$\begingroup$ @qazwx: Just how would this not vaporize the nozzle if there was any decent thrust? H2 propellant needs to be heated to an average temperature of ~150,000K for a 40 km/s. Ok use a "cold shield" of gas to protect the engine's walls and/or magnets. But dealing with >>10,000 times the radiant flux of the sun's surface doesn't sound easy. $\endgroup$ – Kevin Kostlan Feb 4 '20 at 2:53