What is RP-1 rocket fuel?

The name stands for Rocket Propellant-1, sometimes written Refined Petroleum-1. It is ordinary kerosene put through much stricter refining: a narrow density band so every tankful behaves the same, and far less sulphur, which would otherwise attack the engine from the inside.

It is dense, safe to handle at room temperature and cheap compared with the alternatives, which is why it has been used since the 1950s. Falcon 9 burns it with sub-cooled liquid oxygen. The mixture runs deliberately fuel-rich, which is why the exhaust is bright orange rather than clear.

Why not hydrogen? Hydrogen is the better fuel on paper — kerosene engines manage roughly 270–360 seconds of specific impulse against 370–465 for hydrogen. But hydrogen has to be kept at −253 °C and is so light that it needs enormous tanks. RP-1 sits in a barrel at room temperature, at 0.81 g per millilitre, so the same energy fits in a far smaller rocket. A Merlin gets 282 seconds at sea level and 311 in vacuum.

It also does a second job on the way to the fire. Before it is burned, the kerosene is pumped through narrow channels in the engine wall to carry heat away — the engine is cooled by its own fuel.

Its drawback is soot. When the engine shuts down, fuel trapped against hot metal bakes into hard carbon deposits, a process called coking, and engines historically had to be stripped down to clean it out. That matters enormously if you intend to fly the booster again, and it is why Starship moved to methane, which burns clean.

The Saturn V first stage burned it too — five F-1 engines drinking RP-1 and liquid oxygen for the first two and a half minutes of every Apollo launch.