[pja]

50 years ago it was very reasonable to think that we'd have worked out how to make a fusion reactor too.

Just because something uses real physics and engineering materials that already exist, doesn't mean that it's actually possible in the real world & getting to the stars requires that we solve three or four very hard problems.

Impossible? Of course not. Much, much harder than the 'whee, we're all going to space!' crowd likes to think? I'm afraid so.

NB. GDP is not a good proxy for available energy for hopefully obvious reasons: Just because you have a high GDP doesn't mean that you have a lot of energy available, it may mean that you use the energy you have very effectively.

[lmm]

>50 years ago it was very reasonable to think that we'd have worked out how to make a fusion reactor too.

Most of the reason we haven't is economic; modern fission reactor designs are far more efficient than even the most optimistic estimates of 50 years ago.

[pja]

Most of the reason we haven't is because it turns out to be really, really hard to contain a high energy plasma long enough to get useful levels of fusion out of it. We still haven't effectively solved the 'how do we efficiently extract the generated energy' problem either.

Perhaps these problems will be solved in the future, but so far we've spent billions and billions of $ with (relatively) little to show for it in terms of output.