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Max Isp
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 to Fuel Ratio
Density (g/ml) .81–1.02
Heat Capacity Ratio
Heat capacity ratio
The heat capacity ratio or adiabatic index or ratio of specific heats, is the ratio of the heat capacity at constant pressure to heat capacity at constant volume . It is sometimes also known as the isentropic expansion factor and is denoted by \gamma or \kappa . The latter symbol kappa is...

Temperature of Combustion 3,670 K

RP-1 (alternately, Rocket Propellant-1 or Refined Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

) is a highly refined form of kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...

 outwardly similar to jet fuel
Jet fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to a standardized international specification...

, used as a rocket fuel. Although having a lower specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 than liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 (LH2), RP-1 is cheaper, can be stored at room temperature, is far less of an explosive hazard and is far denser. By volume, RP-1 is significantly more powerful than LH2 and LOX/RP-1 has a much better Isp
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

-density than LOX/LH2. RP-1 also has a fraction of the toxicity
Toxicity is the degree to which a substance can damage a living or non-living organisms. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a substructure of the organism, such as a cell or an organ , such as the liver...

 and carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic hazards of hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

, 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 (liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

) as the oxidizer, though other oxidizers have also been used. RP-1 is a fuel in the first-stage boosters of the Soyuz-FG
The Soyuz-FG launch vehicle is an improved version of the Soyuz-U, from the R-7 family of rockets, designed and constructed by TsSKB-Progress in Samara...

, Zenit, Delta I-III
Delta rocket
Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...

, Atlas, and Falcon 9
Falcon 9
Falcon 9 is a rocket-powered spaceflight launch system designed and manufactured by SpaceX. Both stages of its two-stage-to-orbit vehicle use liquid oxygen and rocket-grade kerosene propellants...

 rockets. It also powered the first stages of the Energia
Energia was a Soviet rocket that was designed by NPO Energia to serve as a heavy-lift expendable launch system as well as a booster for the Buran spacecraft. Control system main developer enterprise was the NPO "Electropribor"...

, Titan I
Titan I
The Martin Marietta SM-68A/HGM-25A Titan I was the United States' first multistage ICBM . Incorporating the latest design technology when designed and manufactured, the Titan I provided an additional nuclear deterrent to complement the U.S. Air Force's SM-65 Atlas missile...

, Saturn I and IB, and Saturn V
Saturn V
The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...


During and immediately after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

s (primarily ethyl alcohol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, occasionally methyl alcohol) 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
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....

 fuels would increase engine efficiency, due to a slightly higher density, the lack of an oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 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
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...

 and polymerize
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

. 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 (i.e., thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

 would occur) 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
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 compounds were severely restricted
Hydrodesulfurization is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils...

. 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
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...


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
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...


The more desirable isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s 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
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...

 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
A ladderane is an organic molecule containing two or more fused rings of cyclobutane. The name is a portmanteau because the serial cyclobutane rings look like a ladder and are singly bonded like alkanes. The chemical formula for a ladderane with n rings is C2n+2H2n+6...


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
A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...

 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
Flash point
The flash point of a volatile material is the lowest temperature at which it can vaporize to form an ignitable mixture in air. Measuring a flash point requires an ignition source...

, and is less of a fire hazard than gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

/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
Gram per litre
A gram per liter or litre is a unit of measurement of mass concentration that shows how many grams of a certain substance are present in one litre of a usually liquid or gaseous mixture. It is not an SI unit because it contains the non-SI unit litre...

, 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
R-7 Semyorka
The R-7 was a Soviet missile developed during the Cold War, and the world's first intercontinental ballistic missile. The R-7 made 28 launches between 1957 and 1961, but was never deployed operationally. A derivative, the R-7A, was deployed from 1960 to 1968...

-based rockets, the temperature penalty was minor. Facilities were already in place to manage the vehicle's cryogenic liquid oxygen
Liquid oxygen
Liquid oxygen — abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries — is one of the physical forms of elemental oxygen.-Physical properties:...

 and liquid nitrogen
Liquid nitrogen
Liquid nitrogen is nitrogen in a liquid state at a very low temperature. It is produced industrially by fractional distillation of liquid air. Liquid nitrogen is a colourless clear liquid with density of 0.807 g/mL at its boiling point and a dielectric constant of 1.4...

