RP-1

RP-1 (alternately, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 has a lower specific impulse than liquid hydrogen (LH₂), but it is cheaper, stable at room temperature, far less of an explosion hazard, and far denser. RP-1 is significantly more powerful than LH₂ by volume. RP-1 also has a fraction of the toxicity and carcinogenic hazards of hydrazine, another room-temperature liquid fuel. Thus, kerosene fuels are more practical for many uses.

RP-1 is most commonly burned with LOX (liquid oxygen) as the oxidizer, though other oxidizers have also been used. RP-1 is a fuel in the first-stage boosters of the Soyuz-FG, Zenit, Delta I-III, Atlas, Falcon 9, Antares and Tronador II rockets. It also powered the first stages of the Energia, Titan I, Saturn I and IB, and Saturn V. ISRO is also developing a RP-1 fueled engine for its future rockets.

During and immediately after World War II, alcohols (primarily ethanol, occasionally methanol) were the most common fuels for large liquid-fueled rockets. Its high heat of vaporization kept regeneratively cooled engines from melting, especially considering that alcohols would typically contain several percent water. However, it was recognized that hydrocarbon fuels would increase engine efficiency, due to a slightly higher density, the lack of an oxygen atom in the fuel molecule, and negligible water content. Whatever hydrocarbon was chosen, though, would have to replicate alcohol's coolant ability.

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