re: Yo
I have no problem with operating certain types of nuclear engine in the atmosphere. But, even keeping accidents out of it, keeping the exhaust clean is an issue when you are talking about anything for routine use.
Monotomic hydrogen is one of those things that comes up when propulsion people get wistful, kind of like using liquid fluorine for an oxidizer, but as far as I know, a tank full of the stuff is still a bomb waiting to explode. Heck, our guys had enough hassles working with hi-test hydrogen peroxide.