![RP-1](https://upload.wikimedia.org/wikipedia/commons/d/dc/RP-1_.jpg)
RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of
kerosene
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
outwardly similar to
jet fuel
Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...
, used as
rocket fuel
Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine to produce thrust. The energy required can either come from the propellants themselves, as with a chemical ...
. RP-1 provides a lower
specific impulse
Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse i ...
than
liquid hydrogen
Liquid hydrogen (LH2 or LH2) 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 cooled below its critical point of 33 K. However, for it to be in a fully li ...
(LH
2), but is cheaper, is stable at room temperature, and presents a lower explosion hazard. RP-1 is far denser than LH
2, giving it a higher
energy density
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extract ...
(though its
specific energy
Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, sto ...
is lower). RP-1 also has a fraction of the
toxicity
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
and
carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...
ic hazards of
hydrazine
Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
, another room-temperature liquid fuel.
Usage and history
![Apollo 8 Liftoff](https://upload.wikimedia.org/wikipedia/commons/5/57/Apollo_8_liftoff.jpg)
RP-1 is a fuel in the first-stage boosters of the
Electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
,
Soyuz Soyuz is a transliteration of the Cyrillic text Союз (Russian and Ukrainian, 'Union'). It can refer to any union, such as a trade union (''profsoyuz'') or the Union of Soviet Socialist Republics (Сою́з Сове́тских Социалис ...
,
Zenit
Zenit, meaning "zenith", may refer to:
Spaceflight and rocketry
* Zenit (rocket family), a Soviet family of space launch vehicles
* Zenit (satellite), a type of Soviet spy satellite
* Zenit sounding rocket, a Swiss rocket
Sports
* Zenit (sports ...
,
Delta I-III,
Atlas
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographic ...
,
Falcon
Falcons () are birds of prey in the genus ''Falco'', which includes about 40 species. Falcons are widely distributed on all continents of the world except Antarctica, though closely related raptors did occur there in the Eocene.
Adult falcons ...
,
Antares
Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ S ...
, and
Tronador II rockets. It also powered the first stages of the
Energia,
Titan I
The Martin Marietta SM-68A/HGM-25A Titan I was the United States' first multistage intercontinental ballistic missile (ICBM), in use from 1959 until 1962. Though the SM-68A was operational for only three years, it spawned numerous follow-on mod ...
,
Saturn I and IB, and
Saturn V
Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, with multistage rocket, three stages, and powered with liquid-propellant r ...
. The
Indian Space Research Organization (ISRO) is also developing an RP-1 fueled engine for its future rockets.
Development
During and immediately after World War II, alcohols (primarily
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
, occasionally
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
) were commonly used as fuels for large
liquid-fueled rockets. Their high
heat of vaporization
The enthalpy of vaporization (symbol ), also known as the (latent) heat of vaporization or heat of evaporation, is the amount of energy (enthalpy) that must be added to a liquid substance to transform a quantity of that substance into a gas. T ...
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 are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
fuels would increase engine efficiency, due to a slightly higher
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
, the lack of an
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
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
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
, 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
Dissociation in chemistry is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or radicals, usually in a reversible manner. For instance, when an acid ...
and
polymerize
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many for ...
. 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
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way t ...
) 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 the chemists to formulate a heat-resistant hydrocarbon, with the result being RP-1.
During the 1950s, LOX (
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an applica ...
) became the preferred oxidizer to use with RP-1,
though other oxidizers have also been employed.
Fractions and formulation
First, sulfur and sulfur compounds attack metals at high temperatures, and even very small amounts of sulfur
assist polymerization. Therefore, sulfur and sulfur compounds are
kept to a minimum.
Alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s,
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s, and
aromatics
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...
are also held to low levels, as they tend to polymerize at high temperatures and 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 rocket
A solid-propellant rocket or solid rocket is a rocket with a rocket engine that uses solid propellants ( fuel/oxidizer). The earliest rockets were solid-fuel rockets powered by gunpowder; they were used in warfare by the Arabs, Chinese, Persian ...
s, 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
isomer
In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers.
Iso ...
s were selected or synthesized, with
linear alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whic ...
s being reduced in number in favor of greater numbers of cyclic and highly branched alkanes. Just as cyclic and branched molecules improve
octane rating
An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...
in
petrol
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
, they also significantly increase thermal stability at high temperatures. The most desirable isomers are polycyclics such as
ladderane
In chemistry, a ladderane is an organic molecule containing two or more fused cyclobutane rings. The name arises from the resemblance of a series of fused cyclobutane rings to a ladder. Numerous synthetic approaches have been developed for the synt ...
s.
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, although sulfur and sulfur compounds are still minimized.
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
A turbopump is a propellant pump with two main components: a rotodynamic pump and a driving gas turbine, usually both mounted on the same shaft, or sometimes geared together. They were initially developed in Germany in the early 1940s. The purpos ...
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
12 mass. Because of the lack of light hydrocarbons, RP-1 has a high
flash point
The flash point of a material is the "lowest liquid temperature at which, under certain standardized conditions, a liquid gives off vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture". (EN 60079-10-1)
The fl ...
and is less of a fire hazard than petrol.
