Generation Ship SFV Exodus: Revised Designs

[Strato Incendus]

The first thread on my fictional generation ship SFV Exodus has come to a conclusion. So I've tried to take all the advice to heart and revise the design. I've come up with two potential alternatives. In accordance with @jedishrfu, I'm creating a new thread for this (images in spoiler tags).

Fixed Parameters: These are values we've "agreed on" in the first thread, which I would consider "fix" at this point, unless given sufficient reason to put them up for debate again:

Spoiler

Ship Name: SFV Exodus (Solar Federation Vessel)
Target destination: Teegarden b (12.5 light years from Earth)
Travel speed: 0.1 c / 10% light speed
Acceleration: constant
Estimated travel time: 125 years; the first year is used for acceleration, the final year for deceleration
Functionality of ring sections (from back to front): farm ring(s), factory ring, habitat ring, lab ring, public ring

no windows; only cameras showing the surrounding space outside (like the screen on the bridge in Star Trek)
Such screens are also used as surrogate windows in quarters and other leisure areas. Meaning, they can either depict the stars outside, or they can be switched to a landscape image, to resemble life on Earth more closely.

Ring radius: 250 m
Ring diameter: 500 m
Ring width: 64 m (32 m corridor, 16 m wall on both sides)
Number of decks per ring: 5
Crew size: 500 - 1,500 people
Propulsion: Nuclear fusion (potentially adding black-hole drives, using nuclear fusion for electricity supply instead)

Ring cross section:

Spoiler

Suggestion 1: Dumbbell Design (rings outside the ship)

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.

Spoiler

(not quite to scale; I assume the rings would have to be even smaller in comparison to the spheres at both ends)

As you can see, I've increased the number of rings from the original five to six. I ultimately agreed this is easier than to have the farm ring be twice in mass compared to a regular ring, by extending further inwards. Thus, I'd simply work with two farm rings now.

I've also placed the rings closer together, so that they're now just 5 m apart.

We could either have the rings rotate in opposite directions pairwise, i.e. left-right-left-right-left-right,
or we could have them rotate in a 3:3 manner, with the first three rings rotating in one direction, the next three rings rotating in the other direction.

The central pipe (between the spheres, connecting the rings) is still 100 m in diameter.
The hubs of the rings still rotate inside the central pipe, thus creating 0.2 g in there, compared to the 1 g created on the outer rings (500 m in diameter) themselves.

Here, contrary to Dave's suggestion, the two spheres are still equal in size; the one with the ship name is the one at the front.

@DaveC426913 suggested for the water sphere (the one at the front, to protect against radiation) to be 1 km in radius,
while the fuel sphere (the one at the back?) would be 1 km in diameter, i.e. half as big (see quote above).

I thought it would be the other way round? Since we need fuel to move all that mass of the water, too?
So wouldn't the ship need a much bigger fuel tank (=sphere) than water tank?

The upside of this design is that the central pipe - the section between the spheres - could be fairly short.
Since the sphere for the fuel is so massive that it can be stored in there, we should not need an extra-long trunk of the ship to store the fuel.
Unless we do so for redundancy, of course, in case something happens to the spheres.

Keeping the rings on the outside might make it easier to keep the "dismantling process" I envisioned for the eventual landing of the ship.
When the rings unattach from the central pipe, then break down further into subsections that can enter the atmosphere and form the first "buildings" on the surface. That way, the crew can take effectively all of their equipment and infrastructure with them.

I'll have to come up with some handwaving about how these individual sub-sections of the rings turn into oversized pseudo space shuttles with the proper aerodynamic qualities to enter the atmosphere. For starters, I need additional fuel in the ring walls, or at least cavities in which fuel can be transferred from the big fuel sphere, before the dismantling process begins.

An ice shield in front of the ship would still be required, and it would have to be even larger than the spheres themselves.
Hence, it didn't fit on this image anymore. But just imagine it extends beyond the spheres "in height" on both ends.
So the mass of this ice shield would be immense, compared to the other design.
Suggestion 2: Cylindrical Design (rings inside the ship)

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.

A cylinder 250 m in radius is going to have to be around 5 km long to contain a cubic kilometer.

This would lead to a much more "boring", but also more straightforward design - and a somewhat suggestive one, for a generation ship in particular.

Spoiler

Length: 5 km
Cylinder Radius: 250 m
Cylinder Diameter: 500 m

The big difference is that here, the entire outer hull is as wide as the rings in the other design (500 m).
The rings would therefore be nothing but demarcated sections within the cylinder, specialised in certain functions.
The central pipe (the axis in the middle) would be entirely inside the hull, still with a diameter of 100 m.

The question is whether the central pipe would be needed at all, or whether people would just move through the ship on the rotating inner walls, as if they lived in a small O'Neill cylinder.
However, I doubt the entire inside of the cylinder would rotate?

Thus - and this would be most important for keeping the current plot - the ring sections with their dedicated functions (farm, factory, habitat, lab, public) could still exist, but inside the cylinder?

We would still need at least two sections of the cylinder, rotating in opposite directions to prevent the ship from flipping around.
This would be akin to having three rings in succession spin in one direction, the next three rings in the other direction, in Suggestion 1.

In this image I've inserted the ice shield at the front. There is still a sphere at the back, which could serve as a vessel for the artificial black hole if this ship were to use a Kugelblitz drive.

However, would this mess up gravity relations on board? Would everything in the ship "fall" towards the back end, i.e. would gravity point towards the black hole, with the ship standing on top of it like a skyscraper? (Isaac Arthur suggested this in his video on Kugelblitz drives).

The fuel and water would be placed inside the cylinder, in front of the inhabited areas (meaning not just in front of the habitats, but also in front of the lab and public sections).

One upside of the cylindrical design would be the lower mass of the ice shield at the front.
All the fuel would have to be stored in the cylinder, which requires the ship to be much longer.For the mass ratio, let's use these numbers that @John Strickland provided towards the end of the previous discussion:

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.

For the time being, let's just focus on the basic shape of the ship (with rough measurements / estimates) before losing ourselves in further details. ;)

I definitely want the ship to have a memorable look, so that this becomes part of the story's "iconography".
The dumbbell stands out more. And since it kind of looks as if the ship had a "head", this makes it somewhat reminiscent of various Enterprise builds from Star Trek, which would fit the spirit of the story.
The cylinder is more straightforward, but potentially less memorable. Unless people remember it by association with other things (pencils or cigars, of course).

Also, the closer the ship becomes to an O'Neill cylinder, the weaker the arguments get for actually dismantling the ship and landing on the surface at the end; people could just argue for staying inside the O'Neill cylinder instead.

Any thoughts / preferences for Design 1 (dumbbell) or Design 2 (cylinder)?

 

[DaveC426913]

Last thread was amazing. I will miss it. But this one promises to be just as cool.

Any thoughts / preferences for Design 1 (dumbbell) or Design 2 (cylinder)?

Yes. (And pardon my glibness).

Cigar-shaped ships are, historically, so done-to-death that the stink of unimaginativity will stick to your story like poop. Go with anything rather than a cigar. IMHO.

 

[DaveC426913]

As for arranging tanks, a single huge one is OK, but several smaller ones are more efficient in several ways, not the least of which are:

• redundancy in case of puncture
• easier plumbing access to contents
• less "sloshing"
• easier to arrange into a wider, flatter radiation shield

etc.

The design I was suggesting in an earlier post was inspired by Project Daedalus:

 

[DaveC426913]

Rough, not to scale.

 

[DaveC426913]

There is still a sphere at the back, which could serve as a vessel for the artificial black hole if this ship were to use a Kugelblitz drive.

However, would this mess up gravity relations on board? Would everything in the ship "fall" towards the back end, i.e. would gravity point towards the black hole, with the ship standing on top of it like a skyscraper? (Isaac Arthur suggested this in his video on Kugelblitz drives).

Yikes! A bit of a jump there in tech level. Singularity drives, as they're often generically known, are the epitome of Magical Technology.

