explanation as a kid - but guess what, some science nerds calculated this, and apparently proved 4-year-old me right. James Cameron even met with the guys, and acknowledged their criticism. But at the same time, he couldn't have cared less, telling them "It doesn't matter - the dude's going down!"

The point about the ending of Titanic is that it contains a metaphorical truth about evolution, with Jack sacrificing his life to protect rose. This could be explained in terms of the "Selfish Gene", for example. What Jack does is evolutionarily viable, and that's why such stories resonate with us. Against that background, nobody cares how big that door is. If Jack had survived, that would have been more realistic, given the apparent size of the door. But it would have utterly ruined the story.

As I've mentioned previously, my characters actually discuss the Titanic movie in one chapter, where the film is shown one evening, and the danger of collisions with something much smaller destroying the Exodus on its own maiden voyage to a new world is established. Hence, the ice shield in front of the ship would be another interesting parallel - except the ice is what protects the ship, rather than what destroys it.

 

[John Strickland]

"trying to poke holes into my design wherever you can"

Of course my hole poking is intended to try to improve your construct. I use the mathematicians term construct for the imaginary world SF writers create with some difference between it and reality. If there is no such difference, it is not SF, just regular fiction.

"probably caring even more about all of this than I myself do"
The two main things I care about in this context is preserving LIFE itself, by spreading it around on a very large scale, and the very useful functions that Fusion power will be able to provide once it is perfected, including:
Power Production
Terraforming - moving volatiles to a planet or removing them from a planet
Interstellar travel - fusion engines with exhaust velocities in the thousands of km/sec.
Having seen in detail the incredible diversity and complexity of life on Earth, it is obvious that it is precious beyond belief.

So SF is interesting, enjoyable and also a good tool to investigate aspects of the real reality with a model reality.

fuel mass to dry mass ratio:
What I suggest is creating a baseline set of masses for ships in the general size range of your ship, and you then can decide, (depending on the thrust of your fusion engines), how fast the ship can go, based on the fuel mass vs the dry mass. You can use the same ratio for the whole set of ship masses as long as the dry to wet mass is kept the same. I used a complicated volumetric mass estimate system that since your book is fiction, you do not need to use, but you do need a dry mass to wet mass ratio and a total thrust amount. The mass ratio is very basic for any rocket system. If you then know the density of your fuel, you can then estimate its volume. Higher density fuel like Borane is better since the tanks are smaller and lighter! However, you need the right fuel for the reaction you are using.

"How would you propose to expand the cylinder then"

The most obvious method would be fuel tanks that are not inside the cylinder, but inside the cylindrical hull volume which is slightly narrower than the ice disk. The cylinder should act as part of the main structure of the ship, (along with the struts in the hull. It should be motionless between the engine end and the ice disk end. If the fuel tanks are all in the cylinder, a major blow to the ice disk could rupture ALL of the fuel tanks. If you have a bunch of tanks, the fuel storage is thus modular, and a single accident will not cause the loss of fuel from all of them (unless it is catastrophic).

Remember I said that the fuel mass must be much larger than the dry mass, just like a regular chemical rocket. The rocket equation rules space transport! So if you have a million ton (dry) ship, for example, you would need at least 100 million tons of propellant or more. It all depends on how fast you want to get to the destination star system. My ship was 4 million tons with 1 billion tons of propellant. The fuel tanks were like metal balloons.

(Site did not allow image to be shown)

You could reduce mass by having a series of tori of smaller and smaller radii inside the inner radius of each larger torus to fill each ring space. This also compartmentalizes the tori better in case of an air leak.

"If the hub is on the outside, there would be no gravity in the entire central pipe."

Why do you need gravity in the central pipe at all? It would be very low gravity anyways. Having microgravity in the central pipe would be more realistic. If you remember the only place that had gravity in the movie 2001 was inside the sphere with the rotating section. You could not have gravity in the bays that house the planetary ferries. A ship with a central structure that rotates and also has multiple counter rotating sections would seem like a Rube Goldberg device, and would probably drive some spacecraft engineers crazy!

"How do you get into the lifts going down the spokes, then?"

