How Can a Ship Safely Brake in Space Without Crushing Its Inhabitants?

[Melbourne Guy]

But as long as it can turn around...

There's your journey extender, @Strato Incendus. A gyroscope buried deep in the ship won't respond to the "stop" command and the ship slowly spins on its long axis over and over. They miss their braking point, the sperm bank gets fried, havoc ensues, and it takes a long time for the engineers to cut their way through to the broken part and sort the problem out. You're welcome

 

[Filip Larsen]

One approach I suggested in the first thread was to have all the rings rotate in the same direction, and thereby have the ship flip around its own axis naturally. Of course, the ring rotation would have to be stopped as soon as the flip has been accomplished. And after that, half of the rings would have to rotate in the opposite direction to the other half again, to keep the ship stable on its new path.

So as I understand you: 1) total angular momentum is near zero at first (e.g. half the rings rotate opposite each other, or nothing rotates), 2) all rings (and possibly the rest too perhaps) are made to spin in same direction, 3) ship being in unstable rotation transits over time to stable end over end rotation, 4) half the rings is spun up again, 5) ship again being in unstable rotation transits to "normal" rotation, 6) ship now points engines the other way relative 1).

Going from 1 to 2 means changing your total angular momentum which requires expending reaction mass, i.e. you need rocket propulsion thrusting in a tangential direction. The same is change is then also needed to go from 3 to 4.

Both transitions 3 and 5 will take time and will likely be very unpleasant for the crew as local gravity constantly moves around. There is a 50% chance the flip at 5 will take the ship back to the original rotation, i.e. back to same orientation as before 3 unless this is carefully monitored with a (planned) control loop to nudge the ship towards flipping to the "right" side.

All in all this sounds a lot of complexity with no benefits compared to simply do: 1) total angular momentum is near zero, 7) two rocket pulses starts and stop a rotation of the whole ship so it points the other way, using only a fraction of the reaction mass needed for 2 and 4.

In short, as long as the total angular momentum is near zero there is no "rotational difficulty" in simply change the attitude of the ship as desired. Granted, you need at least one rocket that can be made to thrust 90 deg from the main thrust direction, but that sounds fairly simple and is anyway also required for 2 and 4.

But maybe I misunderstand your setup?

 

[Melbourne Guy]

Granted, you need at least one rocket that can be made to thrust 90 deg from the main thrust direction, but that sounds fairly simple...

It also seems like something that any spacecraft would need, @Filip Larsen, for in-system manoeuvring at least.

 

[Filip Larsen]

It also seems like something that any spacecraft would need, @Filip Larsen, for in-system manoeuvring at least.

Normally the term maneuvering is referring to propulsion that effectively changes the trajectory of the spacecraft , whereas as propulsion that only has a net change for angular momentum is referred to as attitude control. So when you refer to maneuvering in the middle of discussing flipping the attitude of a ship I am unsure what you mean.

If you are just commenting that its common for spacecraft s to use propulsion for some kind of attitude control then yes, but a generation ship is hardly normal, so that is why I just wanted to mention that the main propulsion system could act as the primary attitude control system on top of any rotation whenever the main thrust is "on" anyway.

 

[AllanR]

Gravity assist around object will only help you so much. Your high approach velocity will mean you only have a short time close to the object, and you still have to endure the g-force as you change direction. (and if your ship is only designed to take acceleration force along the long axis, the twist around the planet might cause problems to the structural integrity)

 

[Filip Larsen]

Gravity assist around object will only help you so much. Your high approach velocity will mean you only have a short time close to the object, and you still have to endure the g-force as you change direction. (and if your ship is only designed to take acceleration force along the long axis, the twist around the planet might cause problems to the structural integrity)

While you are right that gravity assist (even with a very lucky alignment of planets) are not going to help much if the ship has high hyperbolic excess speed, the rest of this reads wrong unless you are specifically thinking about aerobreaking?

 

[Melbourne Guy]

If you are just commenting that its common for spacecraft s to use propulsion for some kind of attitude control then yes, but a generation ship is hardly normal

My bad, @Filip Larsen, what you said

In terms of using the main propulsion system to also act as the primary attitude control system, I know Cassini's main engine nozzles were steerable, but not whether @Strato Incendus's ship design allows for that. Irrespective, I would say that altitude control can be presumed a 'standard design feature' of any spaceship, so the failure of it might provide the necessary plot device for the generation ship to overshoot its target.

 

[mfb]

Perhaps I should rephrase the question even more generally: What type of thing could occur during the last 25 years of the journey that would unexpectedly extend the journey by another 100 years? It doesn’t necessarily have to involve braking or taking a detour - I’m open to any alternative suggestions at this point.

Severe damage to the propulsion system, lowering the maximal thrust, works nicely.

Nominal profile: We start 2.5 light years away from the target at 0.1 c, coast for 24 years, decelerate at 0.1 g for one year.
New profile: Acceleration is limited to let's say 0.0016 g. You start decelerating immediately but you only come to a stop relative to your target after 60 years, ~0.5 light years behind the target. You use half of your remaining propellant to accelerate towards the target, and the other half to finally approach it.

You could detect this problem during a routine test, or simply from an emergency causing physical damage to it.
Damage to the structure of the ship works, too, then you limit acceleration to avoid the ship breaking apart.

Turning around the ship is both trivial and unavoidable.

So if Tau Ceti were to prove itself uninhabitable, the crew could more easily decide to “spontaneously” travel on to 82 G. Eridani.

That would need an absurd level of extra propulsion capability. It's like carrying a car in a horse carriage just in case you want to change your destination later. At relativistic speeds everything is point-to-point. Once you fly towards something you'll go there or you'll go nowhere.

You don't need to pick an exoplanet we have discovered already. We are essentially blind to Earth-like planets around Sun-like stars, something that should change within one or two telescope generations. Whatever we'll fly to - it's probably not a planet we know about today.
Fly-by at planets is useful if you want to change your velocity by a few kilometers per second, but they won't do anything at relativistic speeds.

Forget dust clouds, supernovae or solar flares: They are not much better than the "dark matter" cloud. There is no star closer than a few hundred light years that could explode in a supernova in the foreseeable future (10,000+ years). I don't see anything else nearby that could plausibly be a relevant over interstellar distances. Solar flares are far too weak for that.

Letting your ship accelerate at 1 g wouldn't be an issue for the story, by the way. The ship will be limited by its delta_v capability anyway, i.e. how much it can change its velocity overall. It doesn't really matter if you accelerate at 0.1 g for a year or 1 g for a tenth of a year before you used up the propellant. A ship designed for 100+ years of travel time would probably aim at even lower accelerations. It doesn't change the travel time much but you can make the engines smaller and reduce stress in the structure, saving even more mass (which can lead to a higher velocity during the cruise phase, and make the trip faster overall).

In terms of an explanation why they have to leave the solar system, just the reminder: In the story, a gamma ray burst is headed for Earth.

A gamma ray burst that hits Earth will also hit a spacecraft tens of light years away and its destination planet. That's essentially the same spot within the scale of that event.

 

[Melbourne Guy]

We are essentially blind to Earth-like planets around Sun-like stars, something that should change within one or two telescope generations. Whatever we'll fly to - it's probably not a planet we know about today.

Too true, @mfb, and that both helps and hinders writing a sci-fi story involving interstellar travel in our local region.

I've created a few thousand colonies for my current series, using real stars and known planets / moons out to 300 light years, but assumed settlers further out in time and distance will rename their colonies rather than use an obvious moniker like "Kapteyn Moon Base" (which is an orbital settlement founded mid-2217 around Kapteyn's Star, so is quite early in the first FTL expansion phase).

Using fictitious names protects your novel against new exoplanets being discovered and current ones being false positives, but it's a lot of work to keep track of when your cast is reasonably large!

 

[Strato Incendus]

Wow, thanks for your many ideas once again! And welcome to everyone "new" who chimed in!
I really appreciate it!

All in all this sounds a lot of complexity with no benefits compared to simply do: 1) total angular momentum is near zero, 7) two rocket pulses starts and stop a rotation of the whole ship so it points the other way, using only a fraction of the reaction mass needed for 2 and 4.

Alright, that sounds much less complex than what I proposed. ^^ Then I'll simply stick to that, thanks!

It might seem to some like we're moving in circles in this thread, but just like that, we do end up solving some of these questions, one by one. We're simply taking small steps.

Severe damage to the propulsion system, lowering the maximal thrust, works nicely.

Nominal profile: We start 2.5 light years away from the target at 0.1 c, coast for 24 years, decelerate at 0.1 g for one year.
New profile: Acceleration is limited to let's say 0.0016 g.