In my "experience" gen ships and singularity drives are at opposite ends of the tech level spectrum. You start messing with singularity drives, and your readers will wonder why you need a gen ship at all.

But if you're determined to go with that, know that, typically, singularity drives are not massive at all. The singularity itself is usually tiny - microscopic. Their mass is not a significant fraction of the ship's mass, and their gravity is inconsequential. Also, you need much less fuel for them, since you can extract an arbitrarily large fraction of total energy from your reaction mass.
Ever played an RPG called Traveller? They got pretty sophisticated about Tech Levels.

2022 is at about Tech Level 9.
Gen Ships are about Tech Level 11.
Singularity Drives are way up the chart somewhere around Tech Level 15-20.

YMMV.

This just barely scratches the surface:
https://wiki.travellerrpg.com/Tech_Level_Comparison_Chart
There's a million charts out there that cover this stuff.

 

[Strato Incendus]

Cigar-shaped ships are, historically, so done-to-death that the stink of unimaginativity will stick to your story like poop. Go with anything rather than a cigar. IMHO.

I’m glad I wasn’t the only one having that thought , and that the dumbbell design would be more iconic as a result.

As for arranging tanks, a single huge one is OK, but several smaller ones are more efficient in several ways

Good point. Couldn’t this same goal be achieved by having further sub-chambers within the giant spheres? So if the hull gets punctured in one place, only everything inside that sub-chamber would be affected, but not the entire sphere.

The 3D model you created looks very much like what I had in mind, great job! Here you can see even more of the details than in my 2D depiction from the side. May I ask what software you used to create this model?

In my "experience" gen ships and singularity drives are at opposite ends of the tech level spectrum. You start messing with singularity drives, and your readers will wonder why you need a gen ship at all.

Okay, I thought black hole drives would still be lower on the scale, since to my knowledge, they would still be sub light speed, or up to light speed at max. Or would they create enough energy output to power space-warping FTL drives, too? Because without the technology to create a “warp bubble” of some sort, all the power output still wouldn’t get a ship beyond light speed.

In the Star Trek: TNG episode “Timescape”, an artificial black-hole is being used…

Spoiler

…by a Romulan ship to power their warp drive, though. There is an alien species that raises its young inside black holes, and they end up confusing this artificial black hole for a real one - which causes the space-time problems that make up the plot.

”Why would they need a gen ship at all?” It’s great that you ask this question, because that’s precisely what the second book is going to hinge on: The protagonist’s son, having realized the inherent ethical problems of generation ships (and being notoriously “unsuccessful” at contributing his “share” to the default mission of creating the next generation), starts looking for a way to get the ship up to light speed. That way, it would arrive at its destination in 2-3 years, and no longer need to be a generation ship. His opposition thinks this is too risky, and would prefer to stick to the “old, tried and tested” way.

If the sphere at the back of the ship, originally used for fuel, could then be used as a holding cell for an artificial black hole, that would be the ideal hull shape for that. In order to make this twist / new plan work, I need the ship to have everything they need on board. Like the inside of the tanks being made of material that can reflect gamma rays. And enough hydrogen to power the lasers from the nuclear fusion reactor.

Which of course begs the question why they didn’t start the mission with a functioning black-hole drive to begin with.

Alternatively, the fuel tanks at the back could consist of several small spheres indeed - so that only one of these would be used for the tiny black hole. And then, for the tank at the front, we could stick to “one large sphere with sub-chambers”. That way, the ship would still look like having a “head”, i.e., more Star-Trek like. Which is actually a pleasant surprise. :). Because I didn’t expect to get away with any ship shape remotely close to those used in Star Trek.To get back to my question from the starting post:
Did I understand you correctly that, in your original post, you were suggesting that the fuel sphere and the water sphere should differ vastly in size? ;)
One having a radius of 1 km, the other a diameter of 1 km?

 

[DaveC426913]

Good point. Couldn’t this same goal be achieved by having further sub-chambers within the giant spheres? So if the hull gets punctured in one place, only everything inside that sub-chamber would be affected, but not the entire sphere.

Sure. Consider the pros and cons of each. (It seems to be from 20th century designs than smaller, multiple systems are generally better than larger singular ones.)

Question though: if the interior is full of sub-chambers, and all the plumbing required to service those sub-chambers then you're got a working system already ... what purpose does the outer spherical shell serve? Why have it at all?

The 3D model you created looks very much like what I had in mind, great job! Here you can see even more of the details than in my 2D depiction from the side. May I ask what software you used to create this model?

I used Blender, which I've been practicing with. That mockup took me about 2 minutes.

Okay, I thought black hole drives would still be lower on the scale, since to my knowledge, they would still be sub light speed, or up to light speed at max. Or would they create enough energy output to power space-warping FTL drives, too? Because without the technology to create a “warp bubble” of some sort, all the power output still wouldn’t get a ship beyond light speed.

At the simplest level, they are nothing more than a source of virtually limitless energy. It doesn't in-itself provide propulsion. What kind of energy you choose to harness is matter of what you do with it to turn it into an actual motor).

Think of a nuclear pile on an aircraft carrier. The pile is not the motor - the pile is just the source of energy to power the motor.

In both cases, the simplest thing to do is extract the heat, turning your reaction mass into expanding superheated vapour. Then you either power a turbine on your carrier, or blow it put the back of your spaceship.

In the Star Trek: TNG episode “Timescape”, an artificial black-hole is being used…

Spoiler

…by a Romulan ship to power their warp drive, though. There is an alien species that raises its young inside black holes, and they end up confusing this artificial black hole for a real one - which causes the space-time problems that make up the plot.

Yep. Handwavium.

But note: your characters are human; we know what tech we have now in 21st century, and you will tell us how far in Earth's future your story is.

You don't have the luxury of how saying "Well, this species evolved in a binary system with a black hole" or "This species took two thousand years to make a working sing-drive".

If the sphere at the back of the ship, originally used for fuel, could then be used as a holding cell for an artificial black hole, that would be the ideal hull shape for that. In order to make this twist / new plan work, I need the ship to have everything they need on board. Like the inside of the tanks being made of material that can reflect gamma rays. And enough hydrogen to power the lasers from the nuclear fusion reactor.

OK, that's several orders of magnitude more wild than I thought - even when you brought up Singularity Drives. (which I personally would put as even more advanced than all of @Algr 's advanced stuff. )

I assumed the SingDrive tech was designed, made and tested on Earth before sticking it under the butts of your hapless colonists. But you're saying they invent, test and build it while en route - without blowing themselves up even once?

You got your work cut out for you. And it's going to be an interesting read!

Did I understand you correctly that, in your original post, you were suggesting that the fuel sphere and the water sphere should differ vastly in size? ;)
One having a radius of 1 km, the other a diameter of 1 km?

Totally arbitrary on my part. If you are concerned about accuracy, the relative sizes of water versus fuel needs at least some cursory analysis.

And that will depend on your motor and how efficiently it uses reaction mass.

A simpler solution is to use water both for reaction mass and for habi...

--- hang on a second ---

Why two tanks**? One is for fuel and the other is for ... ?

**yes, I am aware that I specifically described it. That was Yesterday-Dave. Today-Dave doesn't know what Yesterday-Dave was thinking.

Your life system is (or at least, should be) a closed system. It has a fixed volume of water that goes in and out of food, drinks, plants, cows and toilets. That water is recycled and has virtually zero losses. It may take up room, but it's not like you need a colossal reservoir to hold it.

I think this needs rethinking.@John Strickland : you seem to have a pretty good handle on the technical aspects of ark-sized ships. Do we need a huge mass of water that is somehow separate from the reaction mass?

 

[Strato Incendus]

Question though: if the interior is full of sub-chambers, and all the plumbing required to service those sub-chambers then you're got a working system already ... what purpose does the outer spherical shell serve? Why have it at all?

Given the mass the outer hull would add to the ship, that’s a very fair question. We could argue “as a final layer of protection”? If any of the individual sub-chambers break (for internal reasons), there would be a surrounding layer to catch the contents, rather than having them escape into space right away.

