Simple question, probably very complicated answer:

Without building a test engine/rocket and finding out experimentally, that is to say, based on chemical characteristics alone, how can one compute the expected specific impulse of a given fuel?

  • $\begingroup$ Fuels produce energy, you can estimate how much for given mixtures from known reactions (or study those regions independently). Then you have to know how your engine transforms that into motion (or whatever else it's supposed to do). I would assume this to be an more or less everyday problem for engine engineers... I think there is another stack exchange for that... $\endgroup$ – Martin - マーチン Oct 14 '18 at 10:39

The relevant property is the temperature of the burnt fuel when it leaves the rocket engine nozzle, i.e. when it stops exchanging impulse with the engine.

That temperature you can convert into a velocity distribution of the gas particles, multiply with their molar mass, and that's it. (somewhat less than that of course, because no all gas particles are flying in the exact same direction. depends on the nozzle type, among other influences)

The temperature you can estimate to some extent from the reaction enthalpy, but of course you have radiative and conductive losses. Which is where this becomes an engineering problem.

  • $\begingroup$ "Adiabatic flame temperature" is a thermodynamic state variable, at least given a fixed oxidant composition. It probably could be used to set an upper bound on achievable impulse if you assume 100% efficiencies everywhere. Dicyanoacetylene FTW! $\endgroup$ – Curt F. Dec 18 '18 at 23:50

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