Alright, I like where this is going! Since I've established previously, I want the acceleration to be barely noticeable to the passengers - no "walls turning into floors" and other stuff that would happen at 1 g, like in "Braking Day". And since I don't need any quick back-and-forth trips between planets (like in The Expanse) or solar systems (like in Star Trek), I usually don't have any reason to expose my crew to high g forces anyway - that's the whole point of the journey taking so long.

The only point where they would be exposed to high g forces would indeed be if we still found some solution to make emergency braking work as a plot point. And then, in part 2, when they accelerate up to a higher percentage of light speed (0.77 c) using a black-hole drive, which they create by recalibrating the on-board lasers inside one of the empty spherical water tanks.

A ship designed for 100+ years of travel time would probably aim at even lower accelerations.

Indeed, because such a ship can afford slower acceleration - and is built as a generation ship precisely because, among others, humans can't withstand continuous exposure to several g. As long as this doesn't change the overall journey time to much from the envisioned 125 years, that's fine.

You start decelerating immediately but you only come to a stop relative to your target after 60 years, ~0.5 light years behind the target. You use half of your remaining propellant to accelerate towards the target, and the other half to finally approach it.

Can we bump those 60 years up to 100 years somehow? The perspective shift makes much more of a difference if not even the current generation's children-to-be will live to see the landing on the planet.

You could detect this problem during a routine test, or simply from an emergency causing physical damage to it.
Damage to the structure of the ship works, too, then you limit acceleration to avoid the ship breaking apart.

Sure, in principle, after 100 years of the journey, I could always have some tech parts failing unexpectedly for internal reasons, due to natural wear and tear. If it's at least partly caused from outside, though, that would help to show the "indifference of the universe" towards humanity's endeavour here. Which is kind of crucial for the commander's motivation. That same "indifference of the universe" - the existence of a star that will hit Earth with a gamma ray burst - is what got the mission started in the first place, after all.

Turning around the ship is both trivial and unavoidable.

Yeah, I think I'll simply use the approach suggested by @Filip Larsen for turning the ship around. Which means we probably can't turn this into a cause for the problem we're looking for. Unless those rocket pulses that are supposed to start and stop the rotation fail for some reason.

That would need an absurd level of extra propulsion capability. It's like carrying a car in a horse carriage just in case you want to change your destination later. At relativistic speeds everything is point-to-point. Once you fly towards something you'll go there or you'll go nowhere.

Great, I'm happy there's an explanation that rules out the Tau-Ceti to 82 G. Eridani alternative - because, as I've said, I actually want to stick to Teegarden b as the destination .

Tau Ceti has really been overused in other sci-fi stories already. Teegarden b has only been discovered in 2019, so it's probably "unused" yet. Plus, I keep getting back to that highest Earth-Similarity Index within the local interstellar neighbourhood.

You don't need to pick an exoplanet we have discovered already. We are essentially blind to Earth-like planets around Sun-like stars, something that should change within one or two telescope generations. Whatever we'll fly to - it's probably not a planet we know about today.

Sure, any sci-fi author is free to postulate their own Vulcan. I could do the same with the star that points at Earth with its gamma-ray burst - just in case someone ends up proving once and for all that we're outside WR 104's line of fire (the research seems to be going back and forth on this).

But tying the story to currently-known planets and stars grounds it more in "hard sci-fi", which is more in line with how we're approaching the design of the rest of the ship.

Forget dust clouds, supernovae or solar flares: They are not much better than the "dark matter" cloud. There is no star closer than a few hundred light years that could explode in a supernova in the foreseeable future (10,000+ years). I don't see anything else nearby that could plausibly be a relevant over interstellar distances. Solar flares are far too weak for that.

That's precisely what I feared.

Letting your ship accelerate at 1 g wouldn't be an issue for the story, by the way. The ship will be limited by its delta_v capability anyway, i.e. how much it can change its velocity overall. It doesn't really matter if you accelerate at 0.1 g for a year or 1 g for a tenth of a year before you used up the propellant.

The main issue I have with 1-g acceleration is the "walls turning into floors" effect. I don't find the solution proposed in "Braking Day" - turning all chambers by 90 degrees - convincing at all, since I can't even begin to imagine how that would be set up in terms of the ship's architecture.

If you have a ship that relies on constant acceleration, it will probably be built like a skyscraper, not using rotating rings or cylinders for artificial gravity at all. But this doesn't work for a generation ship, since it spends the majority of its time coasting, without any acceleration - and thus, without any gravity that could arise from it.

A compromise would of course be to have the central pipe of the ship structured like a skyscraper, with rings around it. That would require everyone to live in the ship's central trunk during acceleration and deceleration, and move to the rings once the acceleration phase stops and coasting begins.

That however would require lifts going through the central pipe, everybody would have a quarter in the central pipe AND on the rings, as well as farms, labs, canteens etc. in the central section, too... everything would have to be there twice, since the acceleration phase would still take too long for everyone to just keep avoiding the rings, and everything they have to offer, for all this time.

Hence, simply having the ship accelerate at much lower g forces, so that the rings can create rotational gravity the entire time, while the gravity of the acceleration and deceleration is barely felt by the crew, seems like the more viable solution overall. Especially since, as you said, a generation ship can afford these lower acceleration and deceleration speeds anyway.

It doesn't change the travel time much but you can make the engines smaller and reduce stress in the structure, saving even more mass (which can lead to a higher velocity during the cruise phase, and make the trip faster overall).

Faster perhaps, safer definitely. So lower acceleration (even lower than 0.1 g) sounds like a win-win!

A gamma ray burst that hits Earth will also hit a spacecraft tens of light years away and its destination planet. That's essentially the same spot within the scale of that event.

Couldn't I have Earth's solar system at the edge of the area of effect of the gamma-ray burst? So that our solar system is still in it, but Teegarden's star is outside of it?

WR 104 is in the Sagittarius constellation. It seems like we're looking at the spiral "from above", so that we'd be right in the middle of the burst when it goes off. But my current state of knowledge is that newer measurements show we're actually still looking at the star from somewhat of an angle. That angle might be strong enough for the GRB not to hit Earth at all - that's what the researchers are still debating.

So can't I just find a compromise between "Earth is in the centre of the burst, therefore, Teegarden's star would be caught, too" and "Earth is not in the line of fire at all, therefore we're safe (and so is Teegarden)"? That compromise would be "Earth is still in the line of fire, but not in the centre of the burst; Teegarden is just barely outside of it".

Because consequently, that should also allow the crew members to notice when the burst does go off and hit Earth - which is what we're currently considering to happen at the disaster plot point (2/3rds into the book).

Using fictitious names protects your novel against new exoplanets being discovered and current ones being false positives, but it's a lot of work to keep track of when your cast is reasonably large!

That is something I'm doing anyway, since all the official planet names are hard to remember thus far. Teegarden b is still among the easier ones to recall. But just like any other planet, it goes my many different designations.

I think it's safe to assume that, if we ever singled out a given planet or star system as a candidate for human colonisation, that this system would quickly earn a special spot in collective memory, and that people would constantly start suggesting proper names for these stars and planets. Since in my story, Teegarden's star is the first other solar system to be colonised, people did the obvious thing and started naming the planets according to the Greek gods instead of the Roman ones.

Hence, Teegarden b, the most fertile planet, is called Demeter, the neighbouring Teegarden c, which is colder, is called Persephone (who in the myth was the cause of winter, whenever she went to live with Hades instead of with her mother Demeter). Then I postulate two gas giants to protect the rocky planets from too frequent meteor impacts - those are consequently named Zeus and Chronos, as analogies to Jupiter and Saturn. And as the ship enters the system, they discover an ice giant at its outer edge, similar to Neptune and Uranus, which therefore ends up being called Poseidon.

Should Teegarden b be proven to be uninhabitable during the time frame in which I'm working on the story, I therefore still have the backup plan of replacing any and all references to the official names (Teegarden b and c) with their new names (Demeter and Persephone, respectively).

I just don't want to start there right away, because then people are going to think I'm pulling all of these fictional planets out of my own "ice giant". ;)

 

[Melbourne Guy]

people did the obvious thing and started naming the planets according to the Greek gods instead of the Roman ones

That's only obvious if your culture aligns with it, @Strato Incendus, so would depend on who gets there first But your naming convention makes sense, though I'm not sure an ice giant, even far out in the system, would not have been detected by the time your ship leaves. Teegarden's Star is not that far away.

 

[DaveC426913]

people did the obvious thing and started naming the planets according to the Greek gods instead of the Roman ones.