In my ship designs, all of the tori are in line with the center of the ship, and all are in a row in a large compartment near the rear of the ship. This compartment is in a vacuum to avoid friction with the rotating tori. I had a single counter-rotating pair but you could have a set of 4 or 6 of them. There is an access tunnel (like a short section of your core cylinder) with bearings holding the tori at intervals. Access is from the axis of the hub of each torus along the tunnel and through the wall of the tori compartment into in-vacuum work and storage areas near it. There is no central core running the length of the ship, which is kilometers long filled with huge fuel tanks.
I sure wish I could send you the image of what this looks like that is in book 1, a detailed cross section created by an artist who lives in Tel Aviv. I cannot name the artist or the book as that would be self-promotion and I would be kicked off the forum. You might be able to find copies of the books by searching with my name on certain topics.

MORE TOMORROW as it is late.
Tuesday 3-29-2022

Obviously, the walls will contain more than just water - electricity and heating, too, to start with.
The 10 meters is water in tanks, you want to do a simple calculation of the total water volume, as if it is comparable to the fuel mass, your trip could be close to twice as long in time. You do not need 6 meters of wall space around all of the rings. Office buildings use about a meter.

If 5 metres works, this would of course reduce the mass of the ship even further. But it might just mean that the ship has a longer back end for all the fuel, so the central pipe wouldn't actually get any shorter.

I have no idea of the basis for selecting the 80-meter distance. It almost sounds like trying to evenly space things out along an arbitrarily central cylinder. The main intent for starship design is to reduce mass without compromising safety or structural integrity. Therefor the ship should be no longer than it has to be.
Also the fuel tanks should be in the front, to act as velocity sleet shielding. So If you have a minimum diameter based on the habitat rings, you make the fuel tanks (multiple) fit inside this diameter. I like long tanks aligned with the ships axis, obviously balanced on each side or sector around the central cylinder. My designs have no central cylinder, since the ship has a hull, most of it filled with tanks, but there is a short access tunnel that goes through the bearings that hold the rotating sections in place.

The infants would need artificial 1G gravity created by a ring, so that their muscle and bone structure doesn't deteriorate before they've even begun to learn how to walk and run themselves.

I totally support the need for artificial gravity via rotation and have been frustrated for decades by the lack of attention to it paid by NASA. The NASA leaders are not really interested in space settlement or civilian space travel or they would have done tests on the space station. We also have no idea of the effects of microgravity on gestation to say nothing about growth and development. There is a calculator called spincalc, which is useful for this. I see nothing wrong with a 250 meter radius. You would need at least 5-10 meters wider than that for the Whipple shield hull and its struts.

I am aware how empty it is.
The groups who discuss interstellar flight gave up on this idea decades ago. Isaac Arthur's graphics and narration do help a lot of people understand how stuff in space works.

The sections of the rings then become the first "buildings" on the surface - thereby providing instant shelter to the entire crew.

I assume that you are aware that an object designed for atmospheric entry must have a specific shape (one of a set of shapes) or it will be destroyed by tumbling and melting during entry. One shape is a mercury capsule or cone with a gently curved bottom, other shapes are somewhat like a thick discus, which must enter facing the air stream at 90 degrees, not edgewise. In my opinion, the difficulty of designing and building a slowboat ship, some of whose components are supposed to enter and land, would be enormous. The attempt compromises the function of both roles for the hardware.

This idea brings back a novel I read decades ago by Brian Aldiss, where an interstellar vehicle has been infected and as a last desperate measure, the ship is commanded to divides itself into many ring or tuna-can shaped segments. I was never sure what the utility of this action was, but the name of the novel was "Starship"!

Technical stuff in Star Trek is usually just some made-up gobbledygook, to give Geordi and Data something to say.

I am not talking aboout video SF, I am talking about written SF and those few directors who actually understand what SF is all about, a fiction of IDEAS, but also of humans overcoming obstacles, by figuring out a way to do what they first cannot do.

So when I watched that series, the technical limitations and realism was all I cared about. The characters, meanwhile, couldn't have been more dull and boring (especially the captain).