But this is a little bit of post-hoc reasoning on my part, to preserve the spherical shape (and the resulting Star-Trek aesthetic ^^) on at least one end of the ship .

I used Blender, which I've been practicing with. That mockup took me about 2 minutes.

Ah yeah, heard about that one, my father used it for some video-editing things. I might have to look into that software, too! ;)

At the simplest level, they are nothing more than a source of virtually limitless energy. It doesn't in-itself provide propulsion. What kind of energy you choose to harness is matter of what you do with it to turn it into an actual motor).

Okay, that’s what I thought.

On line of reasoning I could resort to is “the power supply necessary to reach higher speeds is there, but the AI isn’t fast enough yet to deflect all the debris the ship would encounter at velocities higher than 0.1 c”.

That way, the issue would be on the programming side of things, more so than on the raw energy side.

As far as I understand, the big sphere(s) at the front of the ship, whether they contain water or fuel, would only protect against radiation, but not against particles of physical matter.
So we still need the ice shield in front of it to start with, and then probably still a laser-based reflector system on top of that, right? Or does the ice shield alone suffice?

In the latter case, I could transform the argument to state the AI isn’t fast enough to repair the ice shield that quickly. Which would still be a programming issue, albeit one moving more into the physical side of the issue again. Because then we’d have to start wondering about how big the impact holes in the ice shield would be, and how fast those could be patched up etc.

But note: your characters are human; we know what tech we have now in 21st century, and you will tell us how far in Earth's future your story is.

You don't have the luxury of how saying "Well, this species evolved in a binary system with a black hole" or "This species took two thousand years to make a working sing-drive".

Indeed, and this story is not supposed to have any humanoid aliens in it at all. It’s only mentioned that some single-cell organisms were found, in oceans under the ice of Europa (and potentially other, similar moons and planets in our own solar system). That’s a statement about the likelihood of life occurring anywhere in the galaxy in general, though, so I’m not sure if I should keep that part in.

And then, once they arrive at Teegarden b, since it’s postulated to be habitable for humans, there at least have to be plants there - which makes it likely that some animals will be there, too. I’m envisioning this as a prehistoric place (one of the little boys on board gets his hopes up about seeing live dinosaurs). Except it’s in a state of eternal sunrise / sunset, since the planet is tidally locked, and habitable conditions are only in the twilight zone between the day and night side.

I assumed the SingDrive tech was designed, made and tested on Earth before sticking it under the butts of your hapless colonists. But you're saying they invent, test and build it while en route - without blowing themselves up even once?

You got your work cut out for you. And it's going to be an interesting read!

I know this sounds ridiculously ambitious, and part of that is intended. The people working on getting the ship to light speed in the second part are seen by some as visionaries; others think they’re out of their mind, and should focus on “what’s really important” (=simply sire the next generation on board, stick to the way things used to be etc.).

At the same time, though, of course it would be hard to believe that the physicists on board the ship, who were born there, not selected from all of Earth’s population for their expertise, are somehow smarter in figuring this out than everyone back in the solar system.

More importantly, the protagonist’s 19-year-old son plays a vital role in this. And I certainly don’t want to create a second Wesley Crusher. (I never minded him that much personally, but I understand why a lot of Star Trek fans don’t like him - because anytime he shows up, he makes the other officers seem less qualified than they would actually reasonably be.)

The second problem can be solved in two ways:
1) The 19-year-old boy might be a driving force not through expertise about physics or programming, but through a leadership ability, bringing these different minds with knowledge far greater than his own together to work on the problem.
2) He might contribute some ideas not in spite of, but precisely because of his layman status. He could think outside the box in some way that somebody with greater knowledge of the subject matter would already have ruled out prematurely.

However, the general tech level of society - both on board the ship and back on Earth - is a bigger problem. If the approach “black holes exist as an energy source, but the AI can’t keep up with the speed of the ship” doesn’t work, we could make an alternative case:

We could say there have been experiments to create artificial black holes back in the solar system, but these have been dropped for some reason. Perhaps because of public outcries based on misinformation (people claiming these artificial black holes could somehow swallow Earth or something), so that the people doing those experiments lost their funding, even though they would have had the technical expertise to actually develop a black-hole drive.

Your life system is (or at least, should be) a closed system. It has a fixed volume of water that goes in and out of food, drinks, plants, cows and toilets. That water is recycled and has virtually zero losses. It may take up room, but it's not like you need a colossal reservoir to hold it.

If we had water in the front sphere, I assume that would be too “contaminated” by the constant barrage of cosmic radiation to be used for anything affecting organisms directly? Meaning, you couldn’t use it as drinking water, for showers, or for watering plants etc.?

If we only need one sphere at the front, and none at the back, the one at the front would be the one for fuel. Somehow I’m getting associations to a sperm whale, which has all the matter required for deep diving in its head. Of course, that would make the ship look even more Star-Trek like! :) Except nobody would be living in what looks like a “saucer section” from the outside. (I think I’ve already seen Star Trek ships with spherical “heads”, but those didn’t contain the fuel, of course.)

If there is no sphere at the back end of the ship: Suppose someone wanted to “spontaneously construct” a black-hole drive on board, requiring a sphere to hold in that black hole: Would one of the smaller spheres, and empty sub-tank, which has already had all of its fuel used up, be large enough? The black hole itself is tiny, after all.

The main problem would probably be: That sphere would have to be attached to the outside of the ship, wouldn’t it? Or could it just be an energy source inside the ship, like the nuclear fusion reactor?

On that note: The “doughnut“ shaped fusion reactor will probably be small enough to sit “vertically” inside the central pipe, which has a diameter of 100 m.

Of course, a fusion reactor on board only makes sense if singularities haven’t been established as a power source yet - even if they’re not used for propulsion faster than 0.1 c yet.

To be clear: I don’t want this ship to have a higher tech level than necessary. But I do need some way to get it up to light speed (in fact: 0.99 c) in the second book, so that the characters landing on the planet in book 3 are those the reader originally connected with in book one.
Sure, I could have them arrive when the original protagonists are 80+. But I actually want them to still be alive when the first “follow-up settlers from Earth” arrive, using the light-speed technology developed in part II.

 

[DaveC426913]

If we only need one sphere at the front, and none at the back, the one at the front would be the one for fuel. Somehow I’m getting associations to a sperm whale, which has all the matter required for deep diving in its head. Of course, that would make the ship look even more Star-Trek like! :) Except nobody would be living in what looks like a “saucer section” from the outside. (I think I’ve already seen Star Trek ships with spherical “heads”, but those didn’t contain the fuel, of course.)

Only got a moment to respond this morning.

My fanciful sketchbook designs for interstellar ships tend to look a lot like jellyfish - a big dome on the front, with everything tucked in behind the dome, and the drive trailing off in a tail to distance it from the hab module.

If we had water in the front sphere, I assume that would be too “contaminated” by the constant barrage of cosmic radiation to be used for anything affecting organisms directly? Meaning, you couldn’t use it as drinking water, for showers, or for watering plants etc.?

Unless I am utterly mistaken, water does not get radioactively contaminated.

BTW, at the risk of taking you in a wildly different story direction: the solar system has ready-made giant spheres of solid water. We call them comets.

Take the drive and hab section of your ship, plant it pointing nose-down into a comet, and voila! you have a nigh-limitless supply of water, a radiation shield and an ablative particle shield - three birds with one stone!

Of course, Exodus no longer looks like a spaceship - it looks like a flying ice ball with a spaceship nose-first in the back.

 

[DaveC426913]

@Strato Incendus :

"Unfortunately, humans, being the squishy bags of water that they are, don’t really like to spin super fast for long periods, as anyone who has ever gotten sick on a carnival ride can tell you. Science puts that rotational speed limit for discomfort at around 3 RPM. So, to rotate at less than 3 RPM and still have the benefit of a full Earth’s worth of simulated gravity, the structure itself must be a kilometer long."

