just to make it as clear as possible btw:
I don't really have a problem with gas core nuclear thermal rockets in general, a closed-core config could be very useful for high thrust in deep space, just from the sheer heat alone
I just find the idea of using your barely-spent nuclear fuel as reaction mass absolutely insane, thus why I can't take the open cycle ones seriously
imo if you are going open-cycle gas core, you might as well do the most open-cycle gas core possible and go straight for nuclear pulse propulsion, that stuff at least delivers enough TWR for takeoff and can (perhaps with smaller charges to compensate for the air blast enhancing the thrust) be used in atmosphere
we were robbed of greatness (kiloton payloads SSTO, SSTMoon, SSTMars even) by the partial test ban treaty