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chasonreddit t1_j0uvcp9 wrote

Reply to comment by BlueSkyToday in Nuclear fusion breakthrough: What does it mean for space exploration? by Gari_305

All very good points.

No way around launching radioactives. Until we can bootstrap enough to mine radioactives elsewhere.

The conversion bit is my own fantasy. But if you are using heated reaction mass, heat is heat. If you are using electricity well, does it matter the initial source? I'm think big transfer ships here, not optimized earth to orbit type things.

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BlueSkyToday t1_j0vdfzd wrote

Yup, heat-is-heat, but I think that there's about as much overlap between a laser fusion or a tokamak, and a fission plant as there is between a coal fired plant and a fission plant.

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chasonreddit t1_j0vebyu wrote

Well SI per lb of fuel is quite a bit higher for fission, but I understand what you are saying. Since we don't have a fusion reactor, it's hard to say what that might be.

Now I've always been fascinated by the concept of the Bussard ramjet, but that's a whole different animal and fictional as well.

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BlueSkyToday t1_j0wk7ye wrote

I suspect that the mass of the fuel is a very small portion of the mass of the engine.

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chasonreddit t1_j0wnkve wrote

Really depends on the distance and expected acceleration. You would be surprised. With Hohmann orbits, well those are designed to use minimal reaction mass. It's still a huge proportion. To really get around even the solar system you really want constant acceleration and ultimately 1 G acceleration. Even at high ejection velocities that's a lot of reaction mass.

I won't vouch for the math, but I remember reading that even with a 100% mass conversion drive (the ultimate) a ship would use approximately half of it's mass to make a round trip to nearer stars at 1G.

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