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Fusion rockets can not get away with lower efficiency. Either you are using the fusion reaction as a direct propulsion source, in which case it needs to beat ch
by jjk166 1y ago
Fusion rockets can not get away with lower efficiency. Either you are using the fusion reaction as a direct propulsion source, in which case it needs to beat chemical rockets at producing thrust, or you are using it as a power source for electric propulsion, in which case it needs to beat other power sources for energy efficiency. In fact, the problem is so much worse for propulsion, because not only are you concerned about fuel efficiency, you are also concerned about mass.
Right now it takes a building the size of 3 football fields to house the only machine that has ever achieved ignition of a controlled fusion reaction (ie the energy released by the fusion fuel exceeds the energy put into the fuel). This is several orders of magnitude below the point where the energy produced by fusion exceeds the energy needed to run the machine, which is the absolute bare minimum you need to do anything useful. Fusion power output scales with size - keep making your reactor bigger, and you will eventually reach any desired level of power output. The challenge of fusion for power production is making a big enough power plant while remaining economical. Luckily economics is relative - so long as a massive power plant is preferable to the drawbacks of other power sources like pollution or limited fuel, it can be justified.
But for propulsion, you don't just need to satisfy investors, you must overcome gravity. Make your reactor bigger and your mass increases. More mass demands more propulsive power demands a bigger reactor. This is especially the case for launching from planets, where thrust to weight ratio is king, but even for deep space propulsion mass budget is a chief concern (and you still need to get that thing into space somehow). Not only do you need to make a fusion reactor that is competitive with other power sources in terms of efficient power generation, you need to do so with a comparable power density. Further, the only thing fusion really has going for it over fission - that it doesn't produce long lived radioactive waste - is entirely useless when you are flinging stuff out into the void, so you can't even justify a low power density reactor with reduced fuel mass.
No matter how much fusion advances, fission will always be decades ahead and fundamentally simpler. The only form of fusion propulsion that realistically could find practical application is the one form of fusion power humanity has already learned to harness - thermonuclear explosives.
- JumpCrisscross 1y ago> you are using the fusion reaction as a direct propulsion source, in which case it needs to beat chemical rockets at producing thrust Yes. > Right now it takes a building the size of 3 football fields to house the only machine that has ever achieved ignition of a controlled fusion reaction Compare the size of a jet engine to a power plant of a similar size. Size shrinks dramatically when you can blow the heat out of your ass in propellant product. > you don't just need to satisfy investors, you must overcome gravity Why? Burn it in vacuum? > No matter how much fusion advances, fission will always be decades ahead and fundamentally simpler Why. Fusion precursors are more plentiful than fissionable elements. And fission produces heavy nuclei; fusion doesn’t.
- jjk166 1y ago> Compare the size of a jet engine to a power plant of a similar size. Size shrinks dramatically when you can blow the heat out of your ass in propellant product. Powerplants also blow the heat out. What do you think smokestacks are? Jet engines are smaller than gas turbine powerplants because a plane doesn't need as much power as a city. And the size of fusion reactors has nothing to do with heat expulsion - they haven't gotten to the point they're generating fusion heat to expel. The massive size is required for the equipment to produce the conditions for fusion. Whether it's massive lasers or massive magnetic coils, those things are just as necessary for propulsion as for power generation. > Why? Burn it in vacuum? There's still gravity in space. When you want to go from low earth orbit to the moon, or to mars, or to anywhere, you need to fight gravity to do so. > Why. Fusion precursors are more plentiful than fissionable elements. And fission produces heavy nuclei; fusion doesn’t. Fission will always be decades ahead because it was developed decades earlier. Nothing's going to change that. If a fusion plant started operating tomorrow, it would take ~70 years to gain the operational experience we currently have with fission, and in those 70 years fission will have advanced another 70 years forward. And of course the whole premise of this was that fusion propulsion was an easier challenge to tackle than making an economical powerplant on earth. Relative abundance does not matter when both sources are essentially limitless. The actual difference isn't even particularly large. Take all the deuterium out of the oceans and extract the lithium necessary to produce tritium to burn it with and there is about 3 times as much energy as the uranium in Earth's crust. (The deuterium will run out before the lithium). For space applications, in situ production of either fission or fusion fuels is not realistic due to the need for isotope separation, so relative abundance off of earth is completely irrelevant. As I stated, in space it is not a drawback for fission to produce heavy nuclei. On Earth (and also in space I guess), fusion does produce heavy nuclei by irradiating the reactor components. It's not quite as fundamental as the fuel itself becoming waste, but you're still going to be left with a lot of radioactive waste that needs to be handled. Nuclear waste is not really a big technical challenge in either case, but the idea that fusion will jump ahead because it is cleaner and safer is false.