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Let's say that the most powerful nations on Earth (with a couple hundred billionaires) have decided to build a generation ship. The reason for them leaving is out of the scope of this question, but there are a few things you need to know:

  • In case this isn't obvious, this takes place in near future conditions.
  • The Generation Ship must be big enough to hold and support at least 500 people.
  • The Generation Ship must be built within the scope of 4 years, but we can prepare for another 3 before that.
  • The launch sites for ships holding construction materials will be attacked by mobs of people being left behind.
  • You can only rely on the collective wealth of these billionairs a collective wealth of over 300 billion.
  • As long as you can afford it, you can use any materials.
  • The Generation Ship will be built in space, using materials brought there by Material ships.

Abiding by the above rules, what is the largest I can build a generation ship?

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    $\begingroup$ Ship will be assembled in space, not launched where angry mobs would be possible. $\endgroup$
    – JDługosz
    Oct 3, 2016 at 17:49
  • $\begingroup$ @JDługosz exactly? $\endgroup$
    – TrEs-2b
    Oct 3, 2016 at 17:50
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    $\begingroup$ 3rd bullet doesn't make sense $\endgroup$
    – JDługosz
    Oct 3, 2016 at 17:51
  • $\begingroup$ There will be no launch site if ship isn't launched. Now you seem to contradict yourself. Please fix / clarify this. $\endgroup$
    – Mołot
    Oct 3, 2016 at 17:51
  • $\begingroup$ @JDługosz what I mean is that on Earth, riots will occur at launch sites, but you need to get the materials up there somehow. $\endgroup$
    – TrEs-2b
    Oct 3, 2016 at 17:52

3 Answers 3

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There are several possible ways to go, but given current science and technology, a generation ship capable of interstellar travel is not an option.

Using the rest of the question, and stipulating the billionaires are interested in taking 500 or so of their closest friends to Neptune, we can look at conventional launch and "unconventional" launches.

For a conventional launch, we will need to lift and assemble the ship in as few launches as possible. Elon Musk's project using a massive booster to loft a 500 ton interplanetary ship is the model to use, but since we are limited in time and resources, a few modifications are in order:

  1. Revive the Sea Dragon "Big Dumb Booster". It is far cheaper (built to shipyard tolerances, welded out of sheet steel and using "blow down" pressure fed engines), and only needs a large lagoon to launch from. This makes the launch site much less vulnerable from attack as well.

  2. Musk's interplanetary craft is powered by chemical rockets. This is ridiculously inefficient, and should be replaced by one of the many nuclear or electric drive proposals already in advanced development

  3. Each passenger or cargo ship is going to need a lot of reaction mass to get "out there" in any reasonable time (especially if they are hauling tons of building supplies and life supper consumables). Consumables and supplies can also be sent on large pods as well. Each ship will meet up with one or more external fuel tanks (in the manner of the space shuttle) boosted into orbit on a Sea Dragon. This means the spacecraft will have two or more 550 ton tanks with extra reaction mass or other supplies attached.

enter image description here

Interplanetary craft in flight. For this sort of journey, picture several large "external tanks" strapped on to carry extra supplies

Of course, there may be a need to have a more capable fleet of ships, in which case, the building consortium can try to revive the ORION nuclear pulse drive concept. This has the advantage of requiring fewer lifts, and the launch site is literally defended by nuclear explosives (the ORION pulse units). ORION craft as large as 4000 tons were studied in great detail (the 4000 ton ORION was designed as a space battleship, but the internal volume can be used for more constructive purposes).

The advantages here are:

  1. The ORION ship is conceptually much higher performance than almost anything else known to science. Launch can take place at any time without waiting for synodic launch windows, for example.

  2. Since it is built and equipped on the ground, the quality control is better. The ship itself will be much more robust than one launched from a chemical rocket, and have much better shielding and even room for crew amenities (the original ORION team boasted they could bring barber chairs on board if they wanted to).

  3. No need for extra fuel and supplies to be boosted into orbit to meet the spacecraft prior to setting off.

  4. Anyone silly enough to try to riot at the launch site will be vaporized on liftoff. The fact that the nuclear pulse units are small atomic weapons means there will be a very high level of security at the manufacturing plat and launch site.

  5. Longer term, the experience of building and operating ORION ships will translate to building bigger and better ships, eventually culminating in "Super ORION" craft capable of interstellar flight.

