Every Raptor engine on Starship burns liquid methane and liquid oxygen, a combination engineers call methalox. Super Heavy's 33 Raptor engines and the ship's six Raptors all use the same propellant, which simplifies plumbing, tankage and ground systems compared with a vehicle that burns different fuels in different stages. SpaceX says each Raptor has roughly twice the thrust of the Merlin engine that powers Falcon 9, and the full stack holds more than 5,000 tonnes of propellant at liftoff.

That choice was not obvious. Most large rockets in history have burned kerosene and oxygen, or hydrogen and oxygen, and each has advantages. Kerosene is dense and easy to store, hydrogen is the most efficient chemical fuel but brutally cold and bulky. Methane sits between them on performance, but it wins on two criteria that matter uniquely for Starship: reusability and Mars.

Kerosene leaves soot — technically, coking — inside engines, coating injectors, turbopumps and combustion chambers with carbon deposits that must be cleaned or replaced between flights. For an expendable rocket that flies once, that does not matter. For a rocket designed to fly again quickly, like an aircraft, it matters enormously: cleaning soot takes time, labour and inspection, and it limits how fast a vehicle can be turned around.

Methane burns much cleaner, producing far less soot, which is one reason SpaceX chose it for a fully reusable system. Combined with a stainless-steel structure that tolerates heat and a heat shield of more than 18,000 tiles on the ship, the cleaner fuel supports the goal of landing, inspecting, refuelling and flying again without major refurbishment. That goal is still being proven — Flight 14 reached orbit in September 2026, but routine rapid reuse has not yet been demonstrated — yet the fuel choice is what makes the goal physically plausible.

The more dramatic reason for methane is Mars itself. In principle, methane can be produced on Mars from resources already there: carbon dioxide from the thin Martian atmosphere, plus water ice from the ground, can be combined through the Sabatier reaction to make methane and oxygen. A crew that lands on Mars could therefore manufacture the return journey's propellant on site, rather than carrying it all the way from Earth — which would roughly double the mass that must be launched.

This is called in-situ resource utilisation, and it transforms Mars mission planning. Without it, every kilogram of return fuel must be lifted from Earth, landed on Mars, and kept cold for months. With it, the outbound ship needs only enough fuel to get there and power the production plant, and the plant spends the crew's surface stay making fuel for the trip home. No Mars fuel plant has yet been built or tested on Mars, so this remains a plan, not a proven capability — but it is the plan that makes a return journey affordable in mass terms.

Methane is not free of drawbacks. It must be kept extremely cold as a liquid — around minus 162 degrees Celsius — and its density is lower than kerosene, so tanks must be larger for the same mass of fuel. That is part of why Starship is so enormous: its booster alone holds about 3,650 tonnes of propellant, and its ship about 1,600 tonnes, in tanks that dominate the vehicle's structure.

Subcooling helps: chilling the propellant below its normal boiling point increases its density, letting SpaceX pack more fuel into the same tanks and gain performance. The trade-off is complexity in ground systems, which must handle super-cold fluids quickly during fuelling, and in orbit, where tanker Starships must transfer cryogenic methane and oxygen to another ship without boiling off unacceptable amounts. That orbital transfer is one of the programme's hardest unsolved problems, and it is a direct consequence of choosing a cryogenic fuel.

For lunar missions, methane's advantage is reusability and commonality rather than local production: the Moon has no carbon-dioxide atmosphere to make methane from, so lunar Starships will still be fuelled from Earth, via tankers in Earth orbit. The benefit there is operational: one fuel, one engine family, and a vehicle that can be refuelled in orbit and sent onward to the Moon without a new rocket each time.

For a Mars city — SpaceX's long-term stated goal — methane is the enabling chemistry. A fleet of ships, each capable of 100 to 150 tonnes of payload and, in design, up to 15 flights per vehicle, could only sustain Mars traffic if return fuel is made locally. Whether that vision is realised depends on proving orbital refuelling, Mars fuel production and rapid reuse in sequence. Methane does not guarantee any of those, but it is the fuel that makes all three conceivable in the same vehicle — and that is why Starship burns it.