with triso smrs nuclear is safe. helium coolant loops mean you can do turbojets and scramjets without putting radiation into the atmosphere. idk, im not an engineer though, i just spitball ideas. the issue w nuclear is thermal management, but its the only way to beat the rocket equation for ssto. use airbreathing to boost as high into the atmosphere as possible then swap to ion drives. use a hypersonic waverider config. thermal management i think is the primary difficulty to be solved w such a design. ive even considered simply using a helium loop -> high temp ceramic nozzle and extreme high temperature steam. thus your reaction mass is water and you can also use it as coolant for hypersonic flight and reentry.
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