Nasa Should Go All-in on Nuclear Propulsion

2. Energy-density-vs-mass is relevant for the rocket equation when you burn it as a fuel (and, optionally, use as reaction mass), but not vs. PV where you don't burn it and lose it.

e.g. ion drives, magnetic sails of various kinds, launch loops/space elevators and similar, space stations/bases/almost all planetary industry (which can be PV powered), nor solar sails which directly use the momentum of light (usually the sun's nearly isotropic emission, but in a grand space empire anywhere from here to K2 all bar the entropy losses can be pumped into a single direction by lasers).

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.