See, now, I prefer Larry Niven's approach: if you're going turn your back on Earth forever and go light years away to make a new system your home, you're not going to let long dead Earth ghosts dictate the names of your toys - you're going to dang well start your own new culture. It's a rite of ownership.

Home
We Made It
Wunderland
Silvereyes
Plateau
Cueball

 

[Strato Incendus]

Well, those names were decided on long before the generation ship was sent there. Much like we talk about Mars colonisation today; perhaps future Mars settlers will rename their world, but it may just stick around as "the way it's always been".

though I'm not sure an ice giant, even far out in the system, would not have been detected by the time your ship leaves. Teegarden's Star is not that far away.

Well, on the flipside, we aren't even sure whether our own solar system has a planet 9 yet, and if so, that would most likely also be another ice giant. So if we can't find a planet of that size (or prove its absence) within our own solar system yet, doing the same for a star 12.5 light-years away seems like it would be even harder to do.

Sure, within the story, the Breakthrough Starshot 3 probes have been sent to Teegarden's star. But that doesn't mean they necessarily passed every planet within that system on the way to Teegarden b.

 

[DaveC426913]

Well, those names were decided on long before the generation ship was sent there. Much like we talk about Mars colonisation today; perhaps future Mars settlers will rename their world, but it may just stick around as "the way it's always been".

It's up to you to decide what kind of colonists you want to build.

Are they the type who are happy to live under the yoke of an old Earth cultural domination, or are they the type to decide its time to start their own culture? That has thematic consequences for your stories.

 

[Strato Incendus]

This question does come up, but only once the colonists land on the planet (which is at the very end of the story). By that point, a lot of people have built up certain frustrations about people on Earth. For example, as the people in the solar system finally receive the footage of the crimes that happened on the generation ship over the course of the years, they put the ship's anthem on the index back in the sol system.

The colonists on Demeter perceive this as those people who started the whole mess still trying to tell them what they, the settlers, can and cannot do. Nevertheless, the planet name Demeter sticks around. If I changed it that late in the story, probably no reader would remember. Also, it's a reminder of the ship's history for the settlers themselves - which fits to them keeping their anthem, too.Anyways, back to the cosmic disasters:

1) I still kind of like the superflare approach. Perhaps I could "move" one of the Groombridge stars around a little, by having a rogue star pass that system, pulling it closer to the route from Earth to Teegarden?

2) Is there any way for me to find out if Teegarden's star (in Aries) might be in the line of fire of WR 104 (which is in Sagittarius)? Potentially, if it's in the line of fire "too" (assuming that Earth might be)?

I definitely like the general idea that the ship already starts slowing down during the last 25 years of the journey, in order to brake at really low g forces. So if the flare does something to the drive, they'd notice quickly.

 

[DaveC426913]

1) I still kind of like the superflare approach. Perhaps I could "move" one of the Groombridge stars around a little, by having a rogue star pass that system, pulling it closer to the route from Earth to Teegarden?

A superflare and a rogue star.

Your readers' suspension of disbelief will mimic the Tacoma Narrows Bridge.

 

[Strato Incendus]

Well, the rogue star would have to pass the Groombridge star a long time before the plot starts, wouldn’t it? So that wouldn’t necessarily feel like plot convenience if I introduce it in advance (by mentioning that one of the stars ahead is in a slightly different place than it used to be at the beginning of the millennium).

The question is more whether a few hundred years (between now and 2475) would be enough for the star to travel that far. Because this isn’t about the expansion of space, it is still about masses moving due to gravitational influences.

Superflares meanwhile should be common enough with these types of stars that they don’t seem like as much of a contrivance. Supernovae, or even a pre-established gamma-ray burst which just so happens to go off during the time frame of the story, in terms of their actual likelihood should feel much more like plot convenience.

 

[Melbourne Guy]

1) I still kind of like the superflare approach. Perhaps I could "move" one of the Groombridge stars around a little, by having a rogue star pass that system, pulling it closer to the route from Earth to Teegarden?

Is that Groombridge 1830, @Strato Incendus? Is it even on the flight path, it's more than twice as far away than Teegarden's Star.

But how about a wandering black hole causing trouble? They are really hard to spot from a distance! Having one intersect Sol would play havoc with events back home, and you could couple that with sympathisers on the ship who mutiny to turn it around (and fail, obviously, but their antics extend the trip) or just have a local system failure extend the trip.

 

[DaveC426913]

But how about a wandering black hole causing trouble?

On one hand:

I wonder if that could be tied into the plot about their new drive. The wanderer is both curse and a blessing. Although, it presents a paradox: do we head away from it as a curse, or toward it as a blessing? (Is that what is dividing the crew?)On the other hand:

Gravitationally, a black hole is no more destructive than any star of the same mass. It would require some fancy footwork to arrange the story for it to become a cause for a significant detour.On the third hand:

What of the BH directly caused the course change? If they got too close before realizing anything, their course may be altered whether they want it to or not (though not by a lot without tearing the delicate ship apart).On the fourth hand

Any plot involving a rogue black hole seems like a real deus ex machina.

 

[AllanR]

this reads wrong unless you are specifically thinking about aerobreaking?

It is wrong. I was only thinking of momentum and completely ignored gravity.

Any plot involving a rogue black hole seems like a real deus ex machina.

They might be more common than we suspect?
from https://hubblesite.org/contents/news-releases/2022/news-2022-001.html
"Astronomers estimate that there should be 100 million black holes roaming among the 100 billion stars in our galaxy."

 

[Strato Incendus]

Is that Groombridge 1830, @Strato Incendus? Is it even on the flight path, it's more than twice as far away than Teegarden's Star.

There are two Groombridge stars in "relative proximity" to Teegarden's star: Groombridge 1830 (30 light years from Earth) and Groombridge 1618 (16 light years from Earth). Both are flare stars, or at least presumed to be. That's why I said "one of the Groombridge stars".

But how about a wandering black hole causing trouble? They are really hard to spot from a distance!

Interesting that we haven't thought of that yet, isn't it?

I guess it's because my default image of a black hole by now is one with a shining disc around it, i.e. one that you could spot, if only from the matter falling into it.

However, if the black hole is in interstellar space, currently not sucking up any noteworthy amounts of matter, it could indeed be completely dark, couldn't it?

It's gravitational effects should still be noticed by the ship's sensors somehow, though. So the question here is how early they would notice that?

Having one intersect Sol would play havoc with events back home, and you could couple that with sympathisers on the ship who mutiny to turn it around (and fail, obviously, but their antics extend the trip) or just have a local system failure extend the trip.

I don't think I would have it pass by near the Sol system - we already talked about the believability (or lack thereof) of "dual armageddons". Hence, the catastrophe at home should either be caused by a solar flare from our own sun (given the relatively high frequency with which those occur over the centuries), or by WR 104 indeed going off with its gamma-ray burst during the year of the first book's plot.

So for the time being, let's focus on what a black hole could do if it somehow interfered with the ship's trajectory.

I wonder if that could be tied into the plot about their new drive.

That seems like a good opportunity - but how would this work in practice? Kugelblitz drives use artificial black holes, created from light (=lasers). How would they be supposed to catch a real black hole and use it for a drive somehow? They can of course use it for swingby. But using the Hawking radiation it gives off as a means of propulsion?

The point of artificial black holes, as far as I understand it, is that they are so tiny that they give off much more Hawking radiation than a large one. So a natural black hole, with appropriate size, would be useless as a means to power a drive, wouldn't it?

Gravitationally, a black hole is no more destructive than any star of the same mass. It would require some fancy footwork to arrange the story for it to become a cause for a significant detour.

Indeed - the common thought experiment for this is replacing the sun with a black hole of equal mass. That wouldn't do any damage to Earth - at least not from the black hole itself. The sudden absence of heat would of course be a massive problem for all inhabited planets around, but that is not a unique property of having a black hole in the centre - the same would be true around e.g. a white dwarf star.

The main question here is, analogously to having a rogue star pass by:
Could a rogue black hole pull one of the Groombridge stars (1830 or 1618) close enough to the path leading from Earth to Teegarden b for a solar flar from that Groombridge star to hit the ship as it's passing by? More importantly, could this rogue black hole do so in time (between now and 2475)?

What of the BH directly caused the course change? If they got too close before realizing anything, their course may be altered whether they want it to or not (though not by a lot without tearing the delicate ship apart).

I can imagine a rogue black hole pulling the ship off-course with its gravity - without anyone noticing the black hole, because it's completely dark (=not absorbing matter in the interstellar medium, therefore no shining disk around it).

But as I've asked above, how soon would the crew notice that? If the destination star is no longer straight ahead of them, they should pick up on that pretty quickly. Or can I use gravitational lenses somehow to get around this problem?

