Kaylee is a mechanic on a Generational ship. The ship has several means for propulsion and has a plotted course that goes from star to star, molecular cloud to molecular cloud. It has a Bussard Ramjet and Solar Sails that get it through, most of the way. However, at this point in the story, they are in a region of space where either of these methods of propulsion aren't very useful.

They now fall back of their nuclear pulse propulsion units, ion thrusters, and other secondary propulsion units. One of these units is Kaylee's responsibility.

At this point in the story, the ship has skimmed past an asteroid clump, intending to lasso an asteroid to mine it. Unfortunately, an accident happens, and Kaylee's propulsion unit is hit by an asteroid fragment. She has to eject it, otherwise it will blow up and harm the ship.

This is bad news. It will now add a few extra generations to the ships travelling time, but the ship is not doomed.


What should this propulsion unit be, and how would the accident play out? (Actually it doesn't really have to be a propulsion unit, it just has to be anything that would have to be separated from the ship, delaying its arrival by a few generations).

  • $\begingroup$ This does not seem to fall within the idea generation category. However, the question needs more detail like what propulsion engines it has, what its fuel consumption is, how fast is it going. Once these questions are answered (in an edit or comment, preferably the former) we can create a good answer. Until then, I am voting to close as too broad. $\endgroup$
    – Jax
    May 22, 2015 at 13:29
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    $\begingroup$ Close voters and future close voters: please consider your close votes carefully. This question is neither idea-generation (since there are likely only a few correct solutions, and simple ideas do not satisfy an answer) or too broad - the scope is very similar to other popular scenario questions on this site. $\endgroup$
    – ArtOfCode
    May 22, 2015 at 13:51
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    $\begingroup$ Does the ship travel by Newtonian and/or relativistic mechanics? (No handwavium-based FTL propulsion.) Because if it does, keep in mind that absent forces acting on the ship, for all practical purposes in interstellar space, it will happily keep going in whatever orbit it was in when the propulsion unit was damaged. A collision with a rock small enough to "lasso" might realistically cause an unstable attitude (for example, initially uncontrolled rotation) depending on the impact point and vector, but not beyond the ability of a reasonably spec'd set of attitude control thrusters to handle. $\endgroup$
    – user
    May 22, 2015 at 14:08
  • $\begingroup$ @rdtsc Please don't post answers as comments. Instead, post them as answers so that community voting etc. works. (Full disclosure: I was thinking fuel tank problem too. Especially if the fuel tank involved holds some nasty stuff that you don't want leaking elsewhere.) $\endgroup$
    – user
    May 22, 2015 at 14:15
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    $\begingroup$ Now if they're in a system, and in an asteroid belt with the same motion as the asteroids, it makes sense that a small rock coult hit them at a human-scale (relative) speed. Especially if it's of their own making, as part of mining operation! Ramjet only works (1) at high speed and (2) beyond yhe heliopause, so solar sail is a perfect complement to getnaway from the star. But it's ten thousand miles across, and no way will they deploy in the asteroid belt! So they need the ion drive to get them clear first. How's that? $\endgroup$
    – JDługosz
    May 23, 2015 at 23:00

5 Answers 5


A starship engineer named Kaylee? Shiny.

You don't actually need much here. Let's say that Kaylee's thruster is an outer one. You don't even need to eject it, just have it rendered inoperable. This means that firing all other thrusters would imbalance the ship and throw it off course. So it's not just the impact of losing one - you actually lose at least two of the propulsion units. Let's say this cuts the ship's effective top acceleration by around 20%.

Now, in order for them to move close to the asteroids (in order to mine them) let's say they had to cut speed and move off course slightly. These aren't major adjustments, but they'll be significant, because losing 20% of the ship's max acceleration after that means they can't hit their next molecular cloud target directly. Instead they can just skim it, and that will cut the amount they can harvest next time by over 50%. And this will cause the delay because they're forced to change their route past that point, since they won't be able to make the next target after that. They have to pick a new cloud and kind of work their way around.

As for the accident itself, it depends on your exact propulsion methods. I would have the asteroid destroy a key, irreplaceable component. For example, if she's running a fusion drive, maybe the impact destroys a perfectly formed containment unit, something that can only be built in a specialized factory. It's well beyond the generation ship's means to replace or rebuild, and can't be jury rigged.

