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Nuclear thermal rockets also provide a very poor mass fraction of fuel because their propellant has very low density (liquid hydrogen) and the reactors are heavy. So their advantage is much small than it seems. And low thrust makes things even worse, they can be used only for upper stages/space tugs. In the end they were never used mainly because cost outweighed benefits. Imagine, nuclear fuel contains about millionx the energy per mass of chemical fuel, and yet exhaust velocity is only 2x higher, and effective combined ISP of the stage (velocity change vs mass fraction assuming dead weight of stage being zero, and with gravity/ballistic losses subtracted) is only about 50% higher than state of the art chemical systems, at vastly higher cost and risks involved. If i was Elon Musk i won't launch a nuclear rocket unless i had a really good liability insurance, and i was an insurer i'd say nah unless federal government backs me, and if i was uncle Sam i'd say nah, too. It simply doesn't worth it.

Also, problems with space access are mostly market size-related problems. Cheap access to space is possible, only requires a vast market to pay back the investments, which is simply not there.



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