Any plot involving a rogue black hole seems like a real deus ex machina.

If anything, in this case it might be seen as diabolus ex machina - or Murphy's Law: If something can go wrong, it will.

But conversely, it could also be seen as the complete absence of a deus ex machina: The inherent indifference of the lifeless universe I was talking about earlier. This is something the crew is already contemplating due to the gamma-ray burst threat from WR 104: The universe isn't "trying to kill humanity"; it simply doesn't care.

"Astronomers estimate that there should be 100 million black holes roaming among the 100 billion stars in our galaxy."

Great news, then (for my story, at least - not necessarily for those stars)!

So I could have one pass through the galactic disk "vertically", to pull a Groombridge star closer to the plane on which the ship would be travelling from Earth to Teegarden? Because the "vertical" difference (in light years) seems to be the main problem here. When viewed from above, the two Groombridge stars are actually pretty close to Teegarden's star.

 

[DaveC426913]

However, if the black hole is in interstellar space, currently not sucking up any noteworthy amounts of matter, it could indeed be completely dark, couldn't it?

Certainly.

It's gravitational effects should still be noticed by the ship's sensors somehow, though. So the question here is how early they would notice that?

They would likely first notice it as a miniscule acceleration in their velocity - if they are paying attention.
If they aren't, they might notice it as a deflection in their path, but by the time that happens they are too close - and moving too fast - to do much about it.

Another way they might notice at a distance far enough to do something about it is by happening to spot a Newton's Ring as it passes in front of some background star..

But as I've asked above, how soon would the crew notice that? If the destination star is no longer straight ahead of them, they should pick up on that pretty quickly. Or can I use gravitational lenses somehow to get around this problem?

Actually, they are likely studying the destination system all the time, aren't they? Some plucky young scientist might notice a miniscule anomalous Doppler blue-shifting. Her superiors think it's too small to be anything other than a calibration error, and her hypothesis that it's due "an unexplained but nevertheless very real acceleration" is laughed off - until the ship starts drifting off-course ... then comes all the screaming and running around.

If anything, in this case it might be seen as diabolus ex machina - or Murphy's Law: If something can go wrong, it will.
But conversely, it could also be seen as the complete absence of a deus ex machina: The inherent indifference of the lifeless universe I was talking about earlier.

In fiction, a deus ex machina is not about higher powers at all. It is simply a device used by the author to fix a plot problem. The higher power is actually the author, manipulating events implausibly.

A critic or reader calling out a deus ex machina in a story can be akin to calling the author inexperienced or lazy for getting his story into a pickle that he can't plausibly get it out of.

Now, that doesn't mean deus ex machina are always bad. Many great stories revolved around wildly unlikely events. But you can't be frivolous with it need to give it the respect its due - meaning it should be central to the story, not incidental.

If thecrew of Exodus wer to make that BH an intrinsic part of their story (as opposed to just an incidental fix), then that can work well.It's too powerful a tool to use frivolously. Larry Niven has some great advice about such things:
If you have to tell a lie for your story, tell it as soon as practical.
The bigger the lie, the sooner you need to tell it.

What he means is: if your sci-fi story relies on some space fold drive, make sure your reader doesn't find out in chapter 23. If your story relies on magical telepathic piloting, make sure your reader finds out in chapter 2 at the latest.Consider, perhaps, chapter one being a flash forward to the critical moment where Exodus is about to break apart (or whatever) because they're swinging around an undetected mysterious object, then chapter two starts back at the beginning, decades before. This preps the reader for the suspension of disbelief as well as setting up some great foreshadowing.

 

[mfb]

Can we bump those 60 years up to 100 years somehow? The perspective shift makes much more of a difference if not even the current generation's children-to-be will live to see the landing on the planet.

You still need to fly back to the planet, for a total remaining travel time of 125 years or whatever you like depending on how much propellant is left.

Couldn't I have Earth's solar system at the edge of the area of effect of the gamma-ray burst? So that our solar system is still in it, but Teegarden's star is outside of it?

There is no sharp boundary.

1) I still kind of like the superflare approach. Perhaps I could "move" one of the Groombridge stars around a little, by having a rogue star pass that system, pulling it closer to the route from Earth to Teegarden?

Not if you want any sort of realism. We can predict stellar positions millions of years into the future and a few centuries are nothing. There will be no unexpected motion of anything.

[rogue black holes] might be more common than we suspect?
from https://hubblesite.org/contents/news-releases/2022/news-2022-001.html
"Astronomers estimate that there should be 100 million black holes roaming among the 100 billion stars in our galaxy."

"More common" as in "the estimated risk is now 2 in an octillion instead of 1 in an octillion".
That's not a random number, that's actually a calculation result for the ship.
Still beats the dark matter cloud I guess, but it's still far too unlikely to be a serious option.

Could a rogue black hole pull one of the Groombridge stars (1830 or 1618) close enough to the path leading from Earth to Teegarden b for a solar flar from that Groombridge star to hit the ship as it's passing by? More importantly, could this rogue black hole do so in time (between now and 2475)?

No. Stars are far too slow, and you can't accelerate them quickly. You cannot move stars within the timescale of your story.

I can imagine a rogue black hole pulling the ship off-course with its gravity - without anyone noticing the black hole, because it's completely dark (=not absorbing matter in the interstellar medium, therefore no shining disk around it).

If the ship is so close that there is a relevant impact on its course (and see above for the absurd chance of that event) then it will get ripped apart from tidal forces. You would notice that.

The earlier you do a course correction the smaller it can be - the spacecraft would probably measure its motion with meter per second precision throughout the trip. Not because that precision is needed, but because it's easy to do even with current technology. Any deviation would be picked up quickly.

------------

I played around with some numbers. Target distance is 12.5 light years.

Nominal profile: Accelerate with 0.048 m/s² for 24.9 years to reach a target speed of 12.5% the speed of light, cruise for 75 years, decelerate at 0.048 m/s² for 23.9 years. The acceleration is hardly noticeable even in the non-rotating sections of the spacecraft . ISS reboosts are somewhere in that range.

100 years into the trip, at a remaining distance of 1.56 light years, the crew reactivates its main propulsion system - for the first time in the life of most people on board. Something goes wrong and the damage exceeds the safety margins, limiting the acceleration to 0.04 m/s². It will now take 30 years to stop (relative to Teegarden) and the ship will overshoot by 0.3 light years. 18 years into that process it passes the star at ~15,000 km/s - giving a brief view of the star system that no adult in the ship will reach (at zero speed) within their lifetime. The propellant margins are only sufficient for a total velocity change of ~2000 km/s. The first 1000 km/s of that are used to accelerate the ship back towards Teegarden (0.75 years of acceleration). After 93.5 years of cruising the remaining propellant is used to decelerate for 0.75 years, reaching the star system after a total trip time of 225 years, 125 years after the accident, 107 years after the first fly-by.

The fly-by will be a major event for the crew - being so close in space and yet so far in time from their destination. It's easy to change the return time to adjust expectations. Do you want certainty that no one alive will reach it? Make the fuel reserves a bit smaller, increasing the return cruise time. Do you want some children to reach it, but no adults? Assuming life expectancy increases moderately in the future that's fitting to the numbers I used.

* The achieved acceleration is only a bit smaller than the design value, which is much more plausible than the massive reduction I needed before.
* The accident can happen just as the engines are used again, which is the most likely time anyway.
* The propellant margin looks realistic
* The limit on the acceleration can come from the structural integrity of the ship.

Even fusion will lead to exhaust velocities significantly below the cruise speed, which means the ship will be mostly propellant at the beginning of the trip. To maintain a constant acceleration it can drop empty fuel tanks and unnecessary engines over time. An exhaust velocity of 5% the speed of light leads to a total mass fraction of e⁵ = 150 without safety margins, or 167 with the safety margin I used for the mission above. For every tonne of payload the ship starts with 166 tonnes of fusion fuel. During the cruise stage that ratio is lowered to 13.7. After a nominal stop 11% of the ship's mass is backup fuel, that backup is used for the slow return to the star in the modified profile.

 

[Strato Incendus]

@mfb Thanks for taking the time to come up with this elaborate example! I will comment on it bit by bit in the following.

They would likely first notice it as a miniscule acceleration in their velocity - if they are paying attention.
If they aren't, they might notice it as a deflection in their path, but by the time that happens they are too close - and moving too fast - to do much about it.

Acceleration and deflection of the path are precisely what I imagined this to look like so far, too. However, the “too close and too fast to do anything about it once they notice” is of course a problem. Could I have them notice at some point in between?