(To you, long means diameter)

https://scitechdaily.com/a-huge-kil...avity-could-be-launched-from-a-single-rocket/

 

[Strato Incendus]

Are you referring to the inner ring diameter having to be 1 km? (This is about one of the parameters I thought to be "set in stone", but since you provided a specific source for this, I'm always happy to put such things up to debate again, given sufficient reason.)

Reminder: My source for the current size was a discussion thread on Quora. The posts by a user named Robert Frost contained a specific formula to calculate this, using 2 RPM, and ended up with a minimum radius of 223 / 224 metres to create 1 G (I've calculated it for Teegarden b's 1.05 G, then it's 234.5 metres minimum radius).

Since my ring diameter is 500 m (radius = 250 m), based on this formula, I could have the rings rotate even more slowly than 2 RPM. So this should be more than comfortable for humans.

Or are you referring to the length (="forward diameter") of the central pipe? The ship already is close to a kilometre long (around 850 m) with the previous design. However, now that the rings are closer together, we could get away with a shorter length for the central pipe. The sphere at the front being 1 km in diameter of course will make the ship longer than 1 km overall.

 

[Filip Larsen]

Not sure if its already been mentioned, but if the discussion is revisiting the trade-offs regarding crew comfort for various rotation speeds and dimensions then https://www.artificial-gravity.com has a nice list of references that may be interesting to take a peek at, as well as the SpinCalc calculator that might come in handy.

 

[DaveC426913]

Are you referring to the inner ring diameter having to be 1 km?

Yeah, you're OK. I thought I'd add the article to your list of resources since it has some bona fide research data on maximum rotational speeds. Till now, I did not know that there was a max backed by research.

 

[Strato Incendus]

Alright, back to the two spheres vs. one sphere question:

If I understood @DaveC426913 correctly, the one-sphere solution would mean a large sphere (1 km in diameter) at the front of the ship, full of water (either one large sphere with several sub-chambers, or several small spheres). And that water would be used both for fuel (using the hydrogen from it) and to shield against radiation?

That of course creates the issue: The longer the journey takes, the more of the water will have been used up as fuel, so the amount of water remaining to protect against radiation will keep shrinking. Or am I missing something here?

If the fuel is in a separate sphere, meanwhile - be that at the front or the back of the ship - then the protection layer would be independent of the amount of fuel consumed. If we want to keep the look of one sphere at the front (like on the Star Trek ship “USS Pasteur” from the final TNG episode ”All Good Things…”), then that sphere could contain both water for radiation protection (which doesn’t get consumed) and fuel in separate sub-chambers.

But since we mentioned that 10 m of water would suffice to protect the rings from radiation, wouldn’t a “disk” of water (say, 10 m in thickness, 1 km in radius) at the front of the ship suffice to shield the pipe and the rings against the radiation? Assuming that the ice shield, which we are going to need in front of the ship anyway, can’t do that.

Because 1 km radius and 10 m thickness would of course reduce the mass of water considerably, compared to a sphere with a radius of 1 km.

 

[DaveC426913]

Oops. Don't forget the decel phase! Your ship has done turnover phase, and now has a year of deceleration - except your km sphere of protective water is now on the wrong end of the ship! It still needs to face forward.

It is may be cost-effective to design a ship to be disassembled and reconfigured with the right parts in the right place for various phases. In fact, your shield and your hab are in the correct configuration for decel - its only your drive that is at the wrong end.

 

[Strato Incendus]

Oops. Don't forget the decel phase! Your ship has done turnover phase, and now has a year of deceleration - except your km sphere of protective water is now on the wrong end of the ship! It still needs to face forward.

Good point! I guess this is a good time to think about the deceleration phase in general in more detail. Possible ways to do this:

1) There could be a second exhaust at the front of the ship. With the fuel being stored at the front anyway (in order to shield against radiation), it would actually have to take a much shorter journey through the ship body to make it there. That way, the ship could accelerate in the opposite direction in order to slow down - without having to flip around in the first place. This seems like the easiest solution, so in terms of my layman knowledge, Ockham’s razor would point here?

2) If we turn she ship around, we will have to use the dumbbell design. More specifically, a symmetric dumbbell, since the sphere at the back needs to be just as large, to provide just as much shielding against radiation, as the one on the front - even if the sphere at the back only has to do so for a year (which is the time the deceleration from 0.1 c would take, based on our current calculations). However, the actual deceleration phase in the story will be longer, since, as mentioned before, in part II, the ship will have to accelerate all the way up to 0.99 c.

2a) If we turn the ship around, the question is how to do this. One way would be to use the ring rotation to flip the ship “naturally”. However, this would in turn require to stop the ring rotation for a while (suspending gravity everywhere on the ship), and then have them all spin in the same direction until the ship flips. Then half of the rings would have to be stopped again and spin into the opposite direction - before the ship flips another time.If we go with design 1), a single sphere at the front would suffice. The back of the ship might be a demisphere then (which could still be used later for a black-hole drive, since based on Isaac Arthur‘s video, a Kugelblitz engine would require that same shape at the rear end of the ship).

However, one question from my previous post is still pending:
Do we actually need a sphere with a 1 km radius at the front, rather than a disc with a 1 km radius that’s only 10 m thick?

1 km might be required perpendicularly to the central pipe, so that the protective shield extends well beyond the rings (which are only a little over half as wide). But parallel to the central pipe, why do we need the sphere to be 1 km thick? Only to have enough volume in there for the estimated amounts of fuel, I assume?

 

[DaveC426913]

Good point! I guess this is a good time to think about the deceleration phase in general in more detail. Possible ways to do this:

1) There could be a second exhaust at the front of the ship.
...
That way, the ship could accelerate in the opposite direction in order to slow down - without having to flip around in the first place.

You're going to need a little more than a second exhaust at the front. You're going to need an engine there!

Are you planning to build a ship with two engines - one on each end?

Two engines are expensive, considering they likely the most advanced, finicky component of the ship. Especially redundant since you only ever use one at a time, and there's a thousand year gap between using them. Don't you think perhaps a thousand years is sufficient time to reconfigure the ship? (Your ship is modular anyway, since it's meant to be disassembled for materials.)

I think your earlier estimation for the size of the engine is low. It is a ship component in its own right. Alternately, if you envision the engine filling the core/pipe, then it still pointing the wrong way.

IOW, no matter how you cut it, you've got to reconfigure the ship's major components.

With one exception:Back to the two tank design. Now it makes sense.

Your ship looks like this: [Engine][Aft Tanks][Rings][Fore Tanks]

In the accel phase, use fuel from the aft tank.
In the coasting phase crew are protected by the full fore tank.
Before decel phase pipe half the fuel from the fore tank into the aft tank (so the ship is balanced when it gets flipped)
Flip the ship.
Pipe the rest of the fuel from the fore tank into the aft tank, where it will protect crew during decel.

(Sorry if this seems like a wild goose chase that led us back to where we started, but when we came up with the dumbbell design, we had the wrong reasons for it. Now we know why you need two tanks.)

 

[DaveC426913]

However, the actual deceleration phase in the story will be longer, since, as mentioned before, in part II, the ship will have to accelerate all the way up to 0.99 c.

That could turn out to be more than just a wrinkle. You should work out how much fuel that will require, and how long it will take.

One way would be to use the ring rotation to flip the ship “naturally”.

While I think it's a very cool idea to flip the ship this way, it seems needlessly arduous. Not to mention that it puts all the strain directly on the ring bearings.

If the rings are already stopped, a short burst of steam would start it flipping. You could take as long as you want to flip it (which reduce fuel - the only limit is the ship's exposure to radiation sleet, and the strongest parts of the ship take the strain.

Do we actually need a sphere with a 1 km radius at the front, rather than a disc with a 1 km radius that’s only 10 m thick?

Size and shape of the tank(s) is an unknown. I think you are right, it need not be spherical, but it'll need to be thicker than 10m. After all, it's acting as an ablative shield no? Every particle that impacts it will blow off a chunk like a case of dynamite.

There is no reason to have the tank more than a little wider than the rings. So, rather than a sphere, you could look at a cylinder. That'll provide consistent protection across its surface, unlike a sphere. It's also more compact.

