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

[Strato Incendus]

At any point during deceleration really

That’s what I meant: Deceleration only starts during the last year of the planned journey. And that isn’t even so much “slamming on the brakes”; if a ship can decelerate from 1 c within a year at 1 g, then it can decelerate from 0.1 c over one year at 0.1 g.

The plot however starts 100 years into the journey, i.e. 25 years before the planned arrival. So that the children Generation Five are about to have will be young grownups once the ship arrives.

 

[parallax48]

Ships in the expanse are designed so the acceleration of the engine causes a force on the ship's crew towards the floor. I.e they are built like a high-rise with the engineering decks near the bottom, the engines below the building and the front of the ship is the top floor.

Since their engines run on "efficiency" they can thrust at 1G for the entire duration of their trip, with a short moment where they turn the ship end-on-end in the middle of their journey. During this moment or whenever they shut down their engines they crack out their mag-boots for some easy-to-film-on-earth goodness.

If your ship was designed this way, the crew would need to lie on the floor for 9 days, while experiencing 4G. Most likely they would schedule 30 minute breaks where the forces were reduced to 1.5G or similar during that 9 days.

Some writing prompts:

As soon as you "see" (or expect) a dust cloud, the goal is to reduce velocity relative to that dust cloud. Once you have two objects in space it makes a lot of space to discuss relative velocities.

Potentially the creators of the ship were discussing what-if scenarios and decided that while the ship shouldn't have periods of high acceleration, it should be possible for emergencies like this. But since the journey began, generations have jury-rigged various things to improve the ship which can't handle this thrust. Rooms get messed up, extra hardware outside ripped off. Perhaps people are hearing the impact of the dust and the damage done early in the deceleration and this frightens them.

Also note that most ships would be decellerating by turning their engines towards the oncoming dust and essentially blasting the dust with engine exhaust at high velocity. There is a good chance that this exhaust would collide with oncoming dust and push it away from the ship - creating a safe area in the engine plume. Perhaps even running the engines at 1G is enough to "hide" the rest of the ship from dust in the plume, only leaving the engines very vulnerable to large rocks.

Is the ship designed to operate in this way? Are there sensors that can "see past" the engine plume as it is decellerating? They must have been expecting to decellerate for years at the end of the journey as they approached their destination.

If the engine was disabled after some velocity was scrubbed, now the ship is traveling at high velocity with no engine. Further dust damage is occurring. The crew has to rush to fix it before they sustain further damage or even pass their destination. Can they Jury-rig something which uses the dust to help? A sail of some sort?

 

[Strato Incendus]

@parallax48 : Thanks for your ideas; I think they're mainly a response to the opening post, though? ;) We have already reduced the braking force considerably in the meantime (and are now questioning whether braking is even possible at this point in the story to begin with).

Regarding the ship design: This is the problem with discussing separate issues surrounding the same ship "in isolation". I tried to discuss it all in the same thread originally, one by one, but then the discussion just loses focus, since it's about 10 different things at once. Plus, human working memory can only contain 7 +- 2 items anyway, so people won't be able to keep all of this information in mind in parallel, even if it's all in the same thread.

Here is the thread containing the revised designs of the ship: https://www.physicsforums.com/threads/generation-ship-sfv-exodus-revised-designs.1013958/

In short: It uses rings for artificial gravity, and as has been mentioned in the present thread, the drive is turned off after the coasting speed of 0.1 c is reached. Thus, even if it accelerated at 1 g, rather than 0.1 g, that type of artificial gravity would immediately be lost once the drive is turned off and the ship transitions to coasting.

Since their engines run on "efficiency" they can thrust at 1G for the entire duration of their trip, with a short moment where they turn the ship end-on-end in the middle of their journey.

This high acceleration works for The Expanse, since the story mainly requires short bursts of acceleration within the solar system, to get somewhere quickly. But if you maintained 1 G for a year, you would get pretty close to the speed of light, and would then need 1 year to slow down from that again at the end. If the ship can already maintain constant acceleration for that long, however, so that it would end up getting close to the speed of light within a year, the generation-ship premise falls apart - you would get to Teegarden's star in about 12.5 years.

