Generation Ships

[GreylockDS2]

I am currently working on a Cg model of a Generation ship, and I am wondering what the key features would be , at least if I am overlooking anything that may be necessary. Now the story of the vessel is pretty much the basic flee form the destruction of our solar system. The setting is roughly 100 year into the future form now , and the goal is to use fairly realistic technology. And yes its for a table top rpg system I know this may seem more detailed then needed for a drawing.

So far for the drawing in my sketches the design motto would be redundancy's. the key visible features thus far are four habitation modules attached to a ring which rotates around the center axle of the ship to supply artificial gravity. These modules have one central “gardens” within them (two simulating temperate temperature and humanity and the other two simulating tropical temperature and humidity).

Following this on the design would be a mining / manufacturing section on the ship. This is where raw materials found in various solar systems could be brought on-board , refined and then converted into workable materials for repairs, and manufacturing spare parts. (the occupants are traveling though at STL speeds form one system to another , not really knowing where a earth like planet is, so they are going form one system to another and when there extracting as much as they can before departing to a new solar system)

The next section would be power generation / fuel storage. So far the concept for this has been nuclear fusion power by helium-3. Form what I understand its not the most rare of elements which can be found on both our moon and on Uranus. So hopefully they will be able to find enough in each of the solar systems they visit to continue running.

And last is the propulsion section , the idea was going to be solar sails , but as a previous thread of mine showed me that it may not be viable between systems as the lasers that may have been on earth shining on the vessel would have gone dead. This is one of the big question marks so far ..

As for food it is farmed in the four garden modules, water is stored , recycled and mined when possible. The other thing we thought of was radiation and debris in space. Radiation so far is being countered by the hull composition and a EM field surrounding the habitation areas of the ship..

As for possible debris I was thinking a point defense system. Now going through the pros and cons of Dew vs Rail or coil guns for that , or do you think it would be even necessary to put weapons like that on a generation ship not expecting external threats (at least at first)

[Lord of the Abyss]

As for possible debris I was thinking a point defense system. Now going through the pros and cons of Dew vs Rail or coil guns for that , or do you think it would be even necessary to put weapons like that on a generation ship not expecting external threats (at least at first)

Under the circumstances the only reason for such a system would be for anti-debris/meteor defense, and I'd think lasers would be the best choice. A rail/coilgun projectile is much slower than light and it seems to me would be prone to breaking the target into shrapnel which is not really what you want; you're trying to deflect the target, not break it up. A laser would boil away the surface of the object, and the resulting out-gassing will work to shove the object off its present course.

[Batman]

No reason the projectile needs to be a kinetic impactor. You can make it a high/nuclear explosive which will vapourize/shatter the target into fragments too small to be a problem. Plus I wouldn't be surprised a laser powerful enough to change the target's course in any meaningful way (ant least on short notice) would wind up shattering the target anyway.

[Purple]

Random thought. If you have nuclear point defenses why not make them your engine as well? As in an orion drive.

[Imperial528]

Well, if the PD is missile based you'd be wasting missiles that could be used to deflect asteroids. And depending on how high the yield is, you'll either get not as much thrust as from a dedicated Orion system, or you may have to send the missile out further from the plate so it doesn't fry you, which is just wasting a bomb.

A turret based nuclear PD will not be able to do that, not without a significant mass increase that serves little purpose.

It's also very possible that the sort of bombs ideal for nuclear PD would be a very bad choice for an Orion drive.

[Purple]

While all that might be true. I was more referring to the fact that he could tie the two in production vise. Since he already wants to do mining an manufacturing with this thing there is no reason why he would not also have an enrichment reactor to make nukes. And the fact that he can make both point defenses and fuel on the same production line might come in handy.

[Eternal_Freedom]

The "bombs" used for an Orion drive are very different to normal bombs. For one, they're mostly inert Tungsten, and are designed to shape the blast into a long ellipsoid to throw as much energy at the pusher plate as possible. The bombs you'd want for PD work would be usual, spherical detonations. Wrong bombs, simple as.

