thumb|alt=Photo of person holding flask containing reddish liquid|An [[Erlenmeyer flask containing about of RP-1. It is dyed red but has a natural clear to pale yellow color.]]

RP-1 (Rocket Propellant-1 or Refined Petroleum-1) and similar fuels like RG-1 and T-1 are highly refined kerosene formulations used as rocket fuel. Liquid-fueled rockets that use RP-1 as fuel are known as kerolox rockets. In their engines, RP-1 is atomized, mixed with liquid oxygen (LOX), and ignited to produce thrust. Developed in the 1950s, RP-1 is outwardly similar to other kerosene-based fuels like Jet A and JP-8 used in turbine engines but is manufactured to stricter standards. While RP-1 is widely used globally, the primary rocket kerosene formulations in Russia and other former Soviet countries are RG-1 and T-1, which have slightly higher densities.

Compared to other rocket fuels, RP-1 provides several advantages with a few tradeoffs. Compared to liquid hydrogen, it offers a lower specific impulse, but can be stored at ambient temperatures, has a lower explosion risk, and although its specific energy is lower, its higher density results in greater energy density. Compared to hydrazine, another liquid fuel that can be stored at ambient temperatures, RP-1 is far less toxic and carcinogenic.

Usage and history

thumb|alt=Photo of Saturn V rocket lifting off|[[Apollo 8, Saturn V with 810,700 litres of RP-1 and 1,311,100 liters of LOX in the S-IC first stage]]

RP-1 is a fuel in the first-stage boosters of the Electron, Soyuz, Zenit, Delta I-III, Atlas, Falcon, Antares, and Tronador (rocket)| rockets. It also powered the first stages of the Energia (rocket)|, Titan I, Saturn I and IB, and Saturn V. The Indian Space Research Organisation (ISRO) is also developing an RP-1 fueled engine named SE-2000 for its future rockets.

Development

During and immediately after World War II, alcohols (primarily ethanol, occasionally methanol) were commonly used as fuels for large liquid-fueled rockets. Their 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. Regardless of which hydrocarbon was chosen, it would also have to replace alcohol as a coolant.

Many early rockets burned kerosene, but as burn times, combustion efficiencies, and combustion-chamber pressures increased, engine masses decreased, which led to unmanageable engine temperatures. Raw kerosene used as coolant tends to dissociate and polymerize. Lightweight products in the form of gas bubbles cause cavitation, and heavy ones in the form of wax deposits block narrow cooling passages in the engine. The resulting coolant starvation raises temperatures further, and causes more polymerization which accelerates breakdown. The cycle rapidly escalates (i.e., thermal runaway) until an engine wall rupture or other mechanical failure occurs, and it persists even when the entire coolant flow consists of kerosene. In the mid-1950s rocket designers turned to chemists to formulate a heat-resistant hydrocarbon, with the result being RP-1.

During the 1950s, LOX (liquid oxygen) became the preferred oxidizer to use with RP-1, though other oxidizers have also been employed.

Fractions and formulation

RP-1 is outwardly similar to other kerosene-based fuels, but is manufactured to stricter standards. These include tighter density and volatility ranges, along with significantly lower sulfur, olefin, and aromatic content.

Sulfur and sulfur compounds attack metals at high temperatures, and even very small amounts of sulfur assist polymerization which can harden seals and tubing, therefore sulfur and sulfur compounds are kept to a minimum.

Unsaturated compounds (alkenes, alkynes, and aromatics) are also held to low levels, as they tend to polymerize at high temperatures and long periods of storage. At one 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 levels of 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, with linear alkanes being reduced in number in favor of greater numbers of cyclic and highly branched alkanes. Just as cyclic and branched molecules improve octane rating in petrol, they also significantly increase thermal stability at high temperatures. The most desirable isomers are polycyclics such as ladderanes.

In contrast, the main applications of kerosene (aviation, heating, and lighting), are much less concerned with thermal breakdown and therefore do not require stringent optimisation of their isomers.

In production, these grades are processed tightly to remove impurities and side fractions. Ashes were feared likely to block fuel lines and engine passages, and wear away valves and turbopump bearings, as these are lubricated by the fuel. 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 C<sub>12</sub> mass. Because of the lack of light hydrocarbons, RP-1 has a high flash point and is less of a fire hazard than petrol.

All told, the final product is much more expensive than common kerosene. Any petroleum can produce RP-1 with enough refining, though real-world rocket-grade kerosene is sourced from a small number of oil fields with high-quality base stock, or it can be artificially synthesized. This, coupled with the relatively small demand in a niche market compared to other petroleum users, drives RP-1's high price. Military specifications of RP-1 are covered in MIL-R-25576, and the chemical and physical properties of RP-1 are described in NISTIR 6646.

In Russia and other former Soviet countries, the two main rocket kerosene formulations are T-1 and RG-1. Densities are slightly higher, , compared to RP-1 at .

The Soviets also discovered that even higher densities could be achieved by chilling the kerosene before loading it into the rocket's fuel tanks, although this partially defeated the purpose of using kerosene over other super-chilled fuels. However, operationally, 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. The launcher's central kerosene tank is surrounded on four sides and the top by liquid oxygen tanks with a liquid nitrogen tank at the bottom. The kerosene tanks of the four boosters are relatively small and compact, also located between a liquid oxygen and a liquid nitrogen tank. Thus, once the kerosene was initially chilled, it would remain cold for the brief time needed to finish launch preparations.

While the Soviets would eventually abandon chilling their kerosene, decades later SpaceX would revisit the idea for their Falcon 9 rocket. All versions since the Falcon 9 Full Thrust have used sub-cooled RP-1, chilled to , giving a density increase.

Comparison with other fuels

{| class="wikitable" style="margin: 0; float: right; clear: right; margin-bottom: 0.5em; margin-left: 1em;"

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! colspan="2" | LOX/kerosene

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! scope="row" | Specific impulse| at sea level

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! scope="row" | Specific impulse| in vacuum

Soviet formulations are discussed above. In addition, the Soviets briefly used syntin (), a higher-energy formulation, used in upper stages. Syntin is 1-methyl-1,2-dicyclopropyl cyclopropane (). Russia is also working to switch the Soyuz-2 from RP-1 to "naftil" or "naphthyl".

After the RP-1 standard, RP-2 was developed. The primary difference is an even lower sulfur content. However, as most users accept RP-1, there was little incentive to produce and stock a second, even rarer and more expensive formulation.

The OTRAG group launched test vehicles using more common blends. In at least one instance, a rocket was propelled by diesel fuel. However, no OTRAG rocket came even close to orbit.

References

Further reading

  • Lox/Kerosene propellant
  • Rocket Propellants