As to why they decided to send the ship out with components it can't replace - maybe they just didn't have a choice. Or possibly the ship did have some spares, but they were used on previous fusion units that failed, or maybe it turns out the spares were built by a low bidder and have hidden faults.

  • $\begingroup$ Just spin it to keep a useful trajectory. $\endgroup$
    – JDługosz
    May 23, 2015 at 23:01
  • $\begingroup$ @JDługosz: That would cause the ship to corkscrew through space, so you'd still lose acceleration. Also it might cause structural issues if the ship isn't engineered to spin that way. $\endgroup$ May 23, 2015 at 23:08
  • $\begingroup$ As a general rule, there can't be irreplaceable components on a generation ship. All components have a finite lifespan (sometimes hundreds of years, rarely thousands), and generation ships generally have travel times exceeding 1000 years for a short trip. You can reasonably expect that everything on the ship will wear out at least once on the way, So you'd better be able to build spares as needed. Basically, to have reasonably good odds of success, a generation ship should be regarded as a small, mobile planet, with planetary scale manufacturing abilities. $\endgroup$
    – Leliel
    Jun 16, 2016 at 23:43
  • $\begingroup$ @Leliel: Seems reasonable to me that you might take along spares of certain specific components, rather than an entire factory to rebuild them. Each new factory is just more components that need to be maintained and that could fail. $\endgroup$ Jun 17, 2016 at 14:27
  • $\begingroup$ @DanSmolinske It seems to me that we may be talking about a different scale of ship. I'm thinking of a ship with a population of at least 100K people. By the time you have the general manufacturing to support that population, plus the specialist research centers to carry on from earth based knowlege, you can do short run manufacturing of anything you really need, even if you don't have dedicated full time manufacturing.of it $\endgroup$
    – Leliel
    Jun 17, 2016 at 17:35

Since a spacecraft can generally coast once it has reached its cruise velocity, this question actually raises many other ones.

You suggest that the accident happens when the ship is trying to mine some asteroids for resources, which means the ship had to decelerate from its interstellar cruise velocity to match speed with the asteroids (which generally are not moving at speeds which can be measured as fractions of c). This implies the ship either has pretty powerful engines and lots of on board fuel so it is feasible to slow down, do some mining and then accelerate back to cruise speed. Very few systems using known physics allow you to do that, outside of pulsed nuclear systems like ORION or pulsed fusion systems like the one proposed for the Project Daedalus.

That restricts some of the disaster scenarios, but the fusion system has exposed parts like magnetic induction coils and the ignition lasers (or systems of mirrors to focus the laser beams onto the fusion targets), which would certainly leave the ship stuck in place until these devices were fixed. A working pulsed fusion engine would also need large radiators to dispose of waste heat, which is another weak spot for the ship.

The only other system I can think of which would allow for large deviations from a simple straight line trajectory would be some form of beamed power from the home system. Since you are dealing with a power beam in the terrawatt range delivered over many light years, the receiver will be large, perhaps several hundred kilometres in diameter or more. Obviously a chunk of asteroid striking the beam receiver will have some serious consequences, especially since you can't just quickly call home and have them switch off the beam. The laser sail proposed by Robert Forward is a good example of this sort of starship, although to slow down the starship actually has to eject part of the sail (which becomes a mirror to focus laser light onto the remaining sail's face).

  • $\begingroup$ I suppose it doesn't make sense for them to want to mine some astroids in a 'dead stretch' of space when they should just be going through it. Maybe I'll skip that go with some as of yet unexplained massive resistance of matter in front of the ship... $\endgroup$ May 22, 2015 at 17:33
  • $\begingroup$ A beamed power system that might work for you is a "hot beam" of highly energized neutral atoms beamed from Earth to the ship's receiver. Since the receiver would be a large magsail like device, it might be affected by interactions with other magnetic fields in space (perhaps your ship is passing near a magstar; a neutron star with a powerful rotating magnetic field. $\endgroup$
    – Thucydides
    May 23, 2015 at 3:31

The answer depends on the flavor of generation ship involved. Some generation ships might be built with X "managerial reserves" which can be drawn upon if it slows down. Once you consume those reserves, you're dead in the water. A different answer involves constructing the hardware to be more resilient to failures, like organic bodies do. In that case, as you conserve the reserves, the generation ship shifts to a more conservative form, getting less and less effective at achieving its goal, but never stopping.