The other limitation I have in my setting is the response time of the deflector lasers: Even if you could get a ship close to the speed of light (the fastest it ends up going in the second book is 0.77 c), the lasers also need to be fast enough to vaporise any dust particles in the way, or you're just accelerating into an obstacle that blows the ship apart.

Of course, in principle you can maintain the generation-ship premise even on a ship that travels at close to light speed - by simply picking a destination star further away. However, based on our current knowledge, there is no need to: Teegarden b still has the highest Earth Similarity Index - except for one other candidate: KOI-4878.01. But that one is a) unconfirmed and b) 1075 light-years away. So even at light speed, a trip to that planet would not just maintain the generation-ship premise, but overshoot the requirements for the premise by a long margin.

Unless you want to tell a story about an onboard society that is barely recognisable, compared to that on Earth. But my ship's society is the opposite: They still try to maintain connections to Earth as long as possible. After all, the eventual purpose is to have other ships from Earth follow them, to evacuate the solar system, once the first settlers have established the colony on Teegarden b.

As soon as you "see" (or expect) a dust cloud, the goal is to reduce velocity relative to that dust cloud.

That's what I thought / was hoping for initially, too. But as far as I've understood people here in this thread so far, if you see it early enough, you could simply go around the dust cloud, rather than brake in front of it? And if it's so big that you can't go around it without taking a detour of several light years, then the cloud should have been visible already when the ship launched, i.e., known long in advance.Maybe let's get back to that superflare idea first: As far as I know, both Groombridge stars in relative proximity to Teegarden's star are flare stars: Groombridge 1830 and Groombridge 1618. Would it be reasonable for a superflare from either of those stars to get into the ship's path if it's on a straight line from Earth to Teegarden's star?

Viewed from above (as on the map I linked to), Groombridge 1830 seems to be closer to where the ship would reasonably be after having covered 10 light years - the problem is: It's further "above" the plane. Groombridge 1618 in turn would be "lower" above the plane, but would probably require the ship to be closer to Teegarden's star already.

Just barely escaping a superflare 500,000 km in size with a 3-km-long ship, losing the crew working at the rear end of the ship as casualties, with possible damage to the drive and/or aft deflector lasers included, at least sounds like a fairly spectacular event.

The aft deflector lasers are another crucial point I've thought of: The ship needs to have lasers on both ends, because when it turns around to brake, it still needs lasers to vaporise debris and dust in front of it. Those lasers at the rear end of the ship could obviously be damaged by the superflare, too. Perhaps I can twist that into a justification for having to brake early - but I don't know how yet.

 

[Filip Larsen]

Deceleration only starts during the last year of the planned journey.

So if 1 year of nominal deceleration have to become a 100 year of extra time the acceleration reduction have to be around 97,6%. Meaning, if the ship have 1000 thrusters then somehow 976 of them need to fail irreparable. Or all of them fail, but only 24 can be repaired. Or, say, 50 or more can be repaired but over so long time it ends up having same impact.

I guess it is not a completely unrealistic plot device to have in the context of generation ship stories in general, provided one can come up with a good root cause for the failure that nobody saw coming, but maybe its a bad fit for your story.

 

[Algr]

plant life has already been detected on Teegarden b

Finding life (or even free O₂) on the destination planet would be a show-stopper. Consider what happened to the native Americans when the europeans showed up. There is no way to predict what effect the two kinds of life would have on each other - and utter extinction of one or the other is a likely outcome. You might discover that arsenic is a critical part of Teegarden DNA and the life forms have inserted it into every ounce of soil on the planet. Or it might be that the two forms of DNA like each other and turn every life form into chaos goop.

Maybe the reason for the delay is that they reached the planet, but are sending life samples down to see what happens? Or they saw what happens, and it is bad.

On the other hand, if you are willing to settle for a planet with no breathable atmosphere, why leave the solar system? You'd need to explain why they aren't settling on Mars, Luna, or one of the outer moons.

 

[Strato Incendus]

So if 1 year of nominal deceleration have to become a 100 year of extra time the acceleration reduction have to be around 97,6%. Meaning, if the ship have 1000 thrusters then somehow 976 of them need to fail irreparable. Or all of them fail, but only 24 can be repaired. Or, say, 50 or more can be repaired but over so long time it ends up having same impact.