The downside is getting the permissions to build nuclear pulse units in the first place, and launching from Earth (unless the real reason they are leaving is existential, such as the imminent destruction of Earth).

enter image description here

ORION battleship in flight. A transport ship would have the internal volume used for passengers and equipment

So from a practical POV, setting up an assembly line near the shore, building an artificial lagoon for towing out the Sea Dragon and launching it, and using a series of Sea Dragon launches to lift the passenger ships and the supplies is probably the best way to go. Using a bit of a handwave, I suspect that in terms of time and resources, 3 actual spacecraft each attached to 3 pods carrying 550 toms of fuel and supplies should get you everywhere you want to go in the Solar System, so a total of 12 Sea Dragons, 3 spaceships (probably the most expensive part of the project) and 9 "ET"s for fuel and supplies.

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  • $\begingroup$ ITS is bigger then you describe, potentially it can open lot of doors, and do it pretty wild ITS Moon landing payloads and costs., for relatively humane price. I think it deserves more thinking about possibilities of using it. BDB and ORION not needed there, possibilities are a bit more interesting. OP's timeframe is a problem, but I feel it would be more interesting to skip it first(not start w), and try to apply some king money coefficient in same arbitrary wise style, like mobs attacking ship in space. $\endgroup$
    – MolbOrg
    Oct 3, 2016 at 20:00
  • $\begingroup$ Musk's booster has similar capacity as Sea Dragon, but is wildly more complex and expensive. To answer the OP's question we need to find a cheaper substitute. I'm not totally sold on the ITS as it is currently described (all chemical propulsion limits you to Mars), but it is a work in progress. $\endgroup$
    – Thucydides
    Oct 3, 2016 at 20:32
  • $\begingroup$ I do not mention size literary. And ITS ship is also important part here, Sea Dragon did not have one(?), and it(possible landing) allows to have interesting possibilities. In therms of reference spacex example is more interesting in estimation research time and costs in this time and development level. OP's plan should consists from 4 stages: SpaceX, ITS, Moon production, Genship - and this way it is possible to get more for that budget. U jumped almost directly to spending. If interstellar is OP's demand, it would be most speculative part of answer, but the rest allows be more grounded. $\endgroup$
    – MolbOrg
    Oct 3, 2016 at 22:54
  • $\begingroup$ One argument to rename was that it allow theoretically, after some consideration, to unlock solar system, not mars only. I do not believe it was just for hype, they have enough of it. Also I think about that and it looks possible, technically, would we have ship with descent payload with cheap price to the moon, we can use stuff we already use or know, to get even more with little effort in RD. producing LOX on moon as example, we already have all knowledge needed, maybe it will be not optimal but it will work for sure. It is just interesting topic for me, no complains, just in case. $\endgroup$
    – MolbOrg
    Oct 3, 2016 at 22:55
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(Presuming a near-future world where such a thing is possible:)

There is no limit other than the amount of material you can find. Use an asteroid. How’s that any different from just “using” a large moon or planet? The limit that affects your purpose is the ability to move it. How much power do you have? Why not use the planet Neptune as fuel and reaction mass?

Your question does not contain any information that would be applicable for knowing such limits in the context of your story.

From comments (your question is not clear) it seems you mean to build like we did the ISS and other projects to date, sourced only from Earth with no space industry infrastructure.

Look at the cost of the ISS for comparison. So, you could double the size of that! But, what about life support systems (not invented yet and a major development effort), or interstellar propulsion that is fast enough to get somewhere in a thousand years? Not available; would not be small/cheap as indicated.

In short, it’s not possible to even meet the requirements or even make a large enough craft for that budget.

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The most intelligent way of doing this, provided you are on a very strict time scale is to launch as much oxygen, water, soil, and seeds as you can in as simple a package as possible. Build a rudimentary ship that will last for however long you need to get to the moon or asteroid belt and then mine the construction resources out of them.

The size of the ship is only limited then by physics and the amount of oxygen/nitrogen and the amount of thrust you want vs the fuel you brought. which you could probably bring quite a lot. provided you concentrated on that.

Also since you're going to have government and personal funding and you don't really care about waste probably you can easily build multiple sites around the world that launches ships all the time. You just need to coordinate a little to make sure you don't have launches hitting each other.

With only 500 people you don't need much farm space and that's all you'd use. The size of the ship would easily surpass several kilometers and eventually you might even build several ships to spread out or simply to carry more resources.

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