I was wondering if it would make sense to hollow out an asteroid to use for a generation ship? Also, might it be a good idea to use hollow asteroids for any kind of long distance space ship? I'm thinking you can mount engines on the outside, you have some great hull protection, there may be some handy ready resources, it would work for insulation so less energy would be needed to keep things habitable, and going at high speeds small objects would pose a much smaller risk to the integrity of the ship. So other than mass, are there any other big drawbacks?
As already mentioned the problem with this concept is the mass, you're needing to accelerate the dead mass of the asteroid as well as all the working bits. There are possible some drives where that would not be a problem (for example using a warp drive of some kind, although even then you would need to synchronize velocities with the destination when exiting warp) but the options are pretty limited and even warp drives would most likely need more energy for more mass.
The advantage though is the raw materials. When setting off on a voyage where population may expand, advances may be made, etc, then having a huge chunk of raw material available to refine, hollow out, re-purpose, etc could be very useful.
But more useful than just transporting a stockpile of already refined materials? Probably not. Although with the refined materials you do run into the issue and benefit of having to decide in advance what you need rather than just hauling a huge chunk of rock and hoping it has something useful.
Unlike metal or plastic, many types of rock are slightly porous, so over a long time, atmosphere could slowly leak away. This might not matter for a ship making journeys of a year or two, but over generations, there could be a significant loss of gases unless the whole interior was covered in some sort of sealer that would last generations.
Another drawback of an asteroid is that there is a lot of unnecessary mass that would have to be accelerated. Better to build a ship as light as possible in order to reduce the cost of acceleration and deceleration.
On the other hand, a sufficiently large asteroid would also provide good protection against interstellar radiation, and if the species manning it are sufficiently paranoid or justified, an asteroid would provide more camouflage against enemies than a manufactured ship.
So long as we're not bumping into a logistic growth ceiling, I don't see a strong incentive to leave our solar system
Along with generation ships comes the assumption of a non solar power source and the technology to engineer closed ecosystems-- the Main Asteroid Belt would be open to real estate development.
I see several logistic growth ceilings here. I suspect it'd take centuries or millennia to fill the Main Belt. From there the Hildas are natural cyclers between the Main Belt and Sun-Jupiter Trojans. (See this vid where first half shows Hildas, 2nd half Trojans). When the Main Belt and Trojans are filled up, there's the Centaurs as well as moons of gas giants. After those are filled comes the Kuiper Belt. And then the Oort Cloud.
By the time we fill the Oort, our civilization will probably have many millennia of experience building biomes from ice balls as well as using fusion power.
Given a civilization with this infra structure and capabilities, I'd construct the generation ship in the Oort. On the boundaries of the the sun's gravity well, it only takes a small burn to drop the ship to a perihelion deep in the sun's gravity well.
It'd take a few centuries for the ship to fall from the Oort to the inner solar system. But given city states throughout the Oort, spending centuries dwelling in artificial biomes is already the norm.
When 1 A.U. from the sun, the ship would be traveling about 42 km/s. If perihelion is within .1 A.U. from the sun, perihelion speed is about 133 km/s. Doing the burn near the perihelion can give a substantial Oberth benefit. If the ship does a 55 km/s burn near perihelion, it'd pick up 133 km/s Vinf.
Of course 133 km/s is only .0004 C (speed of light). It would take around ten millennia to reach Alpha Centauri and longer for other neighboring stars.
If the ship had a drive capable of reaching .1 C, solar Oberth benefit would be of little consequence. In this case I might depart directly from from an Oort object a light year or two out -- as close as possible to the destination star.
True, you need to match velocities with comet. Intercepting it and bounding in space will be difficult. But once you do, you will have a lot of water, already at pretty nice velocity. So you need to spend fuel only to shot working bits, and get drinking water for "free".
What's nicer, you now have empty fuel and oxidizer tanks. And sunlight. So you can use electrolysis to refuel from asteroid's water. And you can use water as an emergency source for breathing oxygen.
It would be a very good idea, especially if they could be fitted out as mining colonies as well as transports. Where they may be utilized as base-points to bring materials mined throughout the area for processing and conversion into other products, vehicles and/or habitats. The problems would be the energy required for propulsion and the processing of materials such as metals and manufacturing, which would almost have to be nuclear. It certainly would be worth it, considering the cost of launching processed materials and objects into space from the Earth.
Another scenario where they would be extremely beneficial would be in repetitive trips to other planets within the solar system. If they could be arranged into well timed orbits, for example between Earth and Mars, explorers could jump on one as it passed Earth and get off as it passed Mars. If they could be made to rotate then there would be the benefit of centrifugally generated artificial gravity. So if two, three or four asteroids could be arranged in very well-timed and stable orbits then there would be an asteroid passing Earth and Mars on a quarterly schedule. Once arranged the fuel savings would literally become astronomical, since they would not need to be accelerated and decelerated for each trip . They could be used as shielded taxi's and also as depots for spare parts and provisions. Ultimately the asteroid fuel problem should be a priority, since building any very large ship from the ground on Earth would be prohibitive at our current level of tech.
PS Not all asteroids are mud. Some are solid rock and/or metal. I like the idea of nickel/iron as the hull of a ship, since the radiation hazards in space are not negligible and outside of the solar system radiation hazards are apparently downright outrageous. Asteroids are the only way of doing any major travel at our level of tech, mainly because the radiation will kill or blind 20 out of 100 long range astronauts within a year and the hazard dramatically increase with exposure time.