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The idea isn't ludicrous, although the torch-ship speeds of 39-day transits are definitely out of reach for now. The basic idea is, let's attach a nuclear reactor to an ion engine. Further, let's make the propellant hydrogen so we can refuel anywhere in the solar system. If we want to get serious about exploiting the solar system, we'll eventually have to give in and embrace nuclear technology, whether something like VASIMR or a nuclear-thermal design like NERVA. We already routinely park 100MW+ nuclear reactors in port for our navy, why not consider civilian use for space exploration? We already know many ways to mitigate risk for the launch of nuclear material. I think Musk, Zubrin, et al. analyze propulsion technologies from the perspective of how to enable a journey to Mars now. In that light, something like Raptor makes much more sense. You still need a chemical rocket engine to lift off from Earth or Mars, so in the near term a nuclear ion thruster just adds far too much complexity to justify its inclusion. Further, it's difficult to imagine SpaceX obtaining the political backing to put nuclear tech into space as a private company. This is also why Musk would rather power Martian propellant plants with fields of solar arrays instead of the much more mass-efficient space-rated nuclear reactors that NASA has been developing. But imagine if we actually developed a Mars colony with millions of people. The logistics of seeding a colony on Mars with chemical thrusters already boggle the mind. Economics would basically forbid meaningful interplanetary trade unless we develop new technology with much higher specific impulse. It would be even more impactful than moving from air-freight to container ships. |
The real problem for a spacecraft isn't generating 700 MW of thermal heat, which is pretty small and light, but radiating it away continuously. The ISS EATCS radiates about 70 KW and each radiator is in the thousands of sq ft and thousands of pounds. So four more digits would be tens of millions of sq ft and millions of pounds, VERY superficially. However the reactor probably doesn't have to be optimized for human temps, so maybe ten times to hundred times better? It would be quite large at any rate.
There are certain engineering optimizations you make if you have an infinite liquid heatsink like a naval reactor vs incredibly expensive cooling like a space reactor. If you're willing to boil sodium your condenser can radiate a lot more per sq ft than an ammonia based refrigerator for ISS HVAC. That's a little far fetched but the VHTR/HTGR design goal was a cool 1000 C, so the radiators can run quite a bit hotter and smaller than ISS HVAC systems.