All rocket engines throw fire out the back — but under the skin they're wildly different machines, built around one monstrous problem: feeding a fire that drinks a swimming pool of propellant in seconds.
The pump is the hard part
A rocket engine's combustion chamber runs at pressures far beyond any propellant tank, so fuel must be forced uphill into the fire — and the machine that does it is the turbopump, arguably the hardest component in rocketry. The Saturn V's F-1 engine carried a turbopump producing about 55,000 horsepower — the output of a small power station — just to shove kerosene and oxygen into the chamber fast enough. Turbopumps spin at tens of thousands of RPM, inches from cryogenic liquid on one side and flame on the other. When engineers say engines are hard, they mostly mean this.
Open cycle, closed cycle
The pump needs its own engine, and where that mini-engine's exhaust goes defines the family tree. Gas-generator engines — the F-1, the Merlin — burn a little propellant to spin the pump and dump the exhaust overboard: simple, robust, and slightly wasteful, like a truck idling to run its own fuel pump. Staged-combustion engines route that exhaust into the main chamber instead, wasting nothing: the Shuttle's RS-25 and Russia's legendary closed-cycle engines pioneered it, and Raptor pushes it to the limit with full-flow staged combustion, where both propellants pre-burn to drive the pumps. The reward is efficiency; the price is plumbing that routes fire through fire.
The efficiency scoreboard
Engineers score engines by specific impulse — roughly, miles-per-gallon for rockets, measured in seconds. Chemistry sets the ceiling: kerosene engines reach the low-to-mid 300s in vacuum; hydrogen, the lightest exhaust, tops the chemical league near 450 seconds (the RS-25's home turf) but demands enormous tanks and −253 °C storage. Methane sits deliberately between — around the mid-300s, dense enough for reasonable tanks, clean-burning enough to reuse without overhaul. No choice wins outright; every propellant is a bargain struck between performance, tank size, and turnaround.
Why the new engines drink methane
For sixty years methane was the propellant nobody flew — then Raptor, BE-4, and a wave of new engines all chose it at once. The reasons stack: it burns cleaner than kerosene (no soot clogging a reusable engine), stores at temperatures close to liquid oxygen's (simpler tanks), and one more reason that gives the game away — on Mars, carbon dioxide from the air and ice from the ground can be cooked into methane and oxygen by century-old chemistry. A methane rocket is a rocket that can, in principle, refuel on another planet. The engine choice is the mission statement.