Why SpaceX Chose Methane

I have driven out to Starbase four times since 2024. Before I look at any rocket, I look at the tank farm. Two of the frost-covered cylinders hold what Raptor actually runs on: one liquid oxygen, one liquid methane. That second tank is the reason Raptor is not a Merlin.

The Falcon 9 and Falcon Heavy sitting up the coast burn something else. Their propellant is RP-1, a refined kerosene, with liquid oxygen as the oxidizer. The pairing is cheap, stable, and has flown for decades. The Saturn V first stage that carried Apollo off the same kind of pad burned kerosene and oxygen too. So when Raptor turned out to burn methane, the fair question was why change a formula that already put people on the Moon.

The answer is not efficiency. On specific impulse, the standard measure of thrust per unit of propellant per second, hydrogen beats both kerosene and methane. It is the lightest molecule, so for a given amount of energy it leaves the nozzle fastest, which is exactly what you want. That is why Europe's first flight hydrogen engine, the HM7 on the third stage of Ariane 1 in December 1979, ran on it. It is why the Saturn V upper stages and the Space Shuttle main engines did the same.

Hydrogen carries two penalties that get worse the farther you travel.

The first is storage. Hydrogen stays liquid only below about minus 253 Celsius. That is colder than liquid methane at minus 162 or liquid oxygen at minus 183, and it sits roughly twenty degrees above absolute zero. It demands heavy insulation, and the moment that insulation underperforms, the propellant boils off.

The second is leakage. The hydrogen molecule is the smallest that exists. It works its way between metal grains, and no seal stops it entirely. On a Mars transfer of about six months, a hydrogen tank loses propellant from the day it leaves.

Kerosene avoids both problems and is proven hardware. Merlin let SpaceX reland and reuse Falcon boosters, and the F-1 sent crews to the Moon. So methane had to beat kerosene, not only hydrogen. It did so on three counts.

First, reuse. A kerosene engine lays down carbon deposits, called coking, inside the chamber and passages, so it wants a teardown between flights. Early Falcon Merlins needed weeks of work for that reason. Methane is the simplest hydrocarbon, one carbon bonded to four hydrogens, and it burns close to clean, leaving little soot. That is what lets the same engine fly again quickly instead of going back to a bench.

Second, you can make it on Mars. The Martian atmosphere is about 95 percent carbon dioxide. Extract water from subsurface ice, split it into hydrogen and oxygen, and run the Sabatier reaction, CO2 plus 4 H2 yielding CH4 plus 2 H2O, and you have methane for the trip home. Kerosene is theoretically possible on Mars through Fischer-Tropsch chemistry, but that process is far more involved. Methane is the one fuel that is both simple to synthesize there and easy to store.

Third, the environment, read honestly. Methane is a strong greenhouse gas: over a twenty-year window its warming potential is roughly eighty times that of carbon dioxide. But that figure describes methane vented into the air. When Raptor lights, the methane burns, CH4 plus 2 O2 to CO2 plus 2 H2O, so what leaves the nozzle is mostly carbon dioxide and water vapor, not methane. Raptor's full-flow staged combustion runs at high combustion efficiency, which keeps unburned methane low, and its cleaner burn means far less of the stratospheric black carbon that kerosene engines throw off. None of this makes a rocket harmless. It makes methalox the lower-soot option among large engines, at a time when the entire launch industry still accounts for well under a hundredth of a percent of global carbon dioxide.

That is the trade Musk made. Hydrogen owned the best number on a spec sheet. Methane gave him an engine he could reuse, refuel on another planet, and burn cleaner. For a vehicle whose whole reason to exist is reaching Mars and coming back, the spec sheet was never going to be the deciding line.


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