Let’s say the black hole is about a light-week away from the ship when they discover it. Since they’re only traveling at 0.1 c, they would have ten weeks (2.5 months) to course-correct. Would that still work?

Another way they might notice at a distance far enough to do something about it is by happening to spot a Newton's Ring as it passes in front of some background star..

Is that the gravitational lens idea I thought about?

What if they don’t come into dangerous proximity of the black hole at all, but the lens effect distorted the image of Teegarden’s star enough so that they were traveling into the wrong direction this whole time (=since the beginning of the journey)? Not by a lot, of course - they would still have covered sufficient length towards the star over the course of the past 100 years (10 light-years at 0.1 c). Instead, they would have missed the star “laterally”.

Actually, they are likely studying the destination system all the time, aren't they? Some plucky young scientist might notice a miniscule anomalous Doppler blue-shifting. Her superiors think it's too small to be anything other than a calibration error, and her hypothesis that it's due "an unexplained but nevertheless very real acceleration" is laughed off - until the ship starts drifting off-course ... then comes all the screaming and running around.

That sounds like a good way to set this up in advance, without pulling it out of thin air!

The crucial question here is:
How much “reasonable panic” can I have come up as a result of the discovery, without it already being too late to do anything about the black hole?
If they notice so early that they can easily course correct in time, there is no sense of urgency, and thus no suspense.
But if they notice so late that a collision with the black hole is unavoidable, there is no point in trying anymore.

It's too powerful a tool to use frivolously. Larry Niven has some great advice about such things:
If you have to tell a lie for your story, tell it as soon as practical.
The bigger the lie, the sooner you need to tell it.

Coincidentally, I think I just read that precise quote yesterday somewhere on Quora, when I was looking up some of these things.

Perhaps it was when I was trying to find out if the black hole’s Hawking radiation could harm the people on the ship? (As far as I know: No, it couldn’t - if the ship is close enough for the Hawking radiation to become a potential threat to the crew, the black hole itself will already be pulling the ship apart).

What he means is: if your sci-fi story relies on some space fold drive, make sure your reader doesn't find out in chapter 23. If your story relies on magical telepathic piloting, make sure your reader finds out in chapter 2 at the latest.

In my case, that would mean I should already mention early in book 1 that

- There have been experiments with black-hole drives back in the Sol system, but they were canceled for some reason (could be something as simple as not enough trust in the project by investors). That way it doesn’t seem like the people on the ship have to invent something completely new when they use the black-hole drive in book 2; they just need to make something work that other people have already tried.

- The ship’s rings can dismantle, split into the subsections, and land on the surface. This is part of the ship’s construction, so it can be mentioned early on. Of course, that means the ring sections need a shape suitable for entering the atmosphere. For this purpose, they have the same coating on the outside like space shuttles have it on the bottom. Also, the rings can extend wing shapes. How do you fit two flat metal wings inside a curved ring? By having the shuttle wings be comprised of several pieces, a bit like with plate armour, so that the pieces can slide into each other and “bend” around the curvature while in the ring shape. Once the wings are extended from the ring section, they are sturdy.

- This then also explains how the rings were originally built in space: The four sections of each ring were built separately, then flew to the trunk of the ship (spherical tanks and central pipe, with ring hubs and spokes) like regular (oversized) space shuttles. There they retracted their wings and assumed their curved shape, docking onto the lift shaft extending from the central pipe. Thus, the construction of the ship simultaneously served to test whether the dismantling function actually works on all rings and their respective sections.

The problem with that is: All this information will seem redundant / out of place in the beginning of book 1. In other words: Like info-dumping. The reader won’t necessarily know what this is good for, aside from me being able to say later “see, I told you early on, I didn’t pull this out of thin air to fix a plot hole!”

For example, I just recently removed a section in chapter 3 explaining the purpose of the airlocks, as the protagonist runs past one during daily exercise. Because of course, the airlocks are useless during the travel. They were only for the boarding of the Generation-Zero crew, and to eventually leave the ship after landing.
I had them act as graveyards for a while, too - but simply dumping the people who die on the ship out into space, despite all supposed “sea-burial romanticism”, isn’t just illegal - for all I know, it would also result in the ship dragging along those dead bodies with itself.

Consider, perhaps, chapter one being a flash forward to the critical moment where Exodus is about to break apart (or whatever) because they're swinging around an undetected mysterious object, then chapter two starts back at the beginning, decades before. This preps the reader for the suspension of disbelief as well as setting up some great foreshadowing.

That would be a start in medias res. Perhaps it might work as a prologue. But those have fallen out of fashion, recently, even in fantasy, where they are much more common.

My Chapter 1 is already pretty much fixed: It’s the New Year’s celebration of 2475, which is simultaneously the 100th anniversary of the departure from Earth. The commander gives a speech to remember why they left Earth in the first place - this is where the gamma-ray burst threat from WR 104 is introduced. Also, the protagonist saves a kid from climbing over a guardrail on the public ring and falling to the deck below (save-the-cat moment).

I could of course have the commander mention other cosmic dangers alongside WR 104 in her speech - including the great number of rogue black holes roaming between the stars - to hammer home the point that the universe is indifferent to humanity, and would have wiped them out carelessly in the blink of an eye, if it weren’t for their “noble defiance”.

You still need to fly back to the planet, for a total remaining travel time of 125 years or whatever you like depending on how much propellant is left.

Ah yeah, sorry, I wasn’t thinking!

There is no sharp boundary.

I don’t think we necessarily need a sharp boundary - the danger zone could fade out somewhere between the solar system and Teegarden’s star. 12.5 light years are a quite large margin of error, aren’t they? At least for the width of a gamma ray burst. The thinner they are, the further they reach - but that only refers to their length. And that distance is already clear: The beam needs to be long enough to cover the 8,400 light-years from WR 104 to Earth.

Still beats the dark matter cloud I guess, but it's still far too unlikely to be a serious option.

That was one idea I had about the course deflection: The pilot says she keeps having to course-correct, but she can’t figure out why. And the commander might assume, if there’s no other explanation, that it’s just dark matter, and that it would therefore be immaterial to the spaceship. And then oops - it’s a black hole, after all.

The earlier you do a course correction the smaller it can be

Yes, but wouldn’t and early course-correction also lead them further off their intended route?

That was my idea of them having flown into a slightly wrong direction from the beginning of the journey, due to a gravitational lens effect of a disk-less black hole they couldn’t tell was there: At the beginning of the trip, they might just have been a few degrees off, but 100 years later, after already having covered 10 light years, they might be a few light years away from the star laterally (in addition to the 2.5 light years they still have to cover in “length” before they would reach the star anyway).

the spacecraft would probably measure its motion with meter per second precision throughout the trip. Not because that precision is needed, but because it's easy to do even with current technology. Any deviation would be picked up quickly.

Good to know that’s possible today already. But is meter-per-second precision still that great when you’re flying at 30,000 km per second (0.1 c)?

Nominal profile: Accelerate with 0.048 m/s2 for 24.9 years to reach a target speed of 12.5% the speed of light, cruise for 75 years, decelerate at 0.048 m/s2 for 23.9 years. The acceleration is hardly noticeable even in the non-rotating sections of the spacecraft . ISS reboosts are somewhere in that range.

Great reference here with the ISS reboosts! So this level of acceleration shouldn’t be a problem at all.

18 years into that process it passes the star at ~15,000 km/s - giving a brief view of the star system that no adult in the ship will reach (at zero speed) within their lifetime.

18 years into the process, the protagonist’s son (born at the end of book 1) is an adult, and that’s precisely when the second book plays. Shooting past the target star could be the inciting incident for him, inspiring him to try and get the ship back there, while the older generations have already resigned and accepted that they won’t actually land on the planet within their own lifetime.

The first 1000 km/s of that are used to accelerate the ship back towards Teegarden (0.75 years of acceleration).

So far, so good - I was indeed planning for the re-acceleration in book 2 to take about a year.

However, they’re accelerating to 0.77 c this time, using the black-hole drive, so that acceleration needs to be worth it. If the remaining distance they have to cover is so short that they’d basically have to brake again right away, that black-hole drive might seem like overkill. Unless of course they really need that black-hole drive to brake with sufficient strength, rather than to re-accelerate the ship to sufficient speeds so that the current generations will all still get to land on the planet just 2-3 years later.

After 93.5 years of cruising the remaining propellant is used to decelerate for 0.75 years, reaching the star system after a total trip time of 225 years, 125 years after the accident, 107 years after the first fly-by.

This would be the perspective / future outlook of the people in book 1, then? Because it can’t be what actually ends up happening in book 2.

