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What are the properties of a good rocket fuel?

I figure that in a rocket where the oxygen must be carried the combustion of hydrogen provides 13 kJ per gram (of hydrogen plus oxygen) where as the combustion of methane provides 11 kJ per gram. Liquid methane is much easier to produce and is five times as dense as liquid hydrogen (thus requiring a smaller and thus lighter tank). These factors should outweigh the 20% lower enery output.
Rudi Thomas rudi.thomas@utas.edu.au

When choosing rocket fuels, you should consider not only at the enthalpy of combustion per gram of fuel, but also the heat capacity of the exhaust gas. A lower heat capacity means more thrust.

A high heat capacity for the exhaust gas means energy is wasted on heating the exhaust, instead of being used to generate thrust.

CO2 has a much higher heat capacity than steam does, so the exhaust of your methane rocket (which is 1/3 CO2 and 2/3 steam) will have lower temperature (and thrust) than a hydrogen rocket (with only steam in the exhaust gas) if the mass flux for both rockets is identical.

The gases with lowest heat capacities are monatomic and diatomic, so a good rocket fuel tends to have low average molecular weight exhaust.

Author: Fred Senese senese@antoine.frostburg.edu



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