By that I mainly meant that the plot would have to start 1 year before the planned arrival - not 25 years prior. This would make it a story for Generation Six, not Five.

Unless of course the damage happens while Generation Five is young, and it simply becomes foreseeable 25 years before the “braking year” that the ship will zip past the destination star, simply because they won’t be able to repair enough thrusters in the next 50 years (especially if they lose a bunch of technicians in the event that causes the damage to the thrusters). Given that, with the current backstory, the whole ship took about 30 years to build, that might sound somewhat unbelievable. But it’s one thing to repair thrusters when you have an abundance of resources and workers back in the solar system vs. repairing thrusters with only onboard staff and materials.

In principle, though, it sounds like the general idea could be integrated easily with the aforementioned superflare explanation: If the ship barely escapes a superflare, so that the rear end of the ship (where the thrusters are) is still damaged by the flare (including the death of a bunch of people working there).

The problem is: With any kind of “overshoot” scenario, rather than a braking-too-early scenario, my solution in book two (the on-board development of a black-hole drive) no longer fits the problem. Unless that black-hole drive is then used to brake, much earlier than the crew would have been able to if they had to fix all the individual thrusters.

And then, the black-hole drive would simply continue accelerating in the opposite direction, after braking has already been completed, to take the ship back to Teegarden’s star - now at a much faster speed than the original 0.1 c. (As I’ve mentioned in a previous post, the top speed the ship is estimated to reach with the black-hole drive is 0.77 c. Naturally, this requires a hefty upgrade to the ship’s deflector systems, too.)

Finding life (or even free O2) on the destination planet would be a show-stopper. Consider what happened to the native Americans when the europeans showed up.

Okay, but… wouldn’t that logic discredit any type of search for a “second Earth”? Both fictional and real? By what definition could we even consider any planet habitable without sufficient oxygen in the atmosphere? And without photosynthesis, how long would that oxygen remain in the atmosphere?

The protagonist‘s partner works in the on-board biolab, where they regularly create artificial viruses to keep the immune systems of all crew members sharp. In the opening scene, he explicitly references War of the Worlds, and how they wouldn’t want to end up the same way those fictional Martians did when they finally do land on Teegarden b.

Maybe the reason for the delay is that they reached the planet, but are sending life samples down to see what happens? Or they saw what happens, and it is bad.

This would be another story for Generation Six, not Five. In fact, here the story would have to start once the braking is already over, with the ship’s arrival in the target system. At that point, the main story of the ship’s 125-year-long journey would consequently also be over already.

What do you mean by ”life samples”? Samples of human tissue etc., to see how it responds to the atmosphere and temperature on the planet? Yes, that is indeed something I have my android do when he is the first “person” to land on the surface of Teegarden b in book 3.

On the other hand, if you are willing to settle for a planet with no breathable atmosphere, why leave the solar system? You'd need to explain why they aren't settling on Mars, Luna, or one of the outer moons.

By “planet with no breathable atmosphere”, you mean Teegarden b? Has this been confirmed yet?

I thought we were just about to do measurements of atmospheric composition for Proxima Centauri b in the near future: One camera shot with the planet in front of the star, one without it, to then see how the frequency spectrum changes when the light passes through the planet’s atmosphere. (Then again, given how often it must be hit by solar flares from Proxima Centauri, I‘m sceptical whether Proxima b even still has an atmosphere to begin with.)

When you visit Teegarden b in Space Engine, it gives some details about the atmospheric composition - but I have no idea what those would be based on? The planet has only been discovered in 2019. Are there any exoplanets for which we already know the atmospheric composition? If so, is this factored into the Earth-Similarity Index? Because Teegarden b is still in second place, and if it were confirmed that it doesn’t have a breathable atmosphere, I think it should plummet in the ESI ranking pretty quickly.

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. And if it does hit, it would affect the entire solar system. Sure, you can hide underground - which is what most people have to do anyway, since the generation ship can only export a small number of people. But if you go underground, you’re probably going to be safer on Earth than, say, on Mars or the moon. Because at least Earth has a magnetosphere.

