So, as I pet project I'm designing things, and I've come across an issue: I'm trying to design propellant tanks and such for a vehicle, so I need to find the mix ratio of the propellants. In this case, it's Liquid Oxygen and Methane. CH4 + 2O2 → CO2 + 2H2O Using atomic masses and such, it would seem the best mix ratio would be 2:1, by mass, oxidizer to fuel. However, on Encyclopedia Astronautica, consistently the engines with the highest efficiency (specific impulse) have an Oxidizer to Fuel ratio of 3.4 (big list of engines using these propellants) So, I decided, maybe it's by volume. So, by volume, just plugging in the densities, where LOX has 1.14 g/cm3, and CH4 has 0.424 g/cm3, 1 / 1.14 : 1 / 0.424 = 1 : 2.688 Which still doesn't line up with O:F = 3.4 Any help appreciated, thanks.