Re:hydrogen may be inefficient BUT (Score 1)

I would have thought so too. But we hotfired an N2O thruster and sampled the exhaust gases- no detectable NOx. I am no chemical engineer but I suspect it has to do with the temperature and pressure conditions driving the thermochemisty. There is a company that holds a patent on a N2O breathing air generator- clearly if you control things you can avoid getting a lot of undesirable species.