Some updates after some further googling: - the NRX A6 engine was tested for 1 hour, but based on the observed corrosion, NASA extrapolated that the lifetime of

Some updates after some further googling: - the NRX A6 engine was tested for 1 hour, but based on the observed corrosion, NASA extrapolated that the lifetime of the engine could be 2-3 hours ([1] page 10). That would reduce the fuel cost that I estimated above from $50MM per trip to Mars to $20MM per trip. - nuclear thermal rockets can use different propellants. In [2] Nasa looked at some other propellants that could be available on Mars, such as CO2, H2O, CH4, CO, N2, Argon, NH3. They conclude that CO2 would be very convenient, H2O even more so (but were not sure at the time how easy it would be to source water on Mars). CH4 would be very appealing, but one would need to overcome some coking concerns. The other fuels were all inferior (except for H2, obviously). - nuclear thermal rockets are not intended for takeoff from the Earth [3]. The intention is to be carried to space with chemical rockets, and use them for trips starting in LEO. [1] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910017902.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199100... [2] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910012833.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/199100... [3] https://www.energy.gov/ne/articles/6-things-you-should-know-about-nuclear-thermal-propulsion https://www.energy.gov/ne/articles/6-things-you-should-know-...