RP-1 is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket
fuel. Although having a lower specific impulse than liquid hydrogen, RP-1 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.
Usage and history RP-1 is most commonly burned with LOX 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.
During and immediately after World War II, alcohols were the single most common fuel
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. Many early rockets had burned kerosene, but
as burn times, combustion efficiencies, and combustion-chamber pressures grew, and as
engine masses shrank, the engine temperatures became unmanageable. Raw kerosene used as
coolant would dissociate and polymerize. Lightweight products in the form of gas bubbles, and heavy
ones in the form of engine deposits, then blocked the narrow cooling passages. The coolant
starvation raised temperatures further, accelerating breakdown. This cycle would escalate rapidly
until an engine wall ruptured. This occurred even with the entire flow of
kerosene used as coolant. Rocket designers turned to the fuel chemists to formulate a
heat-resistant hydrocarbon. The specification was completed in the mid-1950s.
Fractions and formulation First, sulfur compounds were severely restricted.
Small amounts of sulfur are naturally present in fossil fuels. It was 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. Comparison with other fuels
Chemically, a hydrocarbon propellant will be less efficient than hydrogen fuel because
hydrogen releases more energy per unit mass during combustion, enabling a higher exhaust
velocity. This is, in part, a result of the high mass of carbon atoms relative to hydrogen
atoms. Hydrocarbon engines are also typically run fuel-rich, which produces some CO instead
of CO2 as a consequence of incomplete combustion, although this is not unique to hydrocarbon
engines, as hydrogen engines are also typically run fuel-rich for the best overall performance.
Some Russian engines run their turbopump preburners oxygen-rich, but the main combustion chamber
is still run fuel-rich. All told, kerosene engines generate an Isp in the range of 270
to 360 seconds, while hydrogen engines achieve 370 to 465 seconds.
During engine shutdown, fuel flow goes to zero rapidly, while the engine is still quite
hot. Residual and trapped fuel can polymerize or even carbonize at hot spots or in hot components.
Even without hot spots, heavy fuels can create a petroleum residue, as can be seen in gasoline,
diesel, or jet fuel tanks that have been in service for years. Rocket engines have cycle
lifetimes measured in minutes or even seconds, preventing truly heavy deposits. However,
rockets are much more sensitive to a deposit, as described above. Thus, kerosene systems
generally entail more teardowns and overhauls, creating operations and labor expenses. This
is a problem for expendable engines as well as reusable ones, because engines must be
ground-fired some number of times before launch. Even cold-flow tests, in which the propellants
are not ignited, can leave residues. On the upside, below a chamber pressure of
about 1000 psi, kerosene can produce sooty deposits on the inside of the nozzle and chamber
liner. This acts as a significant insulation layer, and can reduce the heat flow into the
wall by roughly a factor of two. Most modern hydrocarbon engines however run above this
pressure, therefore this is not a significant effect for most engines.
Recent heavy-hydrocarbon engines have modified components and new operating cycles, in attempts
to better manage leftover fuel, achieve a more-gradual cooldown, or both. This still
leaves the problem of non-dissociated petroleum residue. Other new engines have tried to bypass
the problem entirely, by switching to light hydrocarbons such as methane or propane gas.
Both are volatiles, so engine residues simply evaporate. If necessary, solvents or other
purgatives can be run through the engine to finish dispersion. The short-chain carbon
backbone of propane is very difficult to break; methane, with a single carbon atom, is technically
not a chain at all. The breakdown products of both molecules are also gases, with fewer
problems due to phase separation, and much less likelihood of polymerization and deposition.
However, methane reintroduces handling inconveniences that prompted kerosenes in the first place.
The low vapor pressure of kerosenes gives safety for ground crews. However, in flight
the kerosene tank will need a separate system of pressurization, to replace fuel volume
as it drains. Generally, this is a separate tank of liquid or high-pressure inert gas,
such as nitrogen or helium. This creates extra cost and weight. Cryogenic or volatile propellants
generally do not need a separate pressurant; instead, some propellant is expanded into
low-density gas, and routed back to its tank. A few highly-volatile propellant designs do
not even need the gas loop; some of the liquid automatically vaporizes to fill its own container.
Some rockets use gas from a gas generator to pressurize the fuel tank; usually, this
is exhaust from a turbopump. Although this saves the weight of a separate gas system,
the loop now has to handle a hot, reactive gas instead of a cool, inert one.
Regardless of chemical constraints, RP-1 has supply constraints, due to the very small
size of the launch-vehicle industry versus other consumers of petroleum. While the material
price of such a highly refined hydrocarbon is still less than many other rocket propellants,
the number of RP-1 suppliers is limited. A few engines have attempted to use more standard,
wide-distribution petroleum products such as jet fuel or even diesel. By using alternate
or supplemental engine cooling methods, some can tolerate the non-optimal formulations.
Any hydrocarbon-based fuel when burned produces more air pollution than hydrogen. Hydrocarbon
combustion produces carbon dioxide, toxic carbon monoxide, hydrocarbon emissions, and
oxides of nitrogen, while hydrogen reacts with oxygen to produce only water, with some
unreacted H2 also released. RP-1-like fuels
Robert H. Goddard's initial rockets used gasoline. John Drury Clark mentions in "Ignition!" that
while the RP-1 specification was being developed, Rocketdyne was experimenting with diethyl
cyclohexane. However, it offered few advantages over RP-1, and was dropped. In addition, the
military preferred RP-1 because it was processed alongside jet fuels in the same refineries.
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 (C 10H
16). 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. The later, loosely-similar Scorpius rocket
used Jet A, again without ever reaching orbit. References
External links NASA page on propellants
Lox/Kerosene propellant Last Fight of Hydrocarbons