Liquid fuel / Oxygen proportions

Question

A question ive been thinking about: If an engine works of a liquid propellant(Hydrogen in this case) and LOX, is there a specific proportion required for the two chemicals to burn completely? If so what is it? Otherwise, explain how it may be different, say, if it depends on the specific engine. Thanks

Answer 1

There is indeed a ratio of fuel to oxidizer that yields complete combustion, called the stoichiometric ratio. This produces the most thermal energy from combustion, but for a few different reasons, specific engines run at different ratios, most often on the fuel-rich side.

As it happens, a very similar question came up a few days ago which explained some of this; a fuel-rich combustion can produce better efficiency in spite of the lower thermal energy release.

You wouldn't want to run oxidizer-rich, because the oxidizer that doesn't combust with fuel will react with the metal of the combustion chamber and nozzle.

Since RP-1 kerosene is a blend of hydrocarbons, the stoichiometric ratio is difficult to calculate; the optimum performance ratio is about 2.58-2.77:1 (ratios are normally given in form of oxidizer-mass-to-fuel-mass).

For hydrogen engines, the stoichiometric ratio is 8:1 -- oxygen atoms are 16 times as massive as hydrogen atoms, and we're trying to combine two hydrogens with one oxygen to make H₂O. The optimum ratio for engine performance, however, is 4.13-4.83:1, with nearly half the hydrogen uncombusted, but because hydrogen's density is so low, going with a higher oxidizer ratio yields savings in fuel tank volume, so the overall best performance for a hydrolox rocket is somewhere in the 5.5-6.0:1 range.