> Maybe not on earth, but there are applications in deep space.

Depends on how long the interstellar craft is supposed to travel. If it's under 100 years, fission should be able to do the trick of keeping the craft warm and the lights on for the sealed ecosystem to function during the decades of coasting between stars.

Fusion rockets would be more convenient than fission ones because you can store the hydrogen you need in the form of water and water also acts as a great radiation shield while in deep space. Then, to brake, you use your radiation shield as reaction mass for fission or fusion rockets.

If we are talking about much more than that, fusion is probably a better answer as fission fuel will half-life itself into paperweights over a grand transgalactic tour.

> Depends on how long the interstellar craft is supposed to travel.

I didn't have (only) travel in mind. I was thinking of living between the stars.

You'll need a constant power supply for the entirety of the trip. After doing the math, it turns out, fission seems quite viable - all usual fuels, in storage, have half-lives of more than 10,000 years.

So, if you have enough fissiles for keeping the closed ecosystem happy for the duration of the flight, you can go quite far.

The ship/colony will need to enter orbit around a star and drop by a rocky planet at some point, to gather more fissiles and reaction mass (and other materials needed for fixes and upgrades), so it wouldn't be able to stay indefinitely in deep space. If it's fusion-driven, a gas giant may be a good option for both fuel and reaction mass, and icy moons may work well for replacing water.

I'm not talking about travel. I am talking about permanently living between the stars, and essentially living off hydrogen harvested from the interstellar medium.

This is exactly why we need to keep plugging away at fusion - one day we’ll need it in space. It doesn’t have to be cheap, it just has to work.

Of course, whether that's a good investment of resources right now is another question.

Even if we had no other goal than becoming an intergalactic species as soon as possible, we might still benefit from working on other things first.

You are operating under the rather naive assumption that the people and resources used in fusion research are fungible with the people and resources used in other things.

When you have a bunch of people who know how to build nuclear bombs sitting around with nothing to do, you damn well keep them busy before another country finds them a job.

For further discussion, we'd need to be careful whose perspective we are taking and what timescales we are talking about.

You seem to be taking the perspective of individual countries? And not eg humanity.

And timescales of perhaps decades?

On longer timescales: people don't get born knowing how to build nuclear bombs. They are trained up.