PiPaLiPkA t1_j23ljuj wrote

That's not really true. Nuclear thermal rocket development stopped when military funding dried up. Project NERVA was originally project ROVER which was developing NTR for the upper stage of an ICBM. In 1958 it was transferred to NASA after the launch of Sputnik as it seemed more applicable, but as rockets got larger the efficiency gained from the NTR was less relevant for ICBMs and the advent of LH2 didn't help. Obviously there was still strategic value of exploring space, but then the space race ended so there was even less need to develop it, subsequently it was cancled in 1973.