[Simon_Jester]

Plus, for building a starship, the absolute top priority is high drive performance. You build your ship to use whatever drive gets you there the fastest and most reliably, because the longer the ship spends in transit, the greater the risk of breakdowns- no human construct has ever been built to perform a complicated, difficult task for hundreds or thousands of years without breakdowns, and yet a generation ship needs that capability very much.

Orion drives are questionable for this purpose- they could work with an advanced enough bomb design, which is more than can be said for chemical rockets. But they're certainly not the best that can be imagined; the specific impulse of an Orion-drive ship is actually not that impressive compared to, say, fusion. I'm not sure how it compares to more conventional nuclear rockets either.

So it doesn't make a lot of sense to (potentially) shoot yourself in the foot by settling for an inferior drive technology just so that you can somehow achieve parts commonality between that and your defensive armament. Especially since the only thing you use for both is fissile materials, and anyone who can't get their hands on weapons-grade plutonium probably won't be able to build a starship at all.

And EF? It depends. A shaped nuclear charge such as might be used for Orion might actually be better for defensive applications, because it helps get around the biggest problem with nukes in space: the inefficiency of having a spherical blast. Shaping the charge means that more of the device's energy is radiated toward the target, assuming you can aim it properly. And if you can't aim a nuclear shaped charge at this target properly, then you probably wouldn't be able to score a useful hit on it with any other system either.

The idea of using a design similar to those for the Orion propulsion bombs as a weapon did come up during the design studies- "Casaba Howitzer" was the codename, and most of the research is still classified.

[Eternal_Freedom]

That's true, it adds another layer of complexity to the targeting solution though. Making sure the warhead is at the correct angle at the correct place. Not a problem for computers though.

However, the points about drive ineffeciency stand. In my (limited) experience on the subject, Orion drives are better for short-duration missions that require high performance over endurance. They can give you a nice big push but aren't so good for sustained thrust.

Now I have a picture of generation ships using giant, expendable Orion powered booster units to get them started, and a fusion drive to keep them going. And as long as you were careful, you could re-use the boosters for additional ships...hmm, I may have to think some more on that front.

[sirocco]

Following this on the design would be a mining / manufacturing section on the ship. This is where raw materials found in various solar systems could be brought on-board , refined and then converted into workable materials for repairs, and manufacturing spare parts. (the occupants are traveling though at STL speeds form one system to another , not really knowing where a earth like planet is, so they are going form one system to another and when there extracting as much as they can before departing to a new solar system)

I'd add semi-autonomous transporter. Just to avoid that people travelling back and forth between the ship and the planets bring back something that would infect/harm the whole colony. Moreover, it should enable them to explore without needing multiple refuel trips.

[Number Theoretic]

If the ship is big and structually stable enough, you could use Orion drives for steering. That way you could exploit a dual-use uranium enrichment facility and still choose another drive technology for propulsion. Somewhere i picked up that due to it's theoretical maximum specific impulse, a fusion rocket can only accelerate up to 20% of the speed of light. And if not by that, the ship's mass ratio is your enemy which is why i'd suggest a interstellar ramscoop or a RAIR as main propulsion. It circumvents mass ratio problems and can, in principle, get you up to a significant fraction of the speed of light.

[Knife]

A magnetic sail, ion engines, and some sort of rocket system would be good for you. Lets face it, you're going to need multiple systems. A rocket, fusion, anti matter, plasma, whatever, to initial acceleration and manuvering, then ion engines to continue the accleration (if slowly over the years) and a mag sail to keep it going. IIRC, there was a m2p2 system a couple years ago, where a plasma engine and mag coils were used to make a magnetic sail. Kind of a cool concept, not sure if they kept going with the concept or not...



