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chasonreddit t1_j0rw7qj wrote

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

Thank you for this. I really tire of being the Debbie Downer on this topic. It's nice to have someone with some experience express concerns.

It is a great achievement make no doubt. In real terms it is a step toward practical fusion energy. But it is one step on a trip of 10,000 miles. I'm over 60. Fusion energy has been 20 to 30 years away my entire life. It's a hard nut to crack.

As to space travel, why don't we start building fission plant reaction rockets? Or even nuclear powered ionic drives? Sure, you'd have to launch them, but once up there, they could go anywhere. We could convert them to fusion if/when that becomes practical. Not that these things are easy, but they are doable with today's technology. Fusion power is simply not.

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BlueSkyToday t1_j0us0q1 wrote

People are kind of allergic to launching radioactive materials. You're one RUD away from dirty bombing yourself or your neighbors.

I can't see nuclear rockets happening anytime soon. It's hard enough to get small thermoelectric generators approved.

I'm not so sure that there's a conversion from fission to fusion. I don't see much commonality in the designs.

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chasonreddit t1_j0uvcp9 wrote

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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ItsAConspiracy t1_j0z312i wrote

Fission fuel is barely radioactive before you start the reactor. It's just natural uranium, with a modestly higher percentage of U235.

It's the broken-apart atoms you get after fissioning uranium that are the really dangerous stuff. And to a lesser extent, heavier atoms that absorbed neutrons without splitting. So, just don't start the reactor until you're well away from Earth.

That would be way safer than what NASA has actually done multiple times, which is launch deep space missions powered by plutonium-238. That doesn't even need to be fissioned, its radioactivity is what powers the mission.

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BlueSkyToday t1_j106ge8 wrote

Yes, spent fuel rods are a lot more dangerous but the cost of mass to orbit is very high. Every plan that I've heard of calls minimizing the size of the reactor and the fuel load. So we're looking at highly enriched uranium or other possible designs. These are more of a problem than what we normally use in fission reactors.

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ItsAConspiracy t1_j10bji0 wrote

The half-life of U235 is 700 million years. The longer the half-life, the less radioactive something is, so even weapons-grade uranium at over 90% U235 is not particularly dangerous. You wouldn't want to eat it or inhale a large concentration of it, but you wouldn't want to do that with solid rocket fuel either.

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BlueSkyToday t1_j1p3ws0 wrote

In a world where people are deeply upset about the environmental health effects of fragments of shells made out depleted uranium, I don't think that scattering highly enriched uranium is going to fly.

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ItsAConspiracy t1_j1pzpeu wrote

And yet, plutonium-238 has flown multiple times, and that's way more radioactive.

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