All told, the final product is much more expensive than common kerosene. Although any petroleum can produce RP-1 with enough processing, 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, 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 the rocket's fuel tanks, but this partially defeated the purpose of using kerosene over other super-chilled fuels. In the case of the
Soyuz Soyuz is a transliteration of the Cyrillic text Союз (Russian and Ukrainian, 'Union'). It can refer to any union, such as a trade union (''profsoyuz'') or the Union of Soviet Socialist Republics (Сою́з Сове́тских Социалис ...
and
R-7, the temperature penalty was minor. Facilities were already in place to manage the vehicle's cryogenic
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an applica ...
and
liquid nitrogen
Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...
, 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; 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. The latest version of Falcon 9,
Falcon 9 Full Thrust
Falcon 9 Full Thrust (also known as Falcon 9 v1.2, with variants Block 1 to Block 5) is a partially reusable medium-lift launch vehicle, designed and manufactured by SpaceX. Designed in 2014–2015, Falcon 9 Full Thrust began launch operations ...
, also has the capability of sub-cooling the RP-1 fuel to −7 °C, giving a 2.5–4% density increase.
Comparison with other fuels
Chemically, a hydrocarbon propellant is 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 CO
2 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 a
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
Carbonization is the conversion of organic matters like plants and dead animal remains into carbon through destructive distillation.
Complexity in carbonization
Carbonization is a pyrolytic reaction, therefore, is considered a complex process ...
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 , 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
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
or
propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
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 C
3 molecule) is very difficult to break; methane, with a single carbon atom (C
1), 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 (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases ...
of kerosenes gives safety for ground crews. However, in flight the kerosene tank needs a separate pressurization system 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 the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
or
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
. This creates extra cost and weight.
Cryogenic
In physics, cryogenics is the production and behaviour of materials at very low temperatures.
The 13th IIR International Congress of Refrigeration (held in Washington DC in 1971) endorsed a universal definition of “cryogenics” and “cr ...
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
A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a ...
to pressurize the fuel tank; usually, this is exhaust from a
turbopump
A turbopump is a propellant pump with two main components: a rotodynamic pump and a driving gas turbine, usually both mounted on the same shaft, or sometimes geared together. They were initially developed in Germany in the early 1940s. The purpos ...
. 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, widely distributed petroleum products such as jet fuel or even diesel, fo
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, widely distributed petroleum products such as jet fuel or even diesel (for example, ABL Space Systems' E2 engine can run on either RP-1 or Jet-A). By using alternate or supplemental engine cooling methods, some engines can tolerate the non-optimal formulations.
Any hydrocarbon-based fuel produces more air pollution when burned than hydrogen alone. Hydrocarbon combustion produces carbon dioxide (CO
2), carbon monoxide (CO), and hydrocarbon (HC) emissions, while hydrogen (H
2) reacts with oxygen (O
2) to produce only water (H
2O), with some unreacted H
2 also released. Both hydrocarbon-based fuels and hydrogen fuel will create oxides of nitrogen (NO
''x'') pollutants, because
rocket exhaust temperatures above 1600°C (2900°F) will
thermally combine some of the nitrogen (N
2) and oxygen (O
2) already present in the atmosphere, to create oxides of nitrogen.
RP-1-like fuels
Robert H. Goddard
Robert Hutchings Goddard (October 5, 1882 – August 10, 1945) was an American engineer, professor, physicist, and inventor who is credited with creating and building the world's first Liquid-propellant rocket, liquid-fueled rocket. ...
's initial rockets used gasoline.
While the RP-1 specification was being developed,
Rocketdyne
Rocketdyne was an American rocket engine design and production company headquartered in Canoga Park, California, Canoga Park, in the western San Fernando Valley of suburban Los Angeles, California, Los Angeles, in southern California.
The Rocke ...
was experimenting with
diethyl cyclohexane
In organic chemistry, an ethyl group (abbr. Et) is an alkyl substituent with the chemical formula, formula , derived from ethane (). ''Ethyl'' is used in the International Union of Pure and Applied Chemistry's IUPAC nomenclature of organic che ...
. While superior to RP-1, it was never adopted for use – its formulation was not finished before development of Atlas and Titan I (designed around RP-1) leading to RP-1 becoming the standard hydrocarbon rocket fuel.
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 four stereoisomers (see below). It has a density of 0.851 g/mL, and a boiling point of 158 °C. Due to the presence of three strained cyclo ...
(russian: link=no, синтин), 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
Soyuz-2 (GRAU index 14A14) is a modernized version of the Soviet Soyuz rocket. In its basic form, it is a three-stage launch vehicle for placing payloads into low Earth orbit. Compared to the previous versions of the Soyuz, the first-stage bo ...
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
OTRAG (german: links=no, Orbital Transport-und Raketen-Aktiengesellschaft, or ), was a German company based in Stuttgart, which in the late 1970s and early 1980s planned to develop an alternative propulsion system for rockets. OTRAG was the firs ...
group launched test vehicles using more common blends. In at least one instance, a rocket was propelled by
diesel fuel
Diesel fuel , also called diesel oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and t ...
. However, no OTRAG rocket came even close to orbit.
References
{{Reflist, 30em
External links
NASA page on propellants
Rocket fuels