Too bad that cinema/video SF either has lousy characters or a lousy story with few good ideas expressed. The Amazon Mars episodes were written with an anti-Mars settlement bias, that is the only rational explanation I can imagine, with so many logical errors in the story.

 

[N1206]

Are you referring to Seveneves by Neal Stephenson, @BvU? My goodness, that was a long, laborious novel

I've been reading sci-fi novels (and trilogies, tetralogies) since the mid-70's. One had a premise of a war that exploded the moon with antimatter into The Arch and screwed up the climate. Religious fundamentalists come to power, and there is struggle. The main fun feature was that the oceans has LOTS of yearly sea ice on them and the characters travelled, traded and warred on iceboats--which as discussions of DWFTTW note-- can travel ridiculously fast.
For as long as the ice lasts each season, at any rate.

 

[N1206]

The name I often use for this type of ship is slowboat, (a very ironic name since they eventually do move incredibly fast).

Larry Niven referred to the Bussard Ramjet-driven ships of his universe as slowboats. Eventually, FTL technology, reactionless drives, and anti-gravity come about--which are much, much faster than the slowboats--hence their name.

 

[John Strickland]

If you go back almost a century, some of the early written SF had effectively FTL type transport in its stories, and the Flash Gordon and Buck Rogers shows definitely went to new planets without going to other star systems. That still provides a lot of credit to the Star Wars series and Lucas, since from the beginning they knew they had to go to other star systems to visit other planets (besides the ones in our own system). Thus there is a line in the first movie, something like: "Entering the Alderon system". To this day there are many movie directors and producers who use the terms "galaxy" and "solar system" interchangeably, as they still have no clue about the size difference and probably never will! sigh.

 

[Melbourne Guy]

The point about the ending of Titanic is that it contains a metaphorical truth about evolution, with Jack sacrificing his life to protect rose.

Well, that's certainly a reductionist view of why Jack died, @Strato Incendus. And I'm not sure whether you're overthinking it or not But in one of my novels, I was more explicit: the hero never gets the girl becuase that's not his role (noting that my hero is in the archetypal 'Man with no name' style):

I had been about ten, precocious and certainly insufferable, and I had found some lurid romantic novel during an urban warfare training course. It was probably intended as a prop but to my female-starved mind, it was like a handbook to the opposite sex. I was waving it around, proclaiming how when I grew up, I would be just like Stone, the novel’s leading man, when one of the more taciturn Gen One’s had surprised me by calling me over. He was hugely muscled, intensely imposing, and I was intimidated by his palpable masculinity.

“Son,” he drawled at me, “You’re going to get things all wrong if you take that book as your gospel. We’re not destined to get the girl. Our destiny is to protect the girl, and if some pleasure can be exchanged in that, fine, go ahead. But you’ll never be Stone because he’s the good guy. And sad as it is to have to tell you this, we’re not the good guys, and never will be.”

I dismissed his offered wisdom, as young people had been doing since older people had a voice to dispense it, but he was right. I was not the good guy, no matter how I might behave. My job was to find trouble and stomp on it so others might live peaceful, orderly lives.

 

[DaveC426913]

but also to have the water run through them

Note that this only need to occur on the front faces of the ship, not so much on the flanks and certainly not much sternward.

Any radiation that might hit the flanks or stern is shifted down toward relatively harmlesser frequencies. And remember, there's no sun nearby. Its very EM dark here in interstellar space.

Also, remember that, at .1c you're really only shifting by one order of magnitude. 1011 nm Xrays that are upshifted to 1010 nm are still Xrays.

 

[N1206]

I think folks have covered most of the issues your initial post raised: time dilation at 0.1c will not be significant, and pencil-shaped rotating objects are not stable. You have been pointed at some online calculators for spin-induced g and the rocket equation. Back of the envelope calculation for the floor of a ring made of steel 500 m in radius, 50 m wide and a centimeter thick comes to 6200 tonnes. So, a million tonnes ship isn't unreasonable all fitted out. The USS Nimitz-class carriers are over 100,000 tonnes with a crew of around 5,000. Would 10 Nimitz-class ships sustain a 1000+ people for 125 years? A million tons might be a low estimate.