The characters in book 1 will be those the reader will get primarily attached to. So the black-hole drive in book 2, shortening the trip (through a combination of more efficient acceleration, higher coasting speed, and braking), is just so that those characters the reader cares about are still the ones who actually end up landing on the planet. But in book 1, I do need that perspective shift of potentially looking at a much longer journey than expected.

 

[mfb]

No "black hole encounter" idea works if you want the books to be in any way based on reality.
* Black holes are far too rare. We are not talking about a one in a billion chance here. We are talking about something like a 1 in 10²⁷ chance. If you see someone flip a coin 90 times and it shows the heads every time and they tell you there is no trick, would you believe them? 2⁹⁰ = 1.2*10²⁷.
* Black holes are not massive enough and the ship is too fast. Let's say we pass a 10 solar mass black hole at 100,000 km, an absurdly close encounter. It will deflect the ship by 400 km/s, or ~1% of its cruise speed (with 13500 g peak acceleration and a timescale of seconds). Even at the slow acceleration I proposed in my previous post the ship can correct this in three months. Make the encounter closer and it will rip the ship apart.
* Significant gravitational lensing is limited to a tiny region around the black hole and it leads to significant time-dependent distortions of the background light. There is no way we would miss this even in the extremely unlikely chance that it happens at all.

This would be the perspective / future outlook of the people in book 1, then?

All times are given with a reference so I'm not sure what is unclear.

However, they’re accelerating to 0.77 c this time, using the black-hole drive, so that acceleration needs to be worth it.

Where do they get all the propellant from? Even if we ignore the energy source completely: Even with a 100% efficient photon rocket it would be difficult to reach this speed and slow down again. Inventing and building a 100% efficient photon rocket (or anything that comes anywhere close to it) on the fly, with a spacecraft that was optimized to save as much mass wherever possible, is absurd.Throwing things away from the spacecraft is no problem. It has been done routinely from spacecraft in Earth orbit. To be extra safe you can do it from the rotating sections, then you release everything with a significant relative velocity. But organic material might be too valuable to throw it away.

 

[Strato Incendus]

No "black hole encounter" idea works if you want the books to be in any way based on reality.

Got it. So back to the "can't brake" / overshoot solutions we go.

Let's recap:
By reducing the acceleration / braking speed, you have already solved the problem of braking happening too late in the story: Now they have to start braking 25 years before the intended arrival, which is when the plot starts. Kudos to you for that!

Your explanation also gets around the problem of the ship not being able to interact with anything around it - no collisions with dust particles which would blow up the entire ship, no gas cloud, solar flare, supernova, black hole, ice cluster etc. Internal technical failures really seem like the only way to cause the crew some trouble.

That's two major issues you have already solved!

Now there is one remaining, which is more about storytelling, or what's believable about the humans on board, rather than the tech side:

If I just tell the reader that, due to the technical failure, the ship will overshoot its target by 0.3 to 0.5 light-years, as you described - but that fly-by the destination star won't happen until 18 years from now - it's hard to believe nobody does anything about that in the meantime. Especially if they have every incentive to - everyone on board has been raised with the perspective of landing on the planet themselves, after all.

Unless... this is where my crew-decimating disaster comes in. You know, the event where a third of the men on board die in proximity to the drive and/or reactor.

Let's assume that the first response to noticing that the braking force is too weak is to actively try to fix that as quickly as possible. Meaning, every pair of hands is needed, because every day that passes with the ship going too fast will take them further away from the destination star in the future. As I said, I need to create a sense of urgency here.

Thus, this isn't just something the people on board try to do - it's something they're ordered to do, for the success of the mission as a whole.

Because the target birth rate per couple has been raised to twice the former number (4 kids instead of 2), in preparation of colonising the planet, only men are sent to the fusion drive to fix it. The reason being that radiation is more dangerous to female than male reproductive organs - we've already found that solution in another thread.

But, as established, something goes wrong while trying to fix the reactor, and a bunch of guys die.
Ideally, the same thing that causes the braking process to fail in the first place is also the thing that causes the death of these crewmen.

Now the commander would have legitimate reasons to ban any further "experiments" on the drive, so that they don't lose even more people. The current solution of braking with less force may take longer, but at least it will work eventually.

Does that make sense?
If so, all we still need now is a believable type of failure in a nuclear fusion drive that could both hinder acceleration (=braking strength) and also kill a bunch of people while trying to fix it. Meaning, it shouldn't be something that just one or two engineers would be taking care of.

Forget everything outside the ship, anything to do with the surrounding space - we're only dealing with the inner workings now.

Where do they get all the propellant from?

Rather than putting the fuel into the fusion drive directly, they use the fusion reactor to power lasers (to create a black hole out of light), placed inside one of the empty spherical sub-tanks. Those have a diameter of 344.5 metres (since there are 12 of them inside a sphere with a 1-km diameter). The sphere is then equipped with gamma-ray-reflective panels.

This of course relies on the energy put into powering the lasers being lower than the energy they're getting from the miniature black hole. But if the legwork for that has already been done on Earth, they don't have to reinvent the wheel on board the ship.

Throwing things away from the spacecraft is no problem. It has been done routinely from spacecraft in Earth orbit. To be extra safe you can do it from the rotating sections, then you release everything with a significant relative velocity.

So if somebody jumped out of an airlock on a rotating habitat ring, especially if doing so without letting the air out of the space between the two doors first, they would get flung away from the ship into the void, just like The 100 would have us believe (whenever they "floated" somebody on the Ark)? Without the ship dragging any space-frozen corpses along with itself?

But organic material might be too valuable to throw it away.

Indeed. This is why dead people in Oyebanji's "Braking Day" are "recycled" somehow.

I guess this is the time to talk about whatever ways would be used on board such a ship to "induce death", for whatever reason.

In "Braking Day", there is capital punishment - people who commit enough offenses become "Dead Weight".

Spoiler

Even though I find that concept unconvincing, since the ship is no heavier just because someone stops being useful. And the children on board aren't being useful yet at all. So I don't see how someone's weight suddenly becomes a problem due to their mere presence on the ship - especially if it is already coasting, and if the total weight of all human beings on board is negligible anyway, compared to the mass of the rings, the tanks, and all the other stuff the ship has to drag around.)

Later though, it is revealed that "everyone turns Dead Weight at age 75". And this just seems to be accepted by the people on board as the way things are. Apparently the idea that a generation ship would make it a custom to "get rid of its elderly crew members" isn't as outlandish as I thought it was, when I implemented that into my setting.

Compared to "Braking Day", my way of handling this is actually pretty lenient.

Spoiler

Yes, most people on the Exodus choose not to live any longer than the age of 65, rather than waiting until 75 - but this is voluntary (even though there is social pressure), and pretty much just a result of the fact that you can never really retire while living on a spaceship. Even if you no longer had to work - you would still have to do a whole lot of exercise every day, for example.

Also, because the crew reproduces at replacement level, you normally don't have more young people than old people. On top of that, the young people also have two children each. You can't have one third of the crew do all the work to provide for the two other thirds of the crew; you can have two thirds of the crew (parents and grandparents) work to provide for the children on the ship, until they become adults and join the workforce.

Criminals, meanwhile, are not executed - they're being kept around, if only because you might still need an extra set of genes in a pinch - but merely locked away separate from the rest of the crew, where they still have to work, much like having a job in prison.

In either case: Whether there is "voluntary induced death" or "involuntarily induced death", both beg the question about the method. Assuming they would want to use something which is both humane for the individual and "practical" for the goals of the mission.

In my setting, they use something called the exit pill, which is a two-chamber capsule, consisting of a narcotic first and a poison second. But if organic matter is so valuable, or dead people even become organ donors by default, then anything poisonous would be off the table.

Perhaps they would use one of these Sarco Pod machines from Switzerland instead?

 

[Nik_2213]

As I understand it, even in our 'Local Bubble', there's enough interstellar density for a wandering black hole to accrete enough to be noticed. That's assuming big enough to not be 'Hawking' radiation-active in its own right.

And, surely, star-ships will have telescopes.

Regarding the acceleration: didn't AC Clarke nail similar issue with Rama, via small angular offset ? Enough to be useful, yet stay behind shielding ??

Um, not relevant to a mega-ship as lack of sufficient power source, but could crew of a 'smaller' craft ride out an extended g-boost using immersion in water tank ?? Clearly, there'd be an upper limit to both acceleration and duration, but it beats living in a 'Top-Gun' G-suit.

