Oxygen is the vast majority of the propellant mass, I'm afraid. Methalox is CH4+2O2->CO2+2H2O, so you need 4 Oxygen atoms for every Carbon atom (with Hydrogen b

Oxygen is the vast majority of the propellant mass, I'm afraid. Methalox is CH4+2O2->CO2+2H2O, so you need 4 Oxygen atoms for every Carbon atom (with Hydrogen being a rounding error).