http://en.wikipedia.org/wiki/Magnetic_sail

Mini-magnetospheric plasma propulsion

In order to reduce the size and weight of the magnet of the magnetic sail, it may be possible to inflate the magnetic field using a plasma in the same way that the plasma around the Earth stretches out the Earth's magnetic field in the magnetosphere. In this approach, called mini-magnetospheric plasma propulsion (M2P2), currents running through the plasma augment and partially replace the currents in the coil. This is expected to be especially useful far from the Sun, where the increased effective size of a M2P2 sail compensates for the reduced dynamic pressure of the solar wind. The original NASA design[4] proposes a spacecraft containing a can-shaped electromagnet into which a plasma is injected. The plasma pressure stretches the magnetic field and inflates a bubble of plasma around the spacecraft. The current in the plasma in this case augments and partially replaces current in the coils. The plasma then generates a kind of miniaturized magnetosphere around the spacecraft, analogous to the magnetosphere that surrounds the Earth. The protons and electrons which make up the solar wind are deflected by this magnetosphere and the reaction accelerates the spacecraft. The thrust of the M2P2 device would be steerable to some extent, potentially allowing the spacecraft to 'tack' into the solar wind and allowing efficient changes of orbit.

In the case of the (M2P2) system the spacecraft releases gas to create the plasma needed to maintain the somewhat leaky plasma bubble. The M2P2 system therefore has an effective specific impulse which is the amount of gas consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. Robert Winglee, who originally proposed the M2P2 technique, calculates a specific impulse of 200 kN·s/kg (roughly 50 times better than the space shuttle main engine). These calculations suggest that the system requires on the order of a kilowatt of power per newton of thrust, considerably lower than electric thrusters, and that the system generates the same thrust anywhere within the heliopause because the sail spreads automatically as the solar wind becomes less dense. However, this technique is less well understood than the simpler magnetic sail and issues of how large and heavy the magnetic coil would have to be[5][6] or whether the momentum from the solar wind can be efficiently transferred to the spacecraft[7] are under dispute.

The expansion of the magnetic field using plasma injected has been successfully tested in a large vacuum chamber on Earth, but the development of thrust was not part of the experiment. A beam-powered variant, MagBeam,[8] is also under development

[Rossum]

How about manufacturing a small fleet of smaller automated ships to accompany the generation ship? The small auto-ships could perform tasks or carry mass that one doesn't want on the main ship or weighing it down.

Say, the factory on Earth builds a large number of automated harvesting ships and launches them out towards various solar systems. Naturally, they should be able to survive the trip but they are both easier to make and more expendable than the generation ship. They could serve other purposes like boosting transmissions to allow communication back to Earth or delivering supplies or parts. Say while the generation ship is traveling, the base back home keeps making supplies and sending it to them via auto-ships which can be launched faster than the generation ship. Even if the ship is self-sufficient, extra supplies could come in handy or the auto-ships get commanded to explore stuff, go ahead to prepare the destination, or get cannibalised for parts.

So, it's possible that some of the ships defences could be deligated to unmanned ships, thus freeing the generation ship to concentrate on moving faster to its goal without added mass.

The engine could be designed to be compatable with parts from the unmanned ships so they can be used in repairs. Or, when the auto-ships reach the next solar system and possibly start replicating or harvesting their parts could be used to repair the ship or build new ones.

Basically, the generation ships purpose is to support life and get its occupants to the next system as safely and quickly as possible. Unmanned ships or probes should be cheaper to make and optomised to handle other tasks for the main ship.

Like, while the main ship has its point defence lasers, it also has smaller probes to deal with threats farther out. Several sensor mounted probes could be spread out to give it a better view for watching the stars for earthlike planets or signals.

And of course, probes sent out ahead to gather resources so that once the ships reach their destination they can start work right away or operate the machines remotly so they can start work as soon as they get in signal range without waiting for the ship to necessarily reach the system.

[Simon_Jester]

That's true, it adds another layer of complexity to the targeting solution though. Making sure the warhead is at the correct angle at the correct place. Not a problem for computers though.

However, the points about drive ineffeciency stand. In my (limited) experience on the subject, Orion drives are better for short-duration missions that require high performance over endurance. They can give you a nice big push but aren't so good for sustained thrust.

Now I have a picture of generation ships using giant, expendable Orion powered booster units to get them started, and a fusion drive to keep them going. And as long as you were careful, you could re-use the boosters for additional ships...hmm, I may have to think some more on that front.

