RP-1 (Refined Petroleum-1) (C₁₂H₂₄ (approximate average)) is a hydrocarbon propellant used as a fuel. Storable at ambient temperature. Typical ISP: 270–340 s (with LOX) seconds.
RP-1 is a highly purified kerosene built specifically to power rockets. Burned with liquid oxygen, it has been lifting heavy machines off the launch pad since the dawn of the space age.
Quick facts
- Type: A fuel made from refined kerosene, burned with liquid oxygen (LOX) as the oxidizer. Together this pairing is nicknamed “kerolox.”
- Density: About 0.81 to 0.82 grams per milliliter (roughly 806 kilograms per cubic meter at 15 degrees Celsius) — far denser than liquid hydrogen.
- Specific impulse with LOX: Roughly 220 to 300 seconds at sea level and about 290 to 340 seconds in vacuum. (Specific impulse is a measure of fuel efficiency; higher means more push per unit of propellant. LOX/hydrogen reaches about 450 seconds.)
- Combustion temperature: About 3,670 kelvin.
- Storage: Stored at ordinary room temperature, not deep-frozen (not “cryogenic”).
- Specification: U.S. military standard MIL-R-25576; chemistry documented in the report NISTIR 6646.
What it is and how it works
RP-1 stands for Rocket Propellant-1 (also called Refined Petroleum-1). It is a complex mixture of hydrocarbons — molecules made of carbon and hydrogen — clustering around 12 carbon atoms each (chemists write this as C12), and dominated by branched and ring-shaped molecules. It is chemically close to jet fuels like Jet A and JP-8, but made to much tighter standards, with far less sulfur and fewer reactive compounds.
RP-1 is the “fuel half” of a two-part propellant. In the engine it is first pumped through channels wrapped around the nozzle and combustion chamber, where it doubles as a coolant — a trick called regenerative cooling, like running cold water through a hot engine block. It is then sprayed (atomized) into the combustion chamber alongside liquid oxygen. The two ignite and burn into hot, high-pressure gas — mostly carbon dioxide and water vapor — that rushes out the nozzle to produce thrust.
The refining is the whole point. Raw kerosene, under the brutal heat and pressure inside an engine, breaks apart and forms gummy deposits. RP-1’s tighter chemistry reduces — though does not eliminate — “coking,” the carbon and soot residue that can clog injectors and cooling passages, especially after shutdown when leftover fuel bakes onto hot metal.
Why it matters
RP-1 with LOX is one of the most important propellant combinations in spaceflight history. Because RP-1 is dense, it fits into smaller, lighter tanks and delivers a strong thrust-to-weight ratio — exactly what a first stage or booster needs to heave a heavy rocket off the ground. And because it stays liquid at room temperature, it is far easier and cheaper to store, handle, and load than super-cold liquid hydrogen, while carrying a lower explosion risk.
The trade-off is efficiency: RP-1’s heavier exhaust molecules cap its specific impulse well below hydrogen’s. So engineers tend to use RP-1 for powerful boosters and save high-efficiency hydrogen for upper stages. Its main drawbacks are coking, which historically meant frequent engine teardowns and complicates reuse, and slightly lower efficiency than newer methane/LOX (“methalox”) systems — one reason several modern launchers are shifting to methane.
Where it is used and notable examples
- Rocketdyne F-1 / Saturn V (Apollo): Five F-1 engines burned RP-1 and LOX, each gulping fuel at roughly 15,471 gallons per minute — the most powerful single-chamber liquid engine ever flown.
- SpaceX Merlin 1D / Falcon 9 and Falcon Heavy: Both stages use rocket-grade RP-1 with LOX. The sea-level Merlin 1D produces about 845 kilonewtons of thrust. To squeeze in more fuel, SpaceX uses sub-cooled or “chilled” RP-1 (around minus 7 degrees Celsius) on Falcon 9 Full Thrust for a roughly 2.5 to 4 percent density gain.
- RD-180 / Atlas V (United Launch Alliance): A Russian oxygen-rich staged-combustion kerolox engine giving about 3,826 kilonewtons of sea-level thrust and around 338 seconds of vacuum specific impulse.
- Rocket Lab Electron: A small commercial launcher whose Rutherford engines burn RP-1/LOX using electric pumps.
- Others: Titan I, Saturn I/IB, the Soviet Energia (RD-170), the Soyuz family, Zenit, Delta, and Antares all rely on RP-1-class kerosene with LOX. Related grades include RP-2 (even lower sulfur) and the Soviet/Russian analogs RG-1 and T-1 (slightly denser, about 0.82 to 0.85 grams per milliliter).
Developed in the 1950s to replace ethanol and provide consistent performance. Specified by the US military for rocket use, it became the dominant hydrocarbon rocket fuel worldwide.
High density, storable, safe handling, low cost, well-understood combustion
Lower Isp than hydrogen, coking at high temperatures, soot production