FWIW: 'Of Filters & Fire-Walls', my take on the Fermi Paradox. See #15 in...
https://www.physicsforums.com/threads/the-short-story-thread-post-yours-here.914630/
;-)

 

[Strato Incendus]

I’m coming back to this thread after a little over a year. While we may have ruled out coming to a halt during the first book of the trilogy, a full stop will still be required — in the second book, when the ship has to deal with having overshot the target.

The second book plays right around the time the ship flies by Teegarden’s Star, due to being too fast, 18 years after the accident. Based on @mfb ’s calculation (which is the grim perspective the crew faces at the midpoint of the first book), it would take another 12 years until the ship comes to a full halt, before it can even begin to accelerate back towards Teegarden’s Star.

This is where, according to the current story plan, the black-hole drive comes into play — which is developed on board, based on previous research from Earth, and using parts that could otherwise serve as replacements for the deflector lasers, in order to build the lasers that create the black hole made of light.

Turns out, I don’t actually need to get my ship up to 0.77 c with that drive — even just getting back to the original 0.125 c would suffice. After all, the ship only overshoots the target by 0.3 light years — and the reason the journey back to Teegarden b is initially projected to take another 100 years is merely because of the limited propellant margins. Where, even though they’ve zipped past Teegarden’s Star at 15,000 km/s, once they’ve accelerated back towards the star, they’d be crawling towards it at a mere 946 km/s.

This problem of the limited propellant margins is what the crew would sidestep — by using the energy from the fusion reactor to power the lasers for the black-hole drive (from which they can then get a lot more energy), rather than directly using the hydrogen as fuel.

(I assume we were talking about ”propellant margin” in the sense of “how much momentum could the remaining fuel create back towards the star?”, not “propellant safety margins”?)
I’m using the “Relativistic Rocket” calculator from Greg’s Space Calculations here again:

First of all, getting to a full stop from the remaining 5% light speed (15,000 km/s), which would take another 12 years with the limited propellant margins, could be accomplished within just a little 108 hours by braking (=accelerating back towards Teegarden’s Star) at 4 g. If they only brake at 4 g for one third of the day (see below), that triples this duration, i.e., 324 hours (=13.5 days). (This is based on the German “Rechneronline” calculator for g acceleration again.)

If the black-hole drive can constantly accelerate at 4 g, it could get back to the original coasting speed of 0.125 c in just a little over 11 days. Of course, everyone spending 11 days lying flat on their back in the trunk of the ship isn’t feasible (even though a Montenegrin crew member jokingly suggests her ancestors might have been able to accomplish that).

Instead, they only accelerate at 4 g during the nighttime, from 22:00 to 06:00, and dial the acceleration back to the standard 0.048 m/s2 during the day. So they’re accelerating at 4 g for 8 hours per day, i.e., one third of each day. This way, they should get back to 0.125 c in about 33 days. The same duration would be required to brake at 4 g at the end of the trip.Of course, the development of the black-hole drive on board will still be a tough sell — it’s a sufficiently difficult task on its own, even if it’s only to get the ship back to its original coasting speed in a fraction of the time. But at least I no longer have to deal with the even less believable perspective of having the ship coast at a significant fraction of the speed of light (the former 0.77 c). Do these numbers make sense so far?

 

[DaveC426913]

...they’ve zipped past Teegarden’s Star at 15,000 km/s

I don't suppose they could keep their speed by sling-shotting around a conveniently positioned mass?

If there were a convenient mass along their (now over-shot) course, they might be able to skim it and come around with their course turned 180 degrees but with all their initial speed.

Consider both fuel savings and time savings. Considering the alternative is wait another 12 years, it might be something that the crew would give consideration. Again, if there were a convenient mass.

You'd have to figure out how big a mass you'd need and how close they'd have to skim it.

Of course - if this is too much of a deviation from your plans for the story - just make sure there is no such convenient mass.

But at least I no longer have to deal with the even less believable perspective of having the ship coast at a significant fraction of the speed of light (the former 0.77 c).

Sorry, remind me why coasting is implausible?

(BTW, of they were coasting, then they could happily develop the black hole drive, say, a hundred kms off their beam, without risk to the ship or crew therein).

 

[DaveC426913]

Incidentally, I just finished reading another colony ship novel and Oh my God is it bad from a technical perspective. The author neither knows nor cares about the constraints of interstellar travel; it is simply a backdrop against which the drama plays out.

The author seems to think an interstellar journey is like a passenger liner on the ocean. The whole plot is driven by the ship's course repeated over and over again in each scene, and yet they have gotten it all so, so wrong (because that's what the author needs to happen to drive the characters' interactions).

Like,

• "If it takes n fuel to get to the halfway point, then it must take n fuel to turn around and go home."

Everything that happens to the ship happens under the constraint of 'we have zero fuel to do anything but go from A to B'. They state so many times how many things they can't do because they don't have any spare fuel - just enough to stop at the destination.​

​

And yet the entire plot is kicked off by a plan to turn around and midpoint and go home. I guess everyone on board forgot that they would need twice as much fuel as they took to get to halfway (and that's assuming their fuel is massless).​

• "If you knock a ship 'off course' with an explosion, it will take off in that new direction, missing its destination."

The ship is a ring ship - it gets its gravity from rotation. Crew sometimes float through the hub. So it's coasting or they would experience "down" as rearward.​

​

An explosion that doesn't vaporize the ship is going to merely tilt the ship so it's not pointing at the destination. But if the engines aren't running then it's still going to be heading toward the destination!​

​

Such a ship takes years to get up to cruising speed. That's a lot of kinetic energy. An explosion big enough to actually redirect the ship's heading (which operates over, not years, but milliseconds) will not go into moving the ship but will simply vaporize it instead.​

• "Ships on long journeys don't bother with steering - or the fuel to steer - they just take great care on launch to aim accurately."

The primary plot point is that they can't steer back on course because​

1. their primary engines were never designed to turn, and
2. their "steering" engines (which are there for fine course correction) can't - er - steer because ... "they're too small".

• The line between "we still have time to can back back on course" and "the window has closed for us to get back on course" is razor sharp - down to the second (countdown trope).

This is on a journey that is only halfway through and still has ten years in the second leg. I guess they calculated orbital insertion trajectory (twenty years ago) down to the yard?​

So, I guess, uh, the takeaway is - keep writing! If this book can make it to publication then yours has no worries!

 

[Strato Incendus]

And yet the entire plot is kicked off by a plan to turn around and midpoint and go home.

Well, coincidentally, a similar thing is part of my backstory. My father proposed that premise to me, and I decided I didn’t want it to be part of the main plot. Instead, I shifted this idea back to the first generation born on the ship.

In my case, it’s a little more plausible, because the rebels from Generation One are trying to turn the ship around before it has reached its full coasting speed. Meaning, not even the fuel for accelerating the ship towards Teegarden has been fully used up yet: There is still remaining fuel in the tank to not only bring the ship to a halt (and get stranded between the stars as a result), but to accelerate in the opposite direction again (=back to Earth).

How long they’d actually need to get there, and whether they’d still have enough fuel to stop when reaching Earth, that is yet another question. I haven’t done the math on that yet.

In short, with our current calculation, the ship accelerates with at 0.048 m/s2 in the beginning of the journey, and does so for the first 25 years. The rebels in Generation One are in their late teens or early twenties, so they have a few years before the ship reaches full coasting speed. By the time it does, it has covered a distance of 1.56 light years, meaning it is still within the Oort Cloud.

The rebels are ultimately stopped by the commanding officers; upon realising that they won’t be able to return to Earth, the rebel leader attempts to destroy the ship by pushing it off course and into an asteroid within the Oort Cloud. He does this with the aim of sparing future generations (most notably, Generation Three) the fate of having to spend their entire lifetime in the void between Earth and Teegarden b.

Sorry, remind me why coasting is implausible?

Coasting isn’t implausible — but a speed of 0.77 c might be much less believable (because much harder to attain) than 0.125 c, won’t it? Even with a black-hole drive.

I’d also have to explain how the ship could withstand such speeds if it wasn’t initially built for them. It might be hard enough to explain how the ship can withstand accelerating at 4 g constantly, if it was designed to accelerate at merely 0.048 m/s2 over the course of 25 years.

You'd have to figure out how big a mass you'd need and how close they'd have to skim it.

Of course - if this is too much of a deviation from your plans for the story - just make sure there is no such convenient mass.

I’m addressing this question from the opposite starting point: How much mass can there be in a system surrounding an M-type red dwarf?

Initially, I postulated at least two gas giants in that system, in order to protect Teegarden b and c from frequent meteor impacts — much like Jupiter protects Earth and the other inner planets from them. It had to be two gas giants, since, for all I know, Saturn was responsible for Jupiter not wandering further into the inner solar system and kicking out all the smaller rocky planets in the process.