Since any sane person would build the ship in orbit (possibly solar orbit in an asteroid field, even), Orion isn't a good choice. The main argument for Orion is its extremely high instantaneous thrust, which is great for kicking things into orbit quickly... the one thing no one would want to use it for on a planet with an atmosphere you want to be able to breathe.

Once you're in orbit, a low acceleration, long-endurance drive is much better and cheaper, and lighter- for the weight of the Orion drive pusher plate and bomb magazines, you can just bolt another fuel tank onto your fusion rocket and get more delta-v out of it. Sure, it'll take longer to impart all that delta-v to the ship, but you already planned to spend a hundred years getting to your destination anyway. What's the rush?

[someone_else]

the occupants are traveling though at STL speeds form one system to another

Specify the speed required.

In any case, a massively overpowered engine is very likely to be needed if it is self-powered (it carries its own fuel).

"massively overpowered"= something that will make the sun look like a firecracker.

[someone_else]

And last is the propulsion section , the idea was going to be solar sails , but as a previous thread of mine showed me that it may not be viable between systems as the lasers that may have been on earth shining on the vessel would have gone dead. This is one of the big question marks so far

The only interstellar propulsion system that makes a modicum of sense imho is a double stream of kinetic impactors. And a spacecraft looking like a torus/doghnut for them to get through.

You mass-produce buttloads of smallish solar sails (a few kg each), you use a bigass laser to accelerate them to relativistic speeds (this requires something significantly smaller than a laser that must accelerate your BIG HONKING HUGE spacecraft).

After the first stream heads towards the destination, you start sending a second stream, slower but still relativistic.

Then you insert the spacecraft in the second stream. The impact of the sailbots on the magnetic nozzle with a gas thruster will turn them into plasma, that is then directed to push the ship.

If they keep sending sailbots you can keep accelerating till you want. The sailbots that were sent before you will BURN THROUGH ANY OPPOSITION so you don't need any weapon to clear your path. You can leave more pass throught the ship without burning them if you encounter a very big issue.

To slow down you have to redirect the first stream (slower one), with an onboard laser or with their own engines (rockets can do it), or with magnetic fields to converge on top of the second stream (where the ship still is, but with the magnetic nozzle facing forward and the second stream passing through the whole ship, that's why it has to be shaped like a torus).

The collision between first and second stream of sailbots is less efficient, but still makes plasma that can be harnessed to decelerate the spacecraft.

There are significant issues, but makes more sense than building Death Stars to accelerate a truck's worth of mass.

And does not need anything at the destination to slow down. Big plus.

Of course it's not my own idea.

[someone_else]

Sorry for the multiple posts but hey, got limited internet time.

The next section would be power generation / fuel storage. So far the concept for this has been nuclear fusion power by helium-3. Form what I understand its not the most rare of elements which can be found on both our moon and on Uranus.

He3 from Moon (and any other un-atmosphered piece of rock) is blatantly outrageously unprofitable, to the extent that you would have to mine 150 million tonnes of regolith to obtain one ton of helium 3. Does not sound massively cost-effective.

For Uranus it's more of a big unknown. If you find gas giants the size of Jupiter you need very powerful engines to get up again so it's a risk. And anyway, none tried atmosphere-mining a gas giant before, so it's likely that there are invisible brick walls somewhere.

If you still want to use He 3 the best bet is to breed it from lithium placed inside a nuclear reactor. That beats hands down any other ways of acquiring the fucker from natural sources.


I'd personally favor nuke reactors running on thorium. Thorium is (relatively) abundant on Earth, Moon and Mars, and to get enough to run the reactor for decades you don't have to burn through million tons of crap.

It's very friendly as far as waste and storing/processing/reprocessing go, but of course you need a good reactor able to burn it (at all and) in space. Which imho is much easier to build than a He3 fusion generator anyway, due to most of the issues being more understood.


Feeling a bit lonely here.

[Number Theoretic]

How about a boron-based fusion process? It's still aneutronic, like He3 but perhaps mining and processing boron is less difficult than Helium-3.