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The highest
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 is ...
chemical rockets use liquid propellants (
liquid-propellant rocket A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Liquids are desirable because they have a reasonably high density and high specific impulse (''I''sp). This allows the volume of the propellant ta ...
s). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into two categories;
hypergolic propellant A hypergolic propellant is a rocket propellant combination used in a rocket engine, whose components spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. Th ...
s, which ignite when the fuel and
oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
make contact, and non-hypergolic propellants which require an ignition source. About 170 different
propellants A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...
made of liquid fuel have been tested, excluding minor changes to a specific propellant such as propellant additives, corrosion inhibitors, or stabilizers. In the U.S. alone at least 25 different propellant combinations have been flown. As of 2020, no completely new propellant has been used since the mid-1970s. Many factors go into choosing a propellant for a liquid-propellant rocket engine. The primary factors include ease of operation, cost, hazards/environment and performance.


History


Development in early 20th century

Konstantin Tsiolkovsky Konstantin Eduardovich Tsiolkovsky (russian: Константи́н Эдуа́рдович Циолко́вский , , p=kənstɐnʲˈtʲin ɪdʊˈardəvʲɪtɕ tsɨɐlˈkofskʲɪj , a=Ru-Konstantin Tsiolkovsky.oga; – 19 September 1935) ...
proposed the use of liquid propellants in 1903, in his article ''Exploration of Outer Space by Means of Rocket Devices.'' On March 16, 1926,
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-fueled rocket. Goddard successfully laun ...
used
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 a ...
(''LOX'') and
gasoline 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 ...
as rocket fuels for his first partially successful
liquid-propellant rocket A liquid-propellant rocket or liquid rocket utilizes a rocket engine that uses liquid propellants. Liquids are desirable because they have a reasonably high density and high specific impulse (''I''sp). This allows the volume of the propellant ta ...
launch. Both propellants are readily available, cheap and highly energetic. Oxygen is a moderate
cryogen 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 ...
as air will not liquefy against a liquid oxygen tank, so it is possible to store LOX briefly in a rocket without excessive insulation. In Germany, engineers and scientists became enthralled with liquid propulsion, building and testing them in the late 1920s within Opel RAK in Rüsselsheim. According to Max Valier's account, Opel RAK rocket designer, Friedrich Wilhelm Sander launched two liquid-fuel rockets at Opel Rennbahn in Rüsselsheim on April 10 and April 12, 1929. These Opel RAK rockets have been the first European, and after Goddard the world's second, liquid-fuel rockets in history. In his book “Raketenfahrt” Valier describes the size of the rockets as of 21 cm in diameter and with a length of 74 cm, weighing 7 kg empty and 16 kg with fuel. The maximum thrust was 45 to 50 kp, with a total burning time of 132 seconds. These properties indicate a gas pressure pumping. The first missile rose so quickly that Sander lost sight of it. Two days later, a second unit was ready to go, Sander tied a 4,000-meter-long rope to the rocket. After 2000 m or rope had been unwound, the line broke and this rocket also disappeared in the area, probably near the Opel proving ground and racetrack in Rüsselsheim, the "Rennbahn". The main purpose of these tests was to develop the propulsion system for the aircraft for crossing the English channel. Also spaceflight historian
Frank H. Winter Frank H. Winter (born 1942) is an American historian and writer. He is the retired Curator of Rocketry of the National Air and Space Museum (NASM) of the Smithsonian Institution of Washington, D.C. Winter is also an internationally recognized hist ...
, curator at National Air and Space Museum in Washington, DC, confirms the Opel group was working, in addition to their solid-fuel rockets used for land-speed records and the world's first manned rocket-plane flights, on liquid-fuel rockets (SPACEFLIGHT, Vol. 21,2, Feb. 1979): In a cabled exclusive to The New York Times on 30 September 1929, Fritz von Opel is quoted as saying: "Sander and I now want to transfer the liquid rocket from the laboratory to practical use. With the liquid rocket I hope to be the first man to thus fly across the English Channel. I will not rest until I have accomplished that." At a speech on the donation of a RAK 2 replica to the Deutsches Museum, von Opel mentioned also Opel engineer Josef Schaberger as a key collaborator. "He belonged," von Opel said, "with the same enthusiasm as Sander to our small secret group, one of the tasks of which was to hide all the preparations from my father, because his paternal apprehensions led him to believe that I was cut out for something better than being a rocket researchist. Schaberger supervised all the details involved in construction and assembly (of rocket cars), and every time I sat behind the wheel with a few hundred pounds of explosives in my rear, and made the first contact, I did so with a feeling of total security ..As early as 1928, Mr. Schaberger and I developed a liquid rocket, which was definitely the first permanently operating rocket in which the explosive was injected into the combustion chamber and simultaneously cooled using pumps. ..We used benzol as the fuel," von Opel continued, "and nitrogen tetroxide as the oxidizer. This rocket was installed in a Mueller-Griessheim aircraft and developed a thrust of 70 kg (154 lb.)." By May 1929, the engine produced a thrust of 200 kg (440 lb.) "for longer than fifteen minutes and in July 1929, the Opel RAK collaborators were able to attain powered phases of more than thirty minutes for thrusts of 300 kg (660-lb.) at Opel's works in Rüsselsheim," again according to Max Valier's account. The Great Depression brought an end to the Opel RAK activities. Valier's, who died while experimenting in 1930, and Sander's work on liquid-fuel rockets was confiscated by the German military, the Heereswaffenamt and integrated into the activities under General
Walter Dornberger Major-General Dr. Walter Robert Dornberger (6 September 1895 – 26 June 1980) was a German Army artillery officer whose career spanned World War I and World War II. He was a leader of Nazi Germany's V-2 rocket programme and other projects ...
in the early and mid-1930s in a field near Berlin. An amateur rocket group, the VfR, co-founded by Max Valier, included
Wernher von Braun Wernher Magnus Maximilian Freiherr von Braun ( , ; 23 March 191216 June 1977) was a German and American aerospace engineer and space architect. He was a member of the Nazi Party and Allgemeine SS, as well as the leading figure in the develop ...
, who eventually became the head of the army research station that designed the
V-2 rocket The V-2 (german: Vergeltungswaffe 2, lit=Retaliation Weapon 2), with the technical name '' Aggregat 4'' (A-4), was the world’s first long-range guided ballistic missile. The missile, powered by a liquid-propellant rocket engine, was develop ...
weapon for the Nazis. Sander was arrested by Gestapo in 1935, when private rocket-engineering became forbidden in Germany, was convicted of treason to 5 years in prison and forced to sell his company, he died in 1938.