You mention a region of space where those methods of propulsion do not help. The funny thing is, the most effective method of moving through a region of space for a generation ship is inertia. If it came across one of these regions, it would simply calmly keep going until it got out of it. This tells me there is something in the way (like a massive amount of resistance from matter in front of the ship), or we're modeling something strange enough that the answer is probably dependent on what plot tool's you're looking to use here.

I would personally expect to see more continuous sources of thrust like ion jets and fewer discrete sources like nuclear pulses. I come to this answer from a logistics perspective and a psychological perspective. The ship's crew has the insane job of trying to pilot a ship for thousands of years straight. Continuous sources often deplete well understood fuel sources (which are likely mined on the way). Things like nuclear pulses are going to involve using up more rare materials which the generation ship could not rely upon finding decade after decade. As well, can you imagine the mental strain of saying "we're going to push this generation ship forward by repeatedly blowing up explosives near the ship?" It's the anathema of any captain, pilot, or engineer who has to work on a ship whose millennia long survival matters. Consider, if a period of using the engines for a solid month causes just 0.00001% damage to the ship, it actually starts looking like a mistake to use an engine. The preference would be for engines with fewer moving parts, and fewer surprises.

And so, just because of your character choice, I am now seeing yellow wisps of ion drive leaking out from behind the ship before it takes off to new frontiers.



No offense, but I see some possible problems with the ideas you've presented.

First - It is very unlikely that a Bussard Ramjet Generation ship would have more than a single backup engine type. There would most definitely be large Hydrogen (or Deuterium/Tritium) fuel tanks used by the main engine when speeds fell below the minimum necessary speed required to facilitate fusion. Your ramscoop is going to be huge, somewhere in the neighborhood of 10km or more in radius. It's unlikely you're going to furl and unfurl something that large to use a solar sail - that would be just too much mass to keep messing with. It's more likely you're going to move in towards the sun itself or into a gas giant and use the increased particle density to do speed adjustments.

You can find a lot of the equations for Bussard ramjets here: http://www.dangermouse.net/gurps/science/ramjet.html

The reason I say you're not likely to have more than 1 type of engine is simply the thrust to weight ratio (link is here: http://en.wikipedia.org/wiki/Thrust-to-weight_ratio ). That's to say - you already have a limited amount of weight you can push with your RamJet. Adding more engines means even more weight dedicated to a system that's normally not used. You'd be better off with spare parts for your Bussard Ramjet than implementing (and then having to carry with you) an entirely new engine type. I mean - you're going to have people on this thing, right? You need life support. Hydroponics. Air scrubbers. Big screen TV's. Water. Food. More water. Computers. Twinkies. Comic-books. The list is endless. You don't want to waste that on more drives - especially since you (hopefully) won't be using them during the flight.

Second - the distance between stars is.... vast. It's called a Generation ship for a reason; depending on your efficiency, it's going to take hundreds of years to get to the next star. There's no 'in this region of space.' There's only 'Vastly huge freaking distance between star A and star B.' What's between those two stars? Nothing. Nada. It's only when you hit the Oort clouds of those stars at about 50,000 AU where you start seeing rocks... and those are really far apart as well. (This is assuming you're moving between G class stars, other stellar sequence stars will have different radii). What's most likely to happen is the huge mother ship comes into a star system and then spits out dozens of factory / extractor ships which scour the area for more raw materials to move to the next system (assuming this one didn't have an Earth-like planet). These factory / scouts would then re-attach and you'd drop towards the sun to start the next trip towards the next closest star.

Now - that being said - all of this goes out the window if we find someway to use mass-less engines like the EMDrive (link here: http://en.wikipedia.org/wiki/EmDrive )


Fragment hit a fuel tank. Tank was "dead" or "going critical" so had to be ejected. Extra time taken in sourcing more fuel. (Xenon, Uranium-238, etc.) Thanks Michael.


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