A sphere 1km in radius has a volume of about 4.2 cubic km (that's 2km "long").
A 1km cylinder with the same volume as that sphere is only about 1.3 km long.

(You want your engine to be protected by the shield during decel or it will get bombarded. Have as little of the exhaust out beyond the shield is practical)And all of this has to be thrown out the window when you factor in:
- accel phase - has full fuel complement, and is going .1c
- decel phase - has much less fuel, and is going .99c
The changing amount of fuel will have been worked out by the designers ahead of time, but the additional mid-journey acceleration will not.

So, what you need to do is
1] work out the fuel requirements of the ship as the designers made it, and then
2] work out how the mid-journey engine upgrade and the change in speed affects your fuel, and then
3] if they have enough left over to decel at the end.
It sure looks like the crew has a pretty cut-and-dry problem: while they may have a cool new motor, they don't have any more fuel than they started with. Literally, in terms of energy budget, where does the extra energy come from to accelerate them to .99c? That's surely a couple of orders of magnitude in fuel requirements. (I guess were the extra energy comes from is that the new motor uses the fuel much more efficiently. i.e. whereas before it was ejecting reaction mass at .1c, it is now ejecting the same amount of reaction mass at .99c)

 

[Strato Incendus]

Especially redundant since you only ever use one at a time, and there's a thousand year gap between using them.

I assume this was hyperbole, but just in case it wasn't, let me remind everyone: The estimated time for the journey is 125 years, not 1,000 years.

In the accel phase, use fuel from the aft tank.
In the coasting phase crew are protected by the full fore tank.
Before decel phase pipe half the fuel from the fore tank into the aft tank (so the ship is balanced when it gets flipped)
Flip the ship.
Pipe the rest of the fuel from the fore tank into the aft tank, where it will protect crew during decel.

Okay, I like this! Gives the crew something to do that can take place entirely within the ship (pumping fuel from one tank into the other), without requiring any space walks, or other stunts outside the ship hull, at 10% light speed.

That could turn out to be more than just a wrinkle. You should work out how much fuel that will require, and how long it will take.

Before doing any specific calculations on fuel consumption, let's see whether we can agree on the maximum speed to begin with.

I thought about this last night, and perhaps 0.77 c would be a more realistic number than 0.99 c. From various discussion threads I've checked, 0.77 c seems to be the speed you could get the ship to within one year while still keeping the g forces for the crew on board bearable. Although I'm uncertain about this, since we're currently already working with 1 year to get the ship up to 0.1 c?

In either case: By the time the ship accelerates beyond its original speed of 0.1 c, there are something between 2.5 and 2 light years left to go. At 0.77 c, this remaining distance would take between 3.25 years (for 2.5 light years) and 2.6 years (for 2 light years) to complete.

However, of course the ship won't be flying at 0.77 c constantly, since it needs to start decelerating again early enough. And I assume that would take at least another year, if it already takes a year to get up to 0.77 c.

While I think it's a very cool idea to flip the ship this way, it seems needlessly arduous. Not to mention that it puts all the strain directly on the ring bearings.

If the rings are already stopped, a short burst of steam would start it flipping. You could take as long as you want to flip it (which reduce fuel - the only limit is the ship's exposure to radiation sleet, and the strongest parts of the ship take the strain.

Okay, so in either case, this means we'll have to stop the ring rotation to flip the ship.

Or are there any alternatives? Because flipping the ship in turn seems to be the only way to slow it down (=accelerating into the opposite direction).

Size and shape of the tank(s) is an unknown. I think you are right, it need not be spherical, but it'll need to be thicker than 10m. After all, it's acting as an ablative shield no? Every particle that impacts it will blow off a chunk like a case of dynamite.

I thought we'd stil have that ice shield @John Strickland suggested in front of the ship?

There is no reason to have the tank more than a little wider than the rings. So, rather than a sphere, you could look at a cylinder. That'll provide consistent protection across its surface, unlike a sphere. It's also more compact.

The design you made in Blender (thanks a lot again for that, I appreciate the effort, since it makes all of this a lot easier to visualise! ) manages to still retain the iconic dumbbell design, which I like. Even though it obviously looks less "Star-Trek"-like now, because it no longer has that typical sphere shape reminiscent of the USS Pasteur.

A sphere 1km in radius has a volume of about 4.2 cubic km (that's 2km "long").
A 1km cylinder with the same volume as that sphere is only about 1.3 km long.

And that sounds like a strong argument in favour of cylindrical tanks. Because while the mass of the fuel contained within would be the same, if the spherical design is longer and therefore requires more ship hull, that will also mean more mass.

It might not look as nice as two spheres, but this just goes back to something I've realized a long time ago: "What's pretty is often impractical." (And what's practical often isn't "pretty"...)

Literally, in terms of energy budget, where does the extra energy come from to accelerate them to .99c? That's surely a couple of orders of magnitude in fuel requirements. (I guess were the extra energy comes from is that the new motor uses the fuel much more efficiently. i.e. whereas before it was ejecting reaction mass at .1c, it is now ejecting the same amount of reaction mass at .99c)

Here I'm getting back to the question of "is it possible to pick up any matter during the travel"?

Normally not, of course. But the reason the acceleration to a higher speed than the original 0.1 c is even necessary is because of a significant loss of speed (down to 0.025 c) occurs in the meantime.

Of course, by now I've realized such a loss of speed can't just happen "by itself" or because of some reactor damage. Rather, there needs to be something causing the ship's commander to choose to decelerate the ship prematurely. Which is perhaps stronger from a storytelling perspective anyway, because the deceleration would be the result of a character's decision, rather than just some external accident.

My idea last night was that a partial collision with some particle (due to a malfunction of the deflector) could damage the fuel tank on the front, so that a lot of the fuel is about to get lost into space. If the ship doesn't use it now, a) the leak might thrust the ship off-course, but even more importantly, b) the ship would lack the fuel to slow down at the end, and therefore shoot right past its target star, even if it remains on course.

The commander would therefore decide to decelerate the ship earlier - and faster - than intended. Here the question is what's the maximum g force at which they could do this. Because it's not a short-term exposure to such a force, since the deceleration from 0.1 c would still take a couple of months at least.

The point is: If the ship has to decelerate earlier than it is supposed to, for the second half of part I, it could look like the ship were "stranded" in the middle of nowhere.

Then, in part II, this disadvantage of the ship having become so slow is that this could be used to pick up hydrogen from the surrounding area. Obviously, not just from the interstellar medium, since it's far too empty. My idea was to either have the ship strand inside a nebula, or in another, uninhabitable star system that has several gas giants. (I personally favour the gas-giant solution at the moment.) The ship could then dive into the upper layers of the gas giant to replenish its hydrogen storage.

They could then fill up the entire tanks like at the beginning of the journey, but they only have 2.5 to 2 light years remaining ahead of them. Which means they could use more fuel per light year than estimated at the beginning.
Additionally, the population size on board drops from book I to book II, so additional hydrogen might be stored in some empty quarters. (Most likely in cans, but potentially, sealing off certain sections of the habitat ring to flood those corridors with hydrogen might work, too?)
Regarding efficiency: Well, they would no longer create propulsion from the nuclear fusion reactor directly. Rather, they would use the nuclear fusion reactor to power several lasers, which create a black hole out of light (a Kugelblitz drive), and then use that to accelerate the ship.

The critical issue is of course, like for us with nuclear fusion right now, to get more energy out of this than you put into it. I could set it up in such a way that, analogously to nuclear fusion in the 21st century, the first experiments towards Kugelblitz drives have been made on Earth - so that the basic technology is known - but energy efficiency is the problem.

 

[Algr]

Or are there any alternatives? Because flipping the ship in turn seems to be the only way to slow it down (=accelerating into the opposite direction).

Could you disconnect the rockets from one side of the ship, and move them to the other?

I think I missed something early on. Some ship proposals don't rotate. Instead they fire the engines continuously to create 1g of acceleration. So inside the ship, all the architecture has "up" as the direction that the ship is flying. Then the ship turns around for the deceleration phase and creates the same gravity to slow down. Did someone find a problem with this design? (Also, .6g might be more manageable.)