By now, though, I’m sceptical about whether a red-dwarf system would even have enough mass to form gas giants, in addition to the two Earth-sized rocky planets it already has. If it doesn’t, while that means there is no huge mass like Jupiter protecting the rocky planets from meteor impacts, it also means there is no huge mass that can draw asteroids from outside the system into it to begin with (aside from the star itself, of course).

But they can hardly slingshot around Teegarden’s Star to get back to Teegarden’s Star, can they? They would need some rogue brown dwarf or something outside the system, and that would feel like a real deus ex machina if that object just so happens to be conveniently placed within the path of the ship.

 

[Filip Larsen]

I don't suppose they could keep their speed by sling-shotting around a conveniently positioned mass?

Just know that the maximum deflection angle possible for a slingshot with high hyperbolic excess speed is not going to be great, not even if the slingshot mass is a very dense object to allow close passage.

 

[DaveC426913]

Just know that the maximum deflection angle possible for a slingshot with high hyperbolic excess speed is not going to be great, not even if the slingshot mass is a very dense object to allow close passage.

Did not know that, no. I mean, obvs there's a lower limit but what might it be?

See, I was thinking of about a rocky planet with no atmo. But no, even skimming nape of earth wouldn't get enough deflection would it?

 

[mfb]

At 15000 km/s you pass Earth on a timescale of 1 second, getting deflected by around 10 m/s = 0.01 km/s.

A fly-by just above the surface of the Sun happens over ~100 seconds, deflecting the spacecraft by ~25 km/s.

If your object has a radius r and surface gravity g then your typical fly-by will take of the order of r/v giving you a deflection of rg/v. The product rg can be written as $\frac{1}{2} v_e^2$ using the escape velocity of the object. We can also divide the expression by the velocity of the spacecraft again to express the deflection as an angle: $\alpha \approx \frac{v_e^2}{2v^2}$. This is ignoring prefactors, but it shows that a 180 degree turn needs an object with an escape velocity of the order of the ship velocity or more. That limits us to neutron stars and black holes. Neutron stars are going to rip apart your ship from tidal forces if you get close enough, so you need a large black hole.

 

[DaveC426913]

 

[Filip Larsen]

I mean, obvs there's a lower limit but what might it be?

Using the circular speed $v_c$ around the massive object at closest approach to normalize the hyperbolic excess speed $v_\infty$ as $v_c = \nu v_\infty$ , I find the deflection angle $\alpha$ of a hyperbolic passage to be
$$\tan\tfrac{1}{2}\alpha = \frac{\nu^2}{\sqrt{2\nu^2+1}}, $$
which gives the plot for $\alpha$ as function of $\nu$ shown below. Note that for large deflection angles we need $\nu\gg1$ which implies $v_\infty \ll v_c$.

 

[Strato Incendus]

so you need a large black hole.

Thanks for your detailed explanation!

Since we’ve already established that the ship encountering any black hole, be it as a cause for an emergency brake, or for slingshotting later, is extremely unlikely — and would therefore come off as a real deus ex machina if used in a story.

The black-hole drive, meanwhile, would rely on the creation of a much smaller, artificial black hole out of light, using several lasers and gamma-reflective panels in one of the spheres that formerly served as a fuel tank.

So, getting back from slingshotting to a black-hole / singularity drive:

First of all, getting to a full stop from the remaining 5% light speed (15,000 km/s), which would take another 12 years with the limited propellant margins, could be accomplished within just a little 108 hours by braking (=accelerating back towards Teegarden’s Star) at 4 g. If they only brake at 4 g for one third of the day (see below), that triples this duration, i.e., 324 hours (=13.5 days). (This is based on the German “Rechneronline” calculator for g acceleration again.)

If the black-hole drive can constantly accelerate at 4 g, it could get back to the original coasting speed of 0.125 c in just a little over 11 days. Of course, everyone spending 11 days lying flat on their back in the trunk of the ship isn’t feasible (even though a Montenegrin crew member jokingly suggests her ancestors might have been able to accomplish that).

Instead, they only accelerate at 4 g during the nighttime, from 22:00 to 06:00, and dial the acceleration back to the standard 0.048 m/s2 during the day. So they’re accelerating at 4 g for 8 hours per day, i.e., one third of each day. This way, they should get back to 0.125 c in about 33 days. The same duration would be required to brake at 4 g at the end of the trip.

Does this calculation / scenario make sense so far?

 

[DaveC426913]

The black-hole drive, meanwhile...

Remind me again how an artificially-generated black hole manages to produce more output energy than they input to make it?

 

[Strato Incendus]

To my understanding, this depends on the size of the artificial black hole: Smaller black holes dissipate faster and give off more Hawking radiation in the process.

I found a chart on that a while ago — various sizes of artificial black holes, their resulting lifetime, and the amount of energy they produce. Sadly, I don’t recall where I found that, but I will look for it again.

The point is: The ship would have to find a sweet spot between the black hole lasting long enough, and still producing enough energy to be worthwhile. There’s also the question whether this “on again, of again” approach (4 G acceleration at night, 0.048 m/s2 during the day) works with such a drive.

Unless of course they accomplish this by repeatedly turning the lasers on and off. Meaning, they would create black holes that only last for 8 hours each time. In that case, going by the previous propellant margins, the acceleration during the day should drop to the former 0.04 m/s2.

Alternatively, the black hole could last for the one month of accelerating back to 0.125 c; then the question is what they’d do with the excess energy during the day. With that scenario, they’d have to reactivate the lasers and create a second black hole to stop during the last month of the trip. This would add some tension to the third book, due to the potential of the second black hole failing to form, which would cause the ship to zip past the destination star yet again. (That tension could be vital, since my protagonist’s internal conflict requires some remaining uncertainty about the safe arrival, otherwise she could already start pursuing her personal goal during the trip.)

 

[DaveC426913]

I found a chart on that a while ago — various sizes of artificial black holes, their resulting lifetime, and the amount of energy they produce. Sadly, I don’t recall where I found that, but I will look for it again.

But they don't produce energy out of nothing.

An artificial BH will only emit as much energy as you put into it (in whatever form, including dropping mass into it). The only way they could produce a BH is if they have that (and a lot more) energy at-hand to drop into it.

All the rest of this:

The point is: The ship would have to find a sweet spot between the black hole lasting long enough, and still producing enough energy to be worthwhile. There’s also the question whether this “on again, of again” approach (4 G acceleration at night, 0.048 m/s2 during the day) works with such a drive.

Unless of course they accomplish this by repeatedly turning the lasers on and off. Meaning, they would create black holes that only last for 8 hours each time. In that case, going by the previous propellant margins, the acceleration during the day should drop to the former 0.04 m/s2.

Alternatively, the black hole could last for the one month of accelerating back to 0.125 c; then the question is what they’d do with the excess energy during the day. With that scenario, they’d have to reactivate the lasers and create a second black hole to stop during the last month of the trip. This would add some tension to the third book, due to the potential of the second black hole failing to form, which would cause the ship to zip past the destination star yet again. (That tension could be vital, since my protagonist’s internal conflict requires some remaining uncertainty about the safe arrival, otherwise she could already start pursuing her personal goal during the trip.)

is based on that first faulty premise.The reason one might use a black hole drive is that it could be constructed at home and "filled up" before leaving dock. Basically, it's a battery. You charge it up at your gas station before you leave home.

You can't construct a battery out in the desert and expect to be able to charge it.



(Talk about mixing metaphors...)

 

[Strato Incendus]

Mmh… so we’d have to go back to the idea of the ship picking up extra hydrogen from a gas cloud or something? Or, potentially, from the destination star itself? Both would be ways of “recharging the battery”.

In Elite: Dangerous, for example, you can hang around a star and use the scoop tool to pick up hydrogen for your drive. An interstellar colony ship traveling at between 0.1 and 0.125 c (by the time it reaches the star, since we’ve established they’re going too fast and will zip past it) won’t be able to spend a lot of time in the vicinity of the star, though. And even for the time that it does, it might still be too fast to pick up anything, even though it is indeed decelerating at this point.

Then again, this could be used to add a sense of urgency: We have a limited time to potentially pick up some new fuel for our drive, and use that to brake faster / re-accelerate back towards the star. Zipping past the star, based on the established calculation in this thread, would happen about 18 years after first discovering that the ship can’t brake fast enough. This happens to be right around the time the second volume of the story is supposed to play.

Even if this works, the question is then whether to simply put that scooped-up hydrogen into the fusion drive, or whether there would be any net gain in energy by using it to power lasers for the black-hole drive instead.