Aside from dissociation/bond energy, nearly all of the energy in the combustion chamber is kinetic. Hill’s Mechanics and Thermodynamics of Propulsion gives us this very useful figure for the energy balance:
A good deal of the energy in the exhaust is still locked up in various high-energy states; these states are primarily related to the degrees of freedom of the gas (and thus gamma) and are more strongly occupied at higher temperatures. I think that the lighter molecular weight gasses have equivalently less energy here, but I’m not entirely sure. This might be something to look into.
Posting this graph has got me confused as well, though. I was going to write about how there’s more energy tied up in the enthalpy of the gas in the exhaust, but that wouldn’t make sense—lower MW propellants have a higher specific heat per unit mass, and thus would retain more energy at the same temperature.
The one thing to note: the ideal occurs where the gas has the highest speed of sound. I really can’t think of any intuitive way to write this other than “nozzles are marginally more efficient at converting the energy of lighter molecular weight gases from thermal-kinetic to macroscopic kinetic.”
Aside from dissociation/bond energy, nearly all of the energy in the combustion chamber is kinetic. Hill’s Mechanics and Thermodynamics of Propulsion gives us this very useful figure for the energy balance:
A good deal of the energy in the exhaust is still locked up in various high-energy states; these states are primarily related to the degrees of freedom of the gas (and thus gamma) and are more strongly occupied at higher temperatures. I think that the lighter molecular weight gasses have equivalently less energy here, but I’m not entirely sure. This might be something to look into.
Posting this graph has got me confused as well, though. I was going to write about how there’s more energy tied up in the enthalpy of the gas in the exhaust, but that wouldn’t make sense—lower MW propellants have a higher specific heat per unit mass, and thus would retain more energy at the same temperature.
I ran the numbers in Desmos for perfect combustion, an infinite nozzle, and no dissociation, and the result was still there, but quite small:
https://www.desmos.com/calculator/lyhovkxepr
The one thing to note: the ideal occurs where the gas has the highest speed of sound. I really can’t think of any intuitive way to write this other than “nozzles are marginally more efficient at converting the energy of lighter molecular weight gases from thermal-kinetic to macroscopic kinetic.”