Crew size: 500 - 1,500 people

How many churches on the Exodus? What happens if a new one crops up and has problematic beliefs?

 

[DaveC426913]

Could you disconnect the rockets from one side of the ship, and move them to the other?

That is what I'm leaning toward too.

I think I missed something early on. Some ship proposals don't rotate. Instead they fire the engines continuously to create 1g of acceleration. So inside the ship, all the architecture has "up" as the direction that the ship is flying. Then the ship turns around for the deceleration phase and creates the same gravity to slow down. Did someone find a problem with this design? (Also, .6g might be more manageable.)

Yes, it requires an amount of fuel even more implausible than the OP's design. The OP's fuel is enough for 2 years of operation, not for 100.

Whether or not there are other ways, that's the premise of the OP's story.

How many churches on the Exodus? What happens if a new one crops up and has problematic beliefs?

"Ah. So you're a Follower of the Great Green ArckleSeizure are you? We've got a chapel all ready for you, recently vacated, right down here. Yes, yes it looks like an airlock, but really just have a peek..."

 

[Algr]

Yes, yes it looks like an airlock, but really just have a peek..."

Yes, government oppression has a great track record against religion.

Yes, it requires an amount of fuel even more implausible than the OP's design. The OP's fuel is enough for 2 years of operation, not for 100.

Given that you ran the ion thrusters at a power that was sustainable for half the journey, what speed and G force would you get?

 

[DaveC426913]

Yes, government oppression has a great track record against religion.

Perhaps, although I'd suggest many forms of freedom with be tightly controlled aboard a ship. And rightly so.

Given that you ran the ion thrusters at a power that was sustainable for half the journey, what speed and G force would you get?

No idea. But the ion thrusters we know of seem to have accelerations ranging in the tiny factions of g's.

 

[Algr]

Perhaps, although I'd suggest many forms of freedom with be tightly controlled aboard a ship. And rightly so.

Oh dear. Good luck getting funding.

 

[DaveC426913]

Oh dear. Good luck getting funding.

I am confident that the circumstances surrounding the need to build an ark ship for a one-way trip to colonize another star system will plausibly be such that funding will not be a bottleneck. After all, it's a project of a scale so vast that it could likely only succeed if the whole planet saw the value in it and pulled together.

I don't think of an ark ship being built with early 21st century sensibilities is very plausible. For the most part, we are too attached to our stable society and to our individual freedoms to get motivated to give it up for a few to escape.

But the expectation of such freedoms is really quite new. It has not been thus throughout much of history, and there's no reason to assume, now that we take that freedom of individuality for granted, that is will not go away again.

And it doesn't have to be government oppression. Citizens have no trouble willingly reorganizing their societies for conformity when circumstances call for it. History - as recent as WWII - has shown this.And a society that sees its stability and freedoms in imminent jeopardy (a little like a world war does) might see things that way.

 

[Algr]

Humans. I'll be 3D printing a better species by next year.

But really I just want to know what Strato will do about the churches.

 

[Algr]

https://spacetravel.simhub.online/

Well this calculator produces some pretty dire numbers. 6000 years to alpha centauri under ideal circumstances.

The fuel mass needs to be less then the payload. I had to assume 0.00001 m/s^2 acceleration to get this to work. It assumes perfectly efficient hydrogen power.

 

[Strato Incendus]

Could you disconnect the rockets from one side of the ship, and move them to the other?

I'm paying more attention to this proposal now, since both of you seem to be supporting this. How would this work in practice? How can you move disconnected objects from one side of the ship to the other while the ship is still flying at a speed of at leat 0.1 c?

The dismantling process of unattaching the ring sections from the central pipe would only take place after the ship has already come to a standstill in the orbit of the target destination planet.

Well this calculator produces some pretty dire numbers. 6000 years to alpha centauri under ideal circumstances.

Thanks a lot for linking to this calculator, very helpful! :) Regarding the specific numbers you report, those are hard to interpret, though, without the rest of the values you entered.

Also, in terms of the established parameters defined in the starting post, I'd say you approached this from the "wrong side" in terms of the story. ;) Meaning, from what's strictly physically feasible, starting with the closest possible star. But as it stands, within the story, Alpha Centauri is not the target destination, and 6000 years is not the intended travel time.

(Though funnily enough, in the very first draft of the premise - before there even were characters or a story - I was indeed envisioning a 6300-year-long journey to Proxima Centauri, only to have it end in the rather bleak conclusion that the planet isn't all that habitable after all. Yeah... that story would have gone nowhere. Literally.
The story still has a lot of dark themes, and some may call it pessimistic, but it's not quite that pessimistic. Especially not in terms of technological advancement. Though obviously, in that regard it's much less ambitious than Star Trek. It's more about retaining - or reviving - the humanist spirit 90s Star Trek used to have.)

The target destination is Teegarden b, so the travel distance is 12.5 light-years. The envisioned travel time at 0.1 c, from the ship's perspective, would therefore be roughly 125 years (where the first year is used for acceleration and the last year for deceleration; @DaveC426913 calculated this in the first thread on this ship design).

Given these strictly plot-relevant factors, now we'd have to determine the not-so-plot-relevant factors like fuel mass, payload etc.

If I use those numbers, and a ship length of 1 km - this is closer to the original design from the first thread, where the ship was around 860 metres long, i.e. before we attached the spheres / cylinders with a radius of up to 1 km - your calculator provides the following numbers for the blank fields:

observer time elapsed during journey: 14.31 years
payload ( spacecraft mass without fuel): 25,000 kilograms (this seems way too light)
fuel conversion rate: 0.008 kg x m x m
fuel mass: 694.4 t (this also seems way too light)

The simulation underneath also doesn't run for 125 years, so... I don't know what I'm doing wrong. Several of these values seem quite far off to me.

How many churches on the Exodus? What happens if a new one crops up and has problematic beliefs?

Okay, this opens up a whole new topic, which basically doesn't have anything to do with the technical side of the ship. Also, as much as I have to say on the matter, this branches off into potential political / societal discussions. So I'll focus on the technical aspects of that question here:

- First, it doesn't necessarily reveal itself to me how you infer the number of religions from the number of people aboard? 500 - 1,500 is less than most small villages on Earth. The entire crew could belong to one and the same religious community (see below). Emergence of several religions in parallel is not a given. Especially not within a time frame of 125 years. With 6000 years or more, yes, I could see that happening. ;)

- In my case, it's actually different, since the original crew was comprised of at least one couple from each sovereign country on Earth. So naturally, that meant people with several different religions came aboard (though mainly the five major world religions).

- The society of the Exodus itself is secular, though, and while it protects religious freedom, it also puts confines on "conversion" etc. through a legal document I've called the Carta of 200 Nations. This goes beyond just religion, and refers to the general extent to which each family can "live out" their Earthen ancestors' culture - namely, only as far as it doesn't infringe on the rights of others to do the same.

Spoiler: A harmless example of a breach of the Carta of 200 Nations - nicely fitting the current Easter season

A male crew member of Polish descent splashes a bucketful of water at an unmarried woman at a celebration during "Ancestral Awareness Week", which is basically the ship's equivalent for carnival. (An adaptation of the Polish custom Śmigus-Dyngus, practiced on Easter Monday.) This would have been societally accepted where his forebears were from, but of course, most other people on board are neither familiar with that custom, nor are they particularly fond of becoming involuntary participants in it.

The Carta represents a common set of values that all the original crew members (the first 250 couples that formed Generation Zero) had to sign. Importantly, they also agreed to raise their children in this same spirit. The society of the Exodus is as multi-ethnic as it can get, since it literally has people from every single country on Earth on board. But it is not multi-cultural, and becomes less so with every passing generation.

(In fact, that's what "Ancestral Awareness Week" is for: To preserve whatever little remnants of the original 250 couples are left, so that the idea of exporting human culture(s) can also be upheld. And ironically, when your family is the only one with ties to a given country on Earth, your national identity, which would normally be a group identity, can merge and blur with your identity as an individual.)

