Methalox is the shorthand for a liquid methane (CH4) and liquid oxygen (LOX) propellant combination that has become the fuel of choice for next-generation launch vehicles. SpaceX's Raptor, Blue Origin's BE-4, Relativity's Aeon R, and Stoke Space's engine all use methalox. It offers a favorable balance of specific impulse, density, and clean combustion compared to kerosene-based fuels.
A key advantage of methalox is reusability: methane burns cleanly, producing far less coking and soot than RP-1, which reduces engine refurbishment between flights. Additionally, methane can theoretically be produced on Mars using the Sabatier reaction with atmospheric CO2 and water ice, making it central to plans for in-situ resource utilization on future Mars missions.