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So I had an idea for a starship.

So I had an idea for a starship. Now, this is not going to be the first sort of ship you launch, especially not if you're going to use it as a colony ship rather than a just an automated platform to prepare new systems for colonization with

Right, so there's this concept called a space highway. Long story short, you put a laser at Tau Ceti, one at Sol, and a bunch of lasers on various rogue planets along the way to keep the path clear of hydrogen (which would be a major hazard, at the speeds you intend to travel down one) and to also help accelerate or decelerate sail craft going along it, and you can sail at one g of acceleration all the way to Tau Ceti if you really want and can convince the lasing stations along the way to help you do it. I propose a ship that can lay down this road behind it as it goes.

Now, this thing's going to be massive--it's going to have to have powerful lasers, mass drivers, factories to make the ships it sends to these bodies, raw materials to build these out of, and, if you intend people to actually live there, you're going to want to house a very large population very comfortably--so what you do is, you take a comet and you reshape it.

Now, there are various ways of shielding your ship from the hazards of interstellar space, and you're going to want to do all of them with this build. Of special note is sending robot sail craft ahead of you (as this has the added bonus of allowing you to get a closer look at things along the way), the giant lasers I already mentioned, and a shield that revolves on a tank tread sort of thing so that you can replace sections of it without exposing your ship to relativistic (from your perspective) space particles.

Now, the thing about this shield is that you could make it so that it has a roughly circular cross section when seen from the front--it just takes multiple tracks--but it'd be simpler to just have a single track, which would give you a square profile. If you do have a society living on-board this thing, you can have one large cylinder surrounded by four skinnier, counter-rotating cylinders in the corners of the square--they're just for growing food, and so can be spun however fast they need to go to counteract the main cylinder's one g rotation.

Thanks to the sail shields (and observation by lasing stations/colonies behind them), the line layer has detailed information on the next rogue planet or other space rock that seems a likely location for a lasing station, and so can tailor the ships it sends to them. If the line layer is going .1 c, then that ought to give it more than two months between rogue planets at the densities scientists expect they exist at. And presumably the ship has seen them far in advance of that, so it has time to customize the package it sends to each, which is then shot out of a mass driver against the direction of acceleration and unfurls a laser sail which is then shot by the line layer's lasers until it becomes relatively stationary relative to the target and can navigate where it wants with fusion drives. If there are flesh and blood human colonists, they're going to have to be decelerated relatively gently, so you're going to have to launch them first. Of course, there's bound to be a lot of delicate equipment needed for the station regardless, so it pays to have very long mass drivers on the line layer regardless, so even with the entire mass of a comet to work with, it's going to be largely empty space.

We haven't talked about how we intend to decelerate the fucking thing at Tau Ceti yet. That's the beauty of it, though--we don't. The line layer drops off, say, ten thousand colonists or so (vs the thousand colonists or so it'd be dropping off at the various rogue planets along the way) and uses Tau Ceti's gravity to slingshot to the next star, laying down a new space highway as it goes.

Running out of mass can be averted by the simple expedient of making every colony/station send back an equivalent mass of raw materials (including the fusion fuel it spent along the way), and the ship will actually grow with the simple expedient of charging interest on those materials. Furthermore, every way in which it interacts with the space highway it's laying down increases its velocity. Every time it sends something backwards (colonizing ships and equipment, or people who belatedly realize they want to go back to Earth/the last star they visited), an equal and opposite reaction happens, and every time it receives something from behind (a mass “payment” or a ship full of immigrants), it's averaging momentum with something that was going faster than it. Eventually this will stop being a desirable effect and start being a nuisance--it can only make colony ships so fast, and you probably want the last star behind you to be reasonably built up before you reach the next, not to mention that the faster you're going the more you have to build up the ship's lateral stress tolerances for a given amount of, erm, “slingshotting” at a given star (not that you'd be making 90 degree turns or anything, but still)--at which point you can build ion drives and just have them constantly going at very low velocities to keep things averaged out on the whole.

Now I've been assuming that people will actually live on this thing because, well, by the time you're seriously considering making something like it you'd have colonized the moon, Mars, the asteroid belt, and presumably also the moons of the gas giants and be making inroads at the Kuiper belt, so you're going to have a lot of people who are very comfortable living in environments like this. While that assumption isn't strictly speaking necessary (the only necessary assumption is that they've figured out cheap and reliable fusion), it makes for faster space colonization, so lets roll with it.

With the numbers I gave thusfar (traveling at .1 c, ~52 rogue planets per lightyear, 1,000 colonists per rogue planet and 10,000 per star), the Tau Ceti leg of the journey is going to need an average population growth, via immigration or natural growth, of 5,250 people per year to make up population losses due to colonization. If we assume a natural growth rate of half of the current US growth rate and negligible immigration, this could be maintained by less than half a million people. If we assume that the habitation cylinder from before is a kilometer in radius and the four support cylinders are a sixth of that, and that each resident requires a hundred thousand cubic meters of space (when you include the plants needed to feed them and keep their air fresh and the factories needed to make their stuff (never mind that the latter can be done in the factory section of the ship--we're being generous here)), a habitation module for half a million people can be less than 29 kilometers in length.

An O'Neill cylinder, in contrast, is 4 km in radius and 32 km long; if you wanted an O'Neil cylinder sized habitat, it could house close to eighteen million people (remember that Gerard O'Neill was not assuming we'd have fusion, or indeed the existence of superstrong materials like kevlar (never mind carbon nanotubes), when he came up with the concept, and so he designed a structure with a single floor and gave half of it over to windows, hence the far lower population figures usually associated with the concept). I wouldn't actually recommend building it at O'Neill width, as that greatly increases the area that the shield has to cover, but I can easily see them expanding the ship later on simply because they can. So yeah, this ship can easily keep up with the demands put on it, once fully stocked.