There are certain values that everyone on board had to agree on a-priori, or the mission would have been doomed from the outset, due to all the foreseeable internal conflicts. Applicants for the mission who refused to agree with this common set of values were simply rejected at the earliest stages of the selection process for the crew. As you can imagine, if each country held a national selection, but only one couple from that country could become crew members, that must have been quite a tough competition.

Yes, government oppression has a great track record against religion.

So does government oppression in the name of religion. There have been quite a few fictional examples of generation ships that are theocracies, founded on the basis of religious communities, rather than having them emerge as a "side effect".

Government oppression on my ship, ironically, is done in the name of science - a "dictatorship of reason". This often involves officials citing the principle "Full Transparency": Governmental decisions which the public does not get to vote on (even in Star Trek, a spaceship is not a democracy, but a military hierarchy, as Captain Janeway points out in one Voyager episode) need to be justified in scientific terms, so that the public at least understand why these steps are necessary.

While this may make intuitive sense to us in everyday life (be it for health or climate concerns, for example), on a philosophical level, using scientific findings to justify political mandates is an attempt to get an ought from an is. The actual "ought" premise often remains unspoken. From the commander's perspective, that is usually "we ought to ensure the survival of humanity".

But that rarely gets mentioned explicitly; people simply assume everybody agrees on that. If the ought were spelled out explicitly, people could disagree with it, or spell out alternative, competing oughts. Such as "we ought to protect the individual freedom of every crew member", or "we ought to maximize the well-being of the crew members" etc.
While the commander is strictly about fighting for the survival of the human species as a collective, it takes the protagonist two thirds of the story to realize that this is not actually what she herself is fighting for.

At its most abstract level, the ethical conflict between the ultimately irreconcilable values of life, well-being, and freedom is what the story is all about.

But really I just want to know what @Strato will do about the churches.

There are no paid or full-time priests on board; I have one specific character in mind as a leader of the ship's Christian community, but he's more like a reverend, who does this alongside his main job.

The protagonist and her partner are (agnostic) atheists, but the protagonist does have a Catholic close friend, a psychologist of Irish descent. So she has her scientific views and her religious views, and needs to reconcile them somehow. For example, this Irish-based psychologist will resort to lines of reasoning like the Kalam argument about how the universe began. And this is why I decided to indeed respond to this "random" question about the ship's social structure here on the PhysicsForums:

In the third book, the Christian idea of a universe with a beginning and an end, and the Buddhist concept of Samsara, are lined up with the competing scientific hypotheses of the Big Rip and the Big Bounce.
This happens in the context of some characters questioning the purpose of the mission, and to what extend it represents progress or not.

Spoiler

The protagonist has been historically optimistic about humanity's ability to move on from its violent past. In her view, if the Big Bounce hypothesis is true, that would ultimately render all human progress pointless, since at some point in the far distant future, all progress would be "reset", a new universe would follow the old one, and wherever intelligent life emerges again, those sentient beings would have to start the evolutionary process from scratch.

A Buddhist security officer of Cambodian descent (the nationality is more relevant for part I of the story) then challenges her to answer the question of what happened before the Big Bang. The Big Bounce hypothesis has an answer to that: There was another universe before the current one. But if there's a beginning and an end, what was before the beginning?

Thus, ironically, even though the protagonist is atheist (which would be reconcilable with Buddhism), her understanding of progress still makes her prefer her Irish friend's perspective of a definite beginning and a definite end of the universe. Which begs the question of who or what caused the beginning.

Of course, the Big Rip would also destroy any remainders of life that might somehow manage to survive that long (in O'Neill cylinders, maybe?). But at least a definite starting and end point allow progress to be measured along that line. Or as the protagonist puts it: "Where there's an end, there's a point." Two points on a line, in fact: A starting point and an end point.

As far as my own layman understanding about the current state of knowledge on these two hypotheses goes - please correct me if I'm wrong here:
For all I know, the currently measured expansion rate of the universe seems to favour the Big Rip hypothesis.
However, I've also seen videos by Anton Petrov like "Interesting proof of another universe before our own" (this was about Hawking radiation from past black supermassive black holes, as they are assumed to only form at the end of a universe's life cycle).
And then, there's that suggestion I've recently heard, that every universe might be a black hole, that nothing ever reaches the singularity, and that every big bang is ultimately indeed just a big bounce.
However, I don't know how this would go along with the idea that the universe itself, in contrast to the stars and planets within it, seems to be "flat"?

I am confident that the circumstances surrounding the need to build an ark ship for a one-way trip to colonize another star system will plausibly be such that funding will not be a bottleneck. After all, it's a project of a scale so vast that it could likely only succeed if the whole planet saw the value in it and pulled together.

As explained above, in my case, all sovereign nations on Earth are involved in the project (every country held a nationwide selection to find a couple they could send to the ship as their representatives, so that as much of human knowledge and culture can be "exported" to the new world).

Hence, since all those countries have a stake in the success of the mission, everyone contributed funding to it, as well. And vice versa: The promise of every country being allowed to have at least one couple on board the Exodus was a tactical step to secure as much funding as possible for this gargantuan undertaking.

But the expectation of such freedoms is really quite new. It has not been thus throughout much of history, and there's no reason to assume, now that we take that freedom of individuality for granted, that is will not go away again.

Seems like you may already have chosen a side in the life vs. well-being vs. freedom conflict?

Ultimately, I'd describe the mood of the story as "Star Trek: The Next Generation meets The 100". The philanthropic spirit of TNG is there, but at the same time, there's the core ethical question of The 100, which is "Survival - at what cost?"

If the goal of a generation ship is to export human civilisation, what type of civilisation would you want to export elsewhere to begin with? If the only way for the mission to succeed is to turn the society on board into a totalitarian nightmare, that is willing to sacrifice personal freedoms (up to the point of human rights) on the altar of securing the survival of the collective, is that mission worth it in the first place?In any case: It has been fun for me to elaborate on this, since the ethical and philosophical questions (including religious once, as far as they become relevant to that) are of course what the story is predominantly about. But in order for readers to be able to suspend their disbelief, and thus be open for that debate, I still want this story to take place in a setting as physically realistic as possible.So with all that said, within this thread, let's get back to the technical design of the ship itself.

An interim summary: With the "two-cylinder dumbbell" proposed by @DaveC426913 , we'd be back to the original ship length of about 3 kilometres:

Each cylinder (1 km in radius, 1.3 km in length) + 6 rings, each 64 metres in width + 5 metres in between the rings, as proposed by @John Strickland .

In sum: 2 x 1.3 km + 6 x 64 km + 5 x 5 km = 2.6 km + 0.409 km

Dave's Blender model still shows some distance between the outer rings and the cylinders.
Could we go with another 5 metres here on both ends? Or do we need more space to fit additional equipment in there?
Keep in mind the central pipe has a diameter of 100 metres (=radius of 50 metres).
So pi * (50 m)² * 5 m = 39.270 m³

I'm still wondering where to put the nuclear fusion reactor. All current builds I am aware of seem to be small enough to easily fit within the central pipe, either vertically or horizontally.
The main difference is of course that there is no gravity inside the central pipe, and 0.2 g within the rotating hubs of each ring in the central pipe.

 

[DaveC426913]

How can you move disconnected objects from one side of the ship to the other while the ship is still flying at a speed of at least 0.1 c?

Other than the hard radiation coming from the bow, there is no way, without careful measurements, to even tell that they are not simply sitting stationary in space.

The dismantling process of unattaching the ring sections from the central pipe would only take place after the ship has already come to a standstill in the orbit of the target destination planet.

Yes, my point simply was that the ship is already designed to be dismantlable at the end, so it makes sense to use this feature to reconfigure the ship as-needed.

(where the first year is used for acceleration and the last year for deceleration; @DaveC426913 calculated this in the first thread on this ship design).

(Yeah, it will take the same length of time to decel but it will use less fuel, because the ship masses less now.)

