RP-1 - Fractions and Formulation

Fractions and Formulation

First, sulfur compounds were severely restricted. Small amounts of sulfur are naturally present in fossil fuels. It had already been known that sulfur and sulfur compounds attack metals at high temperatures. In addition, even small amounts of sulfur will assist polymerization.

Alkenes and aromatics were held to very low levels. These unsaturated hydrocarbons tend to polymerize not only at temperature, but during long periods of storage. At the time, it was thought that kerosene-fueled missiles might remain in storage for years awaiting activation. This function was later transferred to solid-fuel rockets, though the high-temperature benefits of saturated hydrocarbons remained. Because of the low alkenes and aromatics, RP-1 is less toxic than various jet and diesel fuels, and far less toxic than gasoline.

The more desirable isomers were selected or synthesized. Linear alkanes were removed in favor of highly branched and cyclic molecules. This increased resistance to thermal breakdown, much as these isomer types improve octane rating in piston engines. Jet engines and heating and lighting applications, the prior users of kerosene, had been much less concerned with thermal breakdown and isomer contents. The most desirable isomers were polycyclics, loosely resembling ladderanes.

In production, these grades were processed tightly to remove impurities and side fractions. Ashes were feared likely to block fuel lines and engine passages, as well as wear away valves and turbopump bearings which were lubricated by the fuel itself. Slightly too-heavy or too-light fractions affected lubrication abilities, and were likely to separate during storage and under load. The remaining hydrocarbons are at or near C₁₂ weight. Because of the lack of light hydrocarbons, RP-1 has a high flash point, and is less of a fire hazard than gasoline/petrol or even some jet and diesel fuels.

All told, the final product is more expensive than straight-run kerosene. On paper, any petroleum can produce some RP-1 with enough processing. In practice, the fuel is sourced from a small number of oil fields with high-quality base stock. This, coupled with small demand in a niche market compared to other petroleum users, drives the price. Military specifications of RP-1 are covered in MIL-R-25576 and some chemical and physical properties of RP-1 and RP-2 are tabulated here

Soviet and Russian rocket-grade kerosenes are very similar to RP-1 and are designated T-1 and RG-1. Densities are higher, 0.82 to 0.85 g/ml, compared to RP-1 at 0.81 g/ml. For a short period, the Soviets achieved even higher densities by super-chilling the kerosene in a rocket’s fuel tanks, but this partially defeated the purpose of using kerosene over other super-chilled fuels. In the case of the Soyuz and other R7-based rockets, the temperature penalty was minor. Facilities were already in place to manage the vehicle's cryogenic liquid oxygen and liquid nitrogen, both of which are far colder than the kerosene temperature. The launcher's central kerosene tank is surrounded on four sides and the top by liquid oxygen tanks; the liquid nitrogen tank is nearby at the bottom. The kerosene tanks of the four boosters are relatively small and compact, and also between a liquid oxygen and a liquid nitrogen tank. Thus, once the kerosene was chilled initially, it could remain so for the brief time needed to finish launch preparations.