What is regenerative cooling in a rocket engine?

A rocket combustion chamber runs far hotter than the metal it is made of could survive unprotected. Something has to carry that heat away continuously, or the engine melts within seconds.

The neat solution is to use the propellant that is already on board and already cold. Fuel is pumped through narrow passages built into the nozzle and chamber walls before it reaches the injector, so it picks up heat on the way to the fire.

Nothing is wasted, which is where the name comes from — the heat is recovered into the propellant rather than dumped overboard. The fuel arrives at the injector slightly warmed, which actually helps it burn.

A Merlin cools its chamber and nozzle this way with its own RP-1 kerosene, and it has to: the chamber runs at 9.7 MPa, roughly a hundred times atmospheric pressure, and the gas inside is far above the melting point of the walls containing it.

It is not the only method. Some engines use film cooling, deliberately running a fuel-rich layer along the wall so the gas touching the metal is cooler than the gas in the middle. Ablative chambers simply let a liner char away, like a heat shield. Regenerative cooling is preferred where an engine has to run for minutes, or be used again.

The catch is kerosene. Held against hot metal it can bake into hard carbon deposits, a process called coking, which is one reason methane engines are easier to reuse.