BTW, did you fix an accel rate and decel rate? If so, do the rings rotate during these phases? If so, have you accounted for the accel/decel in the strength - and direction - of the artificial gravity?

There are no paid or full-time priests on board; I have one specific character in mind as a leader of the ship's Christian community, but he's more like a reverend, who does this alongside his main job.

In the military, they sometimes have generic chaplins, who cover all needs.

'm still wondering where to put the nuclear fusion reactor. All current builds I am aware of seem to be small enough to easily fit within the central pipe, either vertically or horizontally.

How about a fusion reactor large enough to power a 3km long ship across 12.5 light years? Do you not think it needs to be a little larger?

But if you want the reactor to live in the pipe, then you should probably flip the whole hab section - rings and all - end-over-end at turn-around, rather than detach the rings from the pipe.

 

[Strato Incendus]

But if you want the reactor to live in the pipe, then you should probably flip the whole hab section - rings and all - end-over-end at turn-around, rather than detach the rings from the pipe.

To clarify: The rings so far only detach once the ship has already come to a standstill.

What do you mean by "flipping the hab section"? Flipping the centre of the ship, but leaving the sphere or cylinder at the front and back in place? Sounds easier to flip just the entire ship, including "swapping" the front and back sphere / cylinder around, doesn't it?

 

[DaveC426913]

To clarify: The rings so far only detach once the ship has already come to a standstill.

If you have settled on the two-sphere "dumbbell" configuration then this side-thread is an unnecessary discussion.

(You see, "standstill" is relative. Except for the influx of radiation, the ship is just as standstill during the 123-year coasting phase. If you need to reconfigure the ship for the decel phase, why would the modular design of the larger sections (drive, fuel and life section) not be mirrored in the rings themselves?)

I'll move forward assuming the dumbbell design is fixed.

What do you mean by "flipping the hab section"? Flipping the centre of the ship, but leaving the sphere or cylinder at the front and back in place? Sounds easier to flip just the entire ship, including "swapping" the front and back sphere / cylinder around, doesn't it?

If we settled on two tanks - one at each end - then you can flip the whole craft, yes.(I know this seems like we're going around in circles but that's how it works. Every time a parameter is reconsidered, the whole project needs to be re-assessed because it might change dramatically. We are covering decades of design and engineering of a colony ship in just a few score posts.)So, to recap:
A central pipe, with rings at midship, a tank fore and aft, and a drive out the back.
Use water in aft tank during accel phase, so fore tank provides max protection.
Ship pivots as a whole.
Redistribute all water from fore tank to aft tank.
Use fuel in fore tank during decel phase, so aft tank provides max protection.

(What did you decide about the shape of the two tanks? a] cylinder, b] large sphere, c] a constellation of small spheres in an open lattice?)One problem: if the central pipe contains the fusion drive, then it's midship, not at the stern. You really want the exhaust to be at the very stern; you don't want the exhaust to run through a kilometer of pipe to exit aft of the stern tank.

Maybe the fusion drive should be encased in the aft tank? Then the hab is protected from the drive and the drive vents directly out the stern.

 

[Strato Incendus]

If we settled on two tanks - one at each end - then you can flip the whole craft, yes.

Okay, let’s set this in stone, then. :)

I'll move forward assuming the dumbbell design is fixed.

Yes, it is. The question is still just about the shape of the tanks.

So, to recap:
A central pipe, with rings at midship, a tank fore and aft, and a drive out the back.
Use water in aft tank during accel phase, so fore tank provides max protection.
Ship pivots as a whole.
Redistribute all water from fore tank to aft tank.
Use fuel in fore tank during decel phase, so aft tank provides max protection.

(What did you decide about the shape of the two tanks? a] cylinder, b] large sphere, c] a constellation of small spheres in an open lattice?)

I’m leaning towards the large spheres with sub-chambers (to prevent loss of the entire fuel in case of damage to the outer hull). This would also go the best with the planned conversion into a black-hole drive in part II:

All of the remaining fuel from the aft tank could be pumped into the front tank (it’s unlikely to still be full at that point, otherwise we’d need to store the fuel from the aft tank “elsewhere”, like flooding entire decks of empty quarters on the rings with hydrogen, after the crew has already been reduced in size due to external circumstances). Then, the walls of the sub-chambers could be used to create the gamma-ray reflective panels required for the black-hole drive somehow.

More importantly, though, I think the design with two massive spheres is simply the most iconic from the outside.

Having a bunch of small tanks makes the ship look rather “bumpy” in comparison. Plus, there’s the potential of having characters “dive” through the massive sphere to fix something, or later, when one sphere is empty (like when getting to work on building the Kugelblitz drive in part II), they could be floating inside that sphere (since there’s no artificial gravity within it). I could even see some ”boss fight” going on in there, which would certainly be memorable! ^^

On a practical level, though, if the cylinder design would be more efficient in terms of length - and thus, hull mass - for the same volume of fuel, I need to find some in-story explanation why spheres were used instead of two cylinders at both ends.

One problem: if the central pipe contains the fusion drive, then it's midship, not at the stern. You really want the exhaust to be at the very stern; you don't want the exhaust to run through a kilometer of pipe to exit aft of the stern tank.

Maybe the fusion drive should be encased in the aft tank? Then the hab is protected from the drive and the drive vents directly out the stern.

That would have been my next question: Does the pipe go through the aft tank? There needs to be some way for engineers to get to the fusion reactor, the exhaust, or anything else behind that massive tank.

Having the fusion drive encased in the aft tank sounds like a good idea in principle. The question is just how to reconcile this with the black-hole drive built later. The fusion drive would be what provides the energy for the lasers that in turn create the black hole out of light. So having it nearby certainly wouldn’t hurt. But having it, say, in the centre of the aft tank, that would be a problem - since that’s where the black hole would have to go eventually.

 

[DaveC426913]

I’m leaning towards the large spheres with sub-chambers

You may find this useful:
https://en.m.wikipedia.org/wiki/Sphere_packing_in_a_sphere

I like #8.

 

[Strato Incendus]

Cool, thanks a lot! ;) I wasn’t necessarily thinking of the sub-chambers being spherical, too. But if that in turn makes a big sphere on the outside more viable than a cylinder on the outside, that might solve issues at once.

Reminder: The reason for having the sphere on the outside, rather than just the individual smaller spheres as in the Project Daedalus design, is to have an additional layer of protection - i.e., redundancy.

In case of damage, rather than settling for the contents of one smaller sub-tank escaping into space (and potentially throwing the ship off course in doing so), they would escape into the outer sphere, from where they could be transferred back into other sub-spheres (divided up, since all those spheres will be partially emptied at that point, but no single sub-sphere will be big enough to take up all the contents of one sub-sphere breaking).

Depending on the size of the sub-spheres that follows from the chart you linked to, would one of these sub-spheres be large enough to contain the nuclear-fusion reactor? (Assuming that the outer sphere still has a radius of 1 km.)

If we stick with my idea of eventually using the walls of the sub-spheres to create the gamma-reflective panels for the black-hole drive, the curvature will have to be adjusted from the smaller to the larger sphere (but then again, if the sub-chambers had straight walls, that would also be the case).

 

[DaveC426913]

Depending on the size of the sub-spheres that follows from the chart you linked to, would one of these sub-spheres be large enough to contain the nuclear-fusion reactor? (Assuming that the outer sphere still has a radius of 1 km.)

And the size of a nuclear-fusion reactor is...?
(i ; where i is defined as "as large as Strato needs it to be").BTW, note the the chart contains data of several useful measurements, including the maximum radius of the smaller spheres. For the 8:1 configuration I mentioned, it looks like the inner spheres are 378m in radius.I chose #8 because I like the symmetry of it. It's 4 spheres in a square and then another 4 spheres on top of it at a 45 degree offset (i.e. a diamond). The key is that it is rotationally symmetrical about one axis.

Which would align nicely with the ship's axis.

But if you want one of them to contain a drive, then you should have at least one sphere on the ship's axis.
#9 is almost identical to #8, but with a ninth sphere. on the axis Up to you whether you want to go up or down the chart.