RP-1
Initialism of rocket propellant 1 (a highly refined kerosene; usually used in combination with LOX).
RP-1: kerosene built to fly rockets
RP-1 is a highly refined kerosene fuel specification developed in the 1950s for use in liquid-fueled rocket engines, typically paired with liquid oxygen (LOX) as the oxidizer. The name stands for Rocket Propellant 1. Unlike commercial jet fuel or heating oil, RP-1 is purified to remove sulfur compounds, aromatics, and other contaminants that would corrode engine walls or foul injector plates under the extreme pressures and temperatures of rocket combustion.
The fuel meets strict ASTM D6615 specifications in the United States. RP-1 has a specific gravity between 0.775 and 0.815, a pour point below minus 60 degrees Celsius, and flash point around 38 degrees Celsius. It burns with LOX at a mixture ratio typically between 2.3 and 2.8 parts RP-1 per part oxidizer by weight, depending on engine design. The pairing produces an exhaust velocity sufficient for orbital applications while remaining relatively simple and reliable compared to hydrogen fuel alternatives.
RP-1 is stored in aluminum or steel tanks and handled as a hazardous liquid. It does not spontaneously ignite with LOX at room temperature, unlike hypergolic propellants such as hydrazine, which makes ground handling and launch pad operations marginally safer. However, RP-1 leaves carbon residue in the combustion chamber; engines using it require more frequent inspection of throat liners and injector faces than those burning hydrogen.
Most large Soviet-derived launch vehicles, including the Soyuz and R-7 family, use RP-1 with LOX. American rockets including early Atlas and Delta variants burned RP-1, though modern heavy-lift designs have shifted toward methane or hydrogen. The fuel is also called Kerosene in some aerospace contexts, but RP-1 refers specifically to the aerospace-grade product, not the broader kerosene category. Minor formulation variants exist for different engine architectures and altitude profiles, though ASTM D6615 covers the standard military and commercial grade.
Storage life is indefinite if tanks are sealed and kept cool, but RP-1 absorbs water readily and can stratify over months. Tank inspection, water content testing, and periodic recertification are routine maintenance tasks at launch facilities. Once loaded into a rocket, RP-1 remains a consumable; it is not recovered or recycled after a flight.