Nerva-type reactor engines could probably be safely operated in the atmosphere (provided they didn't blow up!) but development and testing is an inherently much more risky business, and routine operation would eventually run out of luck. Maybe the lunar surface could be exploited for this task. But then, getting them there for testing would be very expensive and would also pose some risks.

It's too bad, this technology is workable and had great promise, and it would make possible big payloads for solar system work. Also, Tom, have you heard of any further research in using monatomic hydrogen as a fuel? Some forecasters, like G. Harry Stine in the 60s, seemed to think the instability problems could be solved. In case this was "before your time", there is an enormous amount of energy in the atomic molecular hydrogen bond, much more than in hydrogen-oxygen combustion, but H1 was extremely unstable. I had hoped with recent advances in cryogenics there might be a way around this.