 

[Algr]

By “planet with no breathable atmosphere”, you mean Teegarden b? Has this been confirmed yet?

I thought you said that in your universe it had been. In real life, it is as you say. "Life samples" would mean live mice, small plants, mosses, fungi, insects, and such. For the overwhelming history of Earth, the only life forms were simple single celled organisms. So it is likely that most other planets with life would just be that. Finding actual animals with internal organs and such would be rare and valuable enough that explorers would think twice about living there, given the likely outcomes.

Yes this is very different then space opera and most sci-fi. If you have to to deviate from the most probable accepted science, it is best to establish your universe's rules in the beginning, so that you can deal with the consequences constantly. If the previews show that there is a time machine made out of a Delorean, that is fine. The same device as a third act surprise solution would not work.

 

[Office_Shredder]

Sorry if I missed this something like this earlier in the thread but here's an off the wall idea. What if the problem isn't that they have to brake, but that they can't brake? They fire up the eighteen reaction mass engines and only five turn on. What was supposed to be a one year decelerating turns into five years, and they overshoot the target by 0.1 light years. The bigger problem is the ship was not designed to go in reverse, so if they keep firing the brakes to go backwards, they have no way of stopping again. Engineering options include figuring out how to turn the ship around mid flight, or coasting slow enough they are inside escape velocity when they reach the planet again. Maybe some slingshotting can help with the maneuvering? All of this assumes they figure out how to move the reaction mass in the thirteen broken engines to the five working ones, or else they don't have enough gas to do anything but die on the ship.

 

[DaveC426913]

The bigger problem is the ship was not designed to go in reverse, so if they keep firing the brakes to go backwards, they have no way of stopping again.

I don't follow. The very premise of such a ship is that it rotates 180 degrees and turns on its engines and engages in "negative accel" i.e. decel. How could it be "not designed" to do so?

Or do you mean they run out of fuel or somesuch?

(One of the things that must be factored in with any solution that results in running out of fuel is that the new "singularity" still needs sufficient reaction mass to work on.)

 

[Filip Larsen]

I don't follow. The very premise of such a ship is that it rotates 180 degrees and turns on its engines and engagers in "negative accel" i.e. decel. How could it be "not designed" to do so?

I lost track of how this particular ship is supposed to rotate, but if it is rotating with a large total angular momentum then the rotating parts cannot be flipped easily without expending mass. Think this has been touched on earlier in this or one of the "associated" threads.

 

[Algr]

They fire up the eighteen reaction mass engines and only five turn on. What was supposed to be a one year decelerating turns into five years, and they overshoot the target by 0.1 light years.

This part above works, but "Not designed to go in reverse" doesn't really mean anything in space. All motion is relative, so from the POV of the ship in mid flight, the planet is approaching you and you have to accelerate to keep it from shooting past you.

If the engines are weaker than expected, you could compensate by firing them longer. The ship would pass the planet, but eventually return to it, thus providing the delay that the author wanted. (_{Playing gravity assist pinball with some of the other planets in the system might help.}) Seeing their planned destination pass by and disappear behind them might be quite a blow to that generation of spacefarers.

 

[Strato Incendus]

I thought you said that in your universe it had been. In real life, it is as you say.

No, as long as we don’t know anything about the atmospheric composition of Teegarden b yet, I took the liberty to “postulate” the remaining things we need for habitability. Mainly, a breathable atmosphere and a magnetosphere. It seems to be uncommon for tidally-locked planets to have a magnetosphere, but it could still have one through tidal heating.

For the overwhelming history of Earth, the only life forms were simple single celled organisms.

Yes, I am aware; that still leaves the question open of how to maintain the oxygen supply for the settlers on the planet, since we still need photosynthesis for that, don’t we?

Would they have to look for a planet that has oxygen right now, but would lose it without plant life emerging in time, and then they bring all the plants themselves? ^^ Flying around the planet’s fertile equator and dropping seeds from the atmosphere all over the planet?

Even if there only are single-cell organisms, obviously the settlers would still be interfering with the “natural” development of the planet, since their arrival might prevent higher life forms from ever developing on that planet. Others on board say that’s precisely the point: Exporting civilisation somewhere and having that other world skip the brutal prehistoric times - without harming any humanoid beings in the process.

