An interstellar ship is indistinguishable from a generational colony ship because there's no way to realistically travel between stars in timelines that don't span generations unless we extend human lifetimes to centuries or longer. That's possible but doesn't change the trael times. It just means you live to the destination rather than your descedants do.
And let's aside the serious ethical issue of you choosing to board such a ship vs the offspring you have who definitely did not consent, some of whom may not even make it to the destination.
So a generational colony ship looks a lot like an O'Neil Cylinder [1]. It can spin to create 1g gravity and support enough people to make it to the destination.
The issue is energy. An orbital can support itself with solar power when around a star and doesn't need a form of propulsion. An interstellar ship will need an alternative energy source and also have a propulsion system that can sufficiently accelerate and decelerate. The energy budget for the propulsion is so large that the life support energy budget is a rounding error.
The only realistic policy I see is solar sails. This avoids the reaction mass issue. You need to decelerate at the other end. Part of that you get from drag in the interstellar medium. You either carry reaction mass for the rest or you go ahead and use automated systems to build the solar sail equivalent on the other end to decelerate you.
It's interesting how frequently the issue of unborn children's consent is brought up. This consent is impossible, it's never existed and never will exist, and the only alternative seems to be nihilism.
Yeah, its not like kids today are consenting towards living on this earth now in this age. As long as the ship isn't just some piece of trash death trap and has decent living standards I don't see the problem.
I strongly recommend "The Conspiracy Against The Human Race" by Thomas Ligotti in the context of this line of thinking. The main point he makes is that acceptance of reality as it is, is unthinkable to almost everyone, and we all use different strategies to avoid facing it, at least some of the time.
This is definitely one way how to do it - but it should be possible to also get fast if you really want to[0][1] + likely also increase human lifespan/hibernate/immortal cyborgs. So there might be whole other class of interstellar ships - much more compact and with its own unique class of moral issues!
Having a fusion engine or fusion reactor is basically hand-waving away the energy problem.
Controlled fusion has a fundamental problem: neutrons. Even if you solve the problem of container destruction (ie neutron embrittlement), which is significant, you still face the problem of significant energy loss to the system through high-energy free neutrons.
Stars solve this problem by simply being really large so a free neutron can't really go that far without hitting another nucleus, particularly because fusion happens at the core.
The hope with commercial fusion research is that we can somehow avoid the container destruction issue and have sufficient energy generation (given the energy inputs) despite the free neutron energy loss but it's unclear if that'll ever happen.
IIRC (Its a long document, and I only skimmed it) they assumed 20% efficiency on behalf of the reactors.
I think they just wanted a plausible future energy source that they didnt have to look at too closely.
My first issue with the design is that it looked like a decent percentage of the planet would have to be launched into space just to build the thing.
A lot of the space math looked alright, but theres several issues of practicality.
>Even if you solve the problem of container destruction
No idea if this is related, but everywhere they needed 1 reactor, they had 4. 2 in active/active failover, and 2 cold spares. It seemed relatively easy for them to perform maintenance on these. But its just heaps more launch mass, heaps more acceleration fuel, and a lot more deceleration fuel.
And let's aside the serious ethical issue of you choosing to board such a ship vs the offspring you have who definitely did not consent, some of whom may not even make it to the destination.
So a generational colony ship looks a lot like an O'Neil Cylinder [1]. It can spin to create 1g gravity and support enough people to make it to the destination.
The issue is energy. An orbital can support itself with solar power when around a star and doesn't need a form of propulsion. An interstellar ship will need an alternative energy source and also have a propulsion system that can sufficiently accelerate and decelerate. The energy budget for the propulsion is so large that the life support energy budget is a rounding error.
The only realistic policy I see is solar sails. This avoids the reaction mass issue. You need to decelerate at the other end. Part of that you get from drag in the interstellar medium. You either carry reaction mass for the rest or you go ahead and use automated systems to build the solar sail equivalent on the other end to decelerate you.
[1]: https://en.wikipedia.org/wiki/O%27Neill_cylinder