Besides shielding and every other problem you might McGuffin away, there's a billion tonnes of propellant to be attached to your design. A cubic meter of water is a tonne, so you are looking at a cubic kilometer of propellant, more or less, since water makes a good radiation shield. Cylinder volume is pi*r²L. What is going to define your ship is how you attach the fuel and how you shield from radiation, both from the environment and from your thrust source, however you McGuffin that to make your trip work. A cylinder 250 m in radius is going to have to be around 5 km long to contain a cubic kilometer.

 

[DaveC426913]

you attach the fuel and how you shield from radiation, both from the environment and from your

I can see a design that is essentially a dumbbell. A sphere of fuel 1km in diameter and a sphere of water 1km in radius. A cylinder with rings squished in the gap between them. Could also be done with an array of smaller tanks. As long as you can't see stars directly bowward, you're protected.

 

[John Strickland]

I can see some others support my position on having a large mass of fuel. If the ship was 1 million tons and the fuel mass was 1 billion tons, the mass ratio would be about 1000: 1,000,000 / 1000. If the ships mass is 4 million tons, with the same fuel load, the mass ratio is about 250, still VERY GOOD. However, as you shrink the ships mass, you have to assign smaller amount of cargo and stores, smaller tori, lighter fuel tanks, etc. This is an engineering trade-off, and exact hard numbers for trip times and velocities will be unknown until we have working fusion engines.

Since the acceleration and deceleration are very low, you do not need a super-strong structure along the axis of the ship, but the structure and bearings to support the rotating sections must be strong. A ship shaped as a narrow cylinder with clusters of long fuel tanks with long axis parallel to the ship's axis makes sense to reduce the ice disk mass. IF you use the fuel mass in front of the habitation section, this and the ice disk shield takes care of all radiation from the front direction. If you can create a cargo space surrounding the habitation area, that takes care of cosmic radiation from the side. A water reservoir in the back of the ship between it and the engines would take care of neutron radiation from the fusion reaction AND cosmic radiation from the rear. This thickness may be different from that for just the GCR.

The fusion engines will almost certainly need some kind of active cooling, or they will melt or vaporize. Very large radiator panels edge on to the ships motion would reduce chances of hits and coolant loss. The panels do not need to be protected from CGR. They are one of the few things that should stick out from behind the ice shield and can have a narrow impact shield along their front edge. The dumbell design would require up to 2 square km of ice shield, whereas the 500 meter thick cylindrical design would require about 1/4 of a square km of ice shield. Narrow designs reduce total ship's mass.

Reference: https://www.physicsforums.com/threads/building-a-generation-ship-the-sfv-exodus.1011521/page-3

 

[N1206]

One other big problem: Constant acceleration.
I think a gigatonne is probably on the low side for a total mass, but you may have solar sails, or boost, or directed coronal mass ejections or some other fanciness on the go, but consider the energy requirements in the first second of the flight at a constant 10 m/s:
K_E = 1/2 mv²
m being a terakilo, and v being ~10 m/s means that you got to have an engine that can produce ~50 TW of usable power. 24 hrs a day, 365 days a year, that's 438000 TWh. In 2019 humanity as a whole was using around 170,000 TWh (https://ourworldindata.org/energy-production-consumption)

So, your accelerations are unlikely to be constant, and your trip time will be different than you first proposed. McGuffin up your power supply and have it putting out constant power. Your acceleration will ramp up as you chew up fuel and reaction mass, but 10 m/s off the start is going to break your science, unless you McGuffin up direct mass-to-K_E conversion reactionless drive technology.

And Newton still applies: for every action there is a an equal and opposite reaction. To get a gigatonne going one direction, something has to get accelerated the opposite direction. I think maximum velocity is ~2.5 the speed of the thrust matter. So you are looking at trying to whistle up stuff to around 0.25 c. In how short a distance could you do that? SLAC is about 3.2 km long. Without mucking about with relativity, you would be looking to apply 50 TW of energy to a reaction mass such that it whistles up to 0.25c. That calculation doesn't look too impossible in terms of firing reaction mass out the back, but every ounce of mass needed to get things going is 100 ounces more of fuel and reaction mass.