, 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. Hydrogen is the lightest molecule; when combusted with oxygen, the H2O product has a low weight, and thus a high exhaust velocity. Hydrogen engines are operated fuel-rich, so some exhaust is unreacted H2, which is even lighter. Hydrocarbons, on the other hand, produce both H2O and CO2. CO2 is over 2.5 times heavier, slowing the exhaust. It can also absorb significant amounts of combustion energy by generating any of several oscillating modes between the atoms. This is energy that could have instead gone into exhaust velocity, and thus, thrust. The heavier oxygen atoms absorb much more energy than the two hydrogens of H2O. American designed hydrocarbon engines are also run fuel-rich, which produces some CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 instead of CO2. But this also results in incomplete combustion, producing some organics of high molecular weight and numerous vibration modes. All told, kerosene engines generate an Isp
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 in the range of 270 to 360 seconds, while hydrogen engines achieve 370–465 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
Carbonization or carbonisation is the term for the conversion of an organic substance into carbon or a carbon-containing residue through pyrolysis or destructive distillation. It is often used in organic chemistry with reference to the generation of coal gas and coal tar from raw coal...

 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 beforehand. Even cold-flow tests, in which the propellants are not ignited, can leave residues.

On the upside, below a chamber pressure about 1000 psi (6.9 MPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

), 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.

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
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 or propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

 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 (a C3 molecule) is very difficult to break; methane, with a single carbon atom (C1), 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 (and to a lesser extent propane) reintroduces handling inconveniences that prompted kerosenes in the first place.

The low vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

 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
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 or helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. This creates extra cost and weight. Cryogenic
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...

 or volatile propellants generally do not need a separate pressurant; instead, some propellant is expanded (often with engine heat) 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
Gas generator
A gas generator usually refers to a device, often similar to a solid rocket or a liquid rocket that burns to produce large volumes of relatively cool gas, instead of maximizing the temperature and specific impulse. The low temperature allows the gas to be put to use more easily in many...

 to pressurize the fuel tank; usually, this is exhaust from a turbopump
A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together...

. 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 (CO2, a greenhouse gas), toxic carbon monoxide (CO), hydrocarbon (HC) emissions, and oxides of nitrogen (NOx), while hydrogen (H2) reacts with oxygen (O2) to produce only water (H2O), with some unreacted H2 also released.

RP-1-like fuels

Robert H. Goddard
Robert Hutchings Goddard was an American professor, physicist and inventor who is credited with creating and building the world's first liquid-fueled rocket, which he successfully launched on March 16, 1926...

 initial rockets used gasoline.

John D. Clark
John Drury Clark, Ph.D. was a noted American rocket fuel developer, chemist, and science fiction writer and fan. He was instrumental in the revival of interest in Robert E. Howard's Conan stories and influenced the writing careers of L. Sprague de Camp, Fletcher Pratt, and other authors.- Life and...

 mentions in "Ignition!" that while the RP-1 specification was being developed, Rocketdyne
Rocketdyne was a Rocket engine design and production company headquartered in Canoga Park, California, United States. The company was related to North American Aviation for most of its history. NAA merged with Rockwell International, which was then bought by Boeing in December, 1996...

 was experimenting with diethyl cyclohexane. However, it offered few advantages over RP-1, and was dropped. In addition, the military (NASA did not yet exist) 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
Syntin is a hydrocarbon with the molecular formula C10H16 used as a rocket fuel. It is a mixture of cis and trans isomers. It has a density of 0.851 g/mL, and a boiling point of 158 °C...

 , a higher-energy formulation, used in upper stages
Multistage rocket
A multistage rocket is a rocket that usestwo or more stages, each of which contains its own engines and propellant. A tandem or serial stage is mounted on top of another stage; a parallel stage is attached alongside another stage. The result is effectively two or more rockets stacked on top of or...

. Syntin is 1-methyl 1,2 dicyclopropyl cyclopropane .

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.

It is possible that some groups experimented with turpentine
Turpentine is a fluid obtained by the distillation of resin obtained from trees, mainly pine trees. It is composed of terpenes, mainly the monoterpenes alpha-pinene and beta-pinene...

 fuels. The isoprene
Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

 monomer would form a fairly robust molecule, and blends of its dimer and trimer would be at kerosene weight. It is also possible that the words "turpentine" and "turps" are merely colloquial terms for kerosene, and such references do not actually refer to the tree extract.

OTRAG , was a German company based in Stuttgart, which planned in the late 1970s and early 1980s to develop an alternative propulsion system for rockets. OTRAG was the first commercial developer and producer of space launch vehicles...

 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.

External links