Yes this is very different then space opera and most sci-fi. If you have to to deviate from the most probable accepted science, it is best to establish your universe's rules in the beginning, so that you can deal with the consequences constantly.

I fully agree! This is why I’ve established Breakthrough Starshot 3 (unmanned probes that drop landing rovers) in the first chapter of the book, so that the crew has the best possible evidence that the planet is habitable, prior to going there themselves. As I’ve said, some other stories go with the approach of just sending people somewhere and having them test the planet’s habitability in the first place. But I would consider that both ethically and financially irresponsible, and therefore also unrealistic: It would be hard to find investors and/or willing Gen-Zero crew members for such a mission if not even the minimum necessary criteria for habitability have been confirmed to be fulfilled yet.

Sorry if I missed this something like this earlier in the thread but here's an off the wall idea. What if the problem isn't that they have to brake, but that they can't brake?

Yes, this is one of the ideas we’re currently contemplating. The problem in a nutshell is that braking would only occur during the last year of the 125-year-long journey. So almost the entire mission would be over - and therefore, almost the entire story one could tell about it - before the crew would even discover that they can’t brake.

Maybe some slingshotting can help with the maneuvering?

(Playing gravity assist pinball with some of the other planets in the system might help.)

Indeed, I was pondering whether we could use that to stop or at least turn the ship around, even in the absence of a high-enough number of working engines? That would also give the ship a good “excuse” to enter into a different random star system (could be another red dwarf that shines too dimly for us to see it from Earth, so I could simply make one up). Just to break up the monotony of the ship constantly being in interstellar space, without the ability to interact with anything. And then they could do a U-turn around a gas giant or so.

I lost track of how this particular ship is supposed to rotate,

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.

Seeing their planned destination pass by and disappear behind them might be quite a blow to that generation of spacefarers.

This would fit metaphorically in some sense: Before that, the ship (and the society on board) was moving “forward”, towards the goal. Once they’re past it, the ship has to move backwards - and so does the society on board. TL;DR: The main problem with all the “can’t brake” approaches is the timing of the discovery of this problem. As I’ve said, the reason I need this twist is to change the perspective of the crew. But the perspective at the start of book 1 is not “we’ll get to land on the planet in one year” (that’s the start of book 3); it’s “we’ll get to land on the planet in 25 years; so we should have more children now, to have a larger grownup population by the time we land, so that we have many hands that can help setting up the colony”. Before that, everyone was only having children at replacement level, to maintain a stable crew size.

 

[Office_Shredder]

This part above works, but "Not designed to go in reverse" doesn't really mean anything in space. All motion is relative, so from the POV of the ship in mid flight, the planet is approaching you and you have to accelerate to keep it from shooting past you.

The ship has an orientation where the engines are in the back and thrust it forward (i guess technically they don't have to be in the back). Not designed to go in reverse just means there is no ability to apply thrust in the opposite direction.

 

[Melbourne Guy]

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.

Could you use gyroscopes? Same concept, but easier to control. But what about the sleet of radiation when you turn around (and as you turn around)? The engines won't be protected.

 

[Strato Incendus]

But what about the sleet of radiation when you turn around (and as you turn around)? The engines won't be protected.

I guess the crucial question here is how long the turning-around process takes? And how much damage radiation could do to crew and equipment in the meantime?

This could actually come in handy for the story: It would be unreasonable to assume that the ship left Earth without a sperm bank, as a backup plan for crewmen dying. But I need to get rid of that thing for plot reasons. Hence, what if it’s stored on one of the rings (the lab ring) and gets damaged by radiation while the ship turns around to brake? Especially when it’s an unplanned braking; at the end of the journey, perhaps it wouldn’t matter that much anymore if the bank gets damaged, since it has fulfilled its purpose by then. But if the journey is unexpectedly extended, they would still need that bank.

@Office_Shredder : As the others have explained, that explanation doesn’t work: The ship already has its thrusters at the rear end only. But as long as it can turn around, it can accelerate in the opposite direction, and that’s all it needs to do to brake.

 

[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."