I keep hearing people say monatomic hydrogen would be an excellent propellant if you could get/handle it... so I did some quick calculations. Someone tell me if I'm wrong please :)
What I found was that it would be good for a monopropellant, but you'd still need an oxidizer to compare to hydrogen+oxygen. I guess this is what people were saying? The heat of formation of liquid H2 (it only gets worse as you get warmer) is -8123 J/mol. Presumably your exhaust then would have around 8123 joules per mole, or 4061500 joules per kilogram. Since E = (1/2)mv^2, v = sqrt(2E/m), so you get an exhaust velocity of 2015.3164 m/s, or around 291 seconds. I'm guessing if you added oxygen to the mix you could add the specific impulse from LOX/LH2 to and get something in the range of 740 seconds. Not bad. Either frozen equilibrium problems are really easy to do when you've only got one exhaust product, or I did it wrong :) Oh, incidentally, I calculated the chamber temperature of monatomic hydrogen monopropellant, assuming you'd use the Cp which is (7/2)R at the temperature I calculated, 279 K!!! This seems like it must be right, since 3000K with hydrogen gives you a 1000 s Isp.
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