Not very. That said, DFD is a technology with tremendous moonshot potential.
Fusion propulsion is inherently easier than fusion power on Earth because you don’t have to worry about converting heat to electricity and the breakeven threshold is far lower; depending on the mission, even Q < 1 could be fine.
Absolutely. I’ve just noticed that a lot of people think, correctly, that fusion power is hard and space is hard so doing them together is stupidly difficult. Not so in this application—the relaxation of requirements on fusion outweigh the difficulties of doing it in space.
Put another way, the dollars going into fusion power might be better spent on DFD.
I think we're also getting better and faster at iteration and design. CAD, modelling, even wind tunnels from 50 years ago made a massive difference over jumping off a cliff with a glider for tests.
I guess my point is, I don't see 50 years as validation of it being hard. And some of those designs were likely dismissed due to tech limits at the time.
A nuclear reactor in space would require an enormous heat sink to get useful energy out.