At 04:56 PM 1/26/2003 -0800, Sean Lynch wrote:
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 :)
I think that perhaps you are using the wrong number for heat of formation. The number you are using is probably the one that applies when you are reacting H2 with something else, right? The energy of the recombination reaction is equal to the amount of energy required to disassociate H2 into monatomic H (simple conservation of energy). I'm sure that energy is available somewhere.
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.
The chamber temp you calculated should be the tipoff. That's near room temperature. I would expect the chamber temp of this reaction to be of the same order as the temperature at which H2 dissociates spontaneously.
-p
Mars or Bust!
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