Feasibility study of a mission to Sedna – Nuclear propulsion and solar sailing

> How close are they?

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.

"Easier" in this context is still ridiculously hard. Fusion rocket designs were first seriously researched 50 years ago and not a single one of the countless designs proposed since then has reached readiness for in-space use.

Note the economics might be better than for terrestrial fusion energy because you're not paying for watts you're paying for thrust and something like D-He3 has a great exhaust velocity.

> "Easier" in this context is still ridiculously hard

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.

Da Vinci and others worked on flying machines, I expect designs go back thousands of years. Yet the result is now thousands of designs and working iterations.

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.

The point is more that 50 years ago, people thought we had sufficient understanding of fusion to build something like this. And thanks to advancements in inertial confinement research at the height of the thermonuclear arms race, they actually had a pretty good reason to believe so back then. There is very little reason to believe these new companies today when they say so, because fusion research is in a deep hole and running on a fraction of the funding it did during the cold war.

If Q < 1, aren't you better off with an ion drive that just shoots the hydrogen out the back? You still need to get most of the energy from somewhere other than the fusion drive.

Energy and thrust are not equivalent. There’s hardly any limit to how much energy you can generate in space (for example using a nuclear reactor), but fuel is the limiting factor.

By "fuel", do you mean reaction mass (stuff that shoots out the back of the rocket) or energy source?

A nuclear reactor in space would require an enormous heat sink to get useful energy out.