 

[John Strickland]

The source I had on fusion engines indicated that they put out a massive amount of heat and a small amount of thrust. My engine set put out 14,000 TW, a staggering amount of energy, but only about 400,000 tons of thrust. IF an anti-energy use person saw this they would freak out. So if you had 1 million tons of thrust, your starting acceleration would be 1/1000 of a G, so mine provided only 0.0004 G ! Yes that acceleration lasted for many decades, but the engines could be shut down and serviced if needed at any time. The habitation area is rotating to provide gravity, so the inhabitants would not really notice the lack of ship acceleration. They might notice the deceleration toward the end of the trip. I allow a cruise period as a buffer for service periods, so the cruise period can be shorter.

 

[DaveC426913]

What is the reason for a linear crew cylinder? Air pressure? Why not slightly curved due to the structural load from the rotation? I assume, that at least the internal floors or levels are made to be perpendicular to the local "gravity" vector.

Apologies to FL for missing this, and apologies to OP for going back to it.

Pretty sure KSR says he took the design directly from a bona fide modern design that uses standardized empty fuel tanks perhaps even from the shuttle. But it was for an interplanetary trip so, so KSR simply scaled it up to interstellar.

The cylinders aren't decked; they're open land, with villages. The land can be quite rugged, even mountainous, so local normal gravity is a matter of standing on a plain or a hillside. In fact, the land surface is so divorced from the cylinder floor that water flows and pools just where they see fit.

Of course, any structures will be plumb.

The reason I did up these sketches in the first place is because the geometry of the ship, as described by the author didn't make sense (15 degrees at each end of the joining bridges. So I asked on an 'Ask a Question About This Novel' forum-y thing. Pretty sure it was the author himself that cleared it up.

 

[John Strickland]

Cylindrical tanks with hemispherical ends are currently a standard industrial structure. Since we have yet to start fabricating (in space) the components of tori for rotating space settlements, curved sections of tori, like a bent tank, are not a standard structure. However, by the time we start fabricating an interstellar vessel, we had better be fabricating tori for multiple uses, including rotating space habitats of varying sizes. Space fabrication is fairly mandatory since launching huge, curved sections of a torus from the ground would be rather difficult!
A typical torus that is 500 meters across would have a tube cross section 50 meters across, with the top "deck" being at the widest point as measured along the radius from the torus's hub. This would give a sky ceiling of 25 meters or about 80 feet, and about 8 levels below decks. The major vs minor diameter ratio (here 10 to 1) is not absolute and some designs use much fatter tori, so that the hub of the structure is much smaller, merging almost into a disk in some designs.

My concepts use a large structure called a jig factory to build a torus continuously until about 90 % of it is done. with the missing 10 % finished externally and then inserted. This system is like a 3-dimensional assembly line, with most of the work being done by robot arms very similar to existing assembly line units. The same factory unit can turn out torus after torus of the same size and shape. A set of identical tori could then be incorporated into a slowboat under construction.

 

[N1206]

Ah, the novel with the exploded moon was:
Ice Prophet by William R. Forstchen. (1983)

 

[John Strickland]

Forstchen is the one who, more recently, wrote the "One Second After" trilogy, showing what would happen if an enemy detonated nuclear bombs IN ORBIT over the USA. The result would be the loss of our power grid, communications, most of our civilian vehicles and within a few months, most of our population due to starvation. To my knowledge, the Feds still have done little to no work on dealing with this problem.

One Second After is almost a "bible" for those of us who keep warning of this danger. The risk is still unmitigated by a lack of attention by both the Administration and the Congress over the last decade or two.

Exploding Moons can make a good story, but most of them rely on "magic" physics to do the exploding. A 500 km wide asteroid hitting the moon would work but it would have to come from outside our solar system. The energy that would need to be released to actually "blow up" the moon is beyond most people's comprehension.

 

[jedishrfu]

This thread is getting rather long and is starting to veer in other directions. Since it has likely answered the many questions from the OP, I think its a good time to close it and thank everyone who has posted here.

Jedi

PS: @John Strickland if you want to continue expanding on your ideas about a generation ship, you are encouraged to start a separate thread on the subject.