World War II era

Germany had very active rocket development before and during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
, both for the strategic
V-2 rocket The V-2 (german: Vergeltungswaffe 2, lit=Retaliation Weapon 2), with the technical name '' Aggregat 4'' (A-4), was the world’s first long-range guided ballistic missile. The missile, powered by a liquid-propellant rocket engine, was develop ...
and other missiles. The V-2 used an alcohol/LOX liquid-propellant engine, with
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
to drive the fuel pumps. The alcohol was mixed with water for engine cooling. Both Germany and the United States developed reusable liquid-propellant rocket engines that used a storeable liquid oxidizer with much greater density than LOX and a liquid fuel that ignited spontaneously on contact with the high density oxidizer. The major manufacturer of German rocket engines for military use, the HWK firm, manufactured the RLM-numbered 109-500-designation series of rocket engine systems, and either used
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
as a monopropellant for ''Starthilfe'' rocket-propulsive assisted takeoff needs; or as a form of thrust for MCLOS-guided air-sea glide bombs; and used in a bipropellant combination of the same oxidizer with a fuel mixture of hydrazine hydrate and methyl alcohol for rocket engine systems intended for manned combat aircraft propulsion purposes. The U.S. engine designs were fueled with the bipropellant combination of
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
as the oxidizer; and
aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile start ...
as the fuel. Both engines were used to power aircraft, the Me 163 Komet interceptor in the case of the Walter 509-series German engine designs, and RATO units from both nations (as with the ''Starthilfe'' system for the Luftwaffe) to assist take-off of aircraft, which comprised the primary purpose for the case of the U.S. liquid-fueled rocket engine technology - much of it coming from the mind of U.S. Navy officer Robert Truax.


1950s and 1960s

During the 1950s and 1960s there was a great burst of activity by propellant chemists to find high-energy liquid and solid propellants better suited to the military. Large strategic missiles need to sit in land-based or submarine-based silos for many years, able to launch at a moment's notice. Propellants requiring continuous refrigeration, which cause their rockets to grow ever-thicker blankets of ice, were not practical. As the military was willing to handle and use hazardous materials, a great number of dangerous chemicals were brewed up in large batches, most of which wound up being deemed unsuitable for operational systems. In the case of
nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...
, the acid itself () was unstable, and corroded most metals, making it difficult to store. The addition of a modest amount of
nitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russia rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium ...
, , turned the mixture red and kept it from changing composition, but left the problem that nitric acid corrodes containers it is placed in, releasing gases that can build up pressure in the process. The breakthrough was the addition of a little hydrogen fluoride (HF), which forms a self-sealing metal fluoride on the interior of tank walls that ''Inhibited'' Red Fuming Nitric Acid. This made "IRFNA" storeable. Propellant combinations based on IRFNA or pure as oxidizer and kerosene or hypergolic (self igniting)
aniline Aniline is an organic compound with the formula C6 H5 NH2. Consisting of a phenyl group attached to an amino group, aniline is the simplest aromatic amine. It is an industrially significant commodity chemical, as well as a versatile start ...
,
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, hydrazin ...
or
unsymmetrical dimethylhydrazine Unsymmetrical dimethylhydrazine (UDMH; 1,1-dimethylhydrazine, НДМГ or codenamed Geptil) is a chemical compound with the formula H2NN(CH3)2 that is used as a rocket propellant. It is a colorless liquid, with a sharp, fishy, ammonia-like smell ...
(UDMH) as fuel were then adopted in the United States and the Soviet Union for use in strategic and tactical missiles. The self-igniting storeable liquid bi-propellants have somewhat lower specific impulse than LOX/kerosene but have higher density so a greater mass of propellant can be placed in the same sized tanks. Gasoline was replaced by different
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 ...
fuels, for example
RP-1 RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 provides a lower specific impulse than liquid hydrogen (LH2), but is cheaper, i ...
a highly refined grade 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 reg ...
. This combination is quite practical for rockets that need not be stored.


Kerosene

The V-2 rockets developed by Nazi Germany used LOX and ethyl alcohol. One of the main advantages of alcohol was its water content which provided cooling in larger rocket engines. Petroleum-based fuels offered more power than alcohol, but standard gasoline and kerosene left too much silt and combustion by-products that could clog engine plumbing. In addition they lacked the cooling properties of ethyl alcohol. During the early 1950s, the chemical industry in the US was assigned the task of formulating an improved petroleum-based rocket propellant which would not leave residue behind and also ensure that the engines would remain cool. The result was
RP-1 RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 provides a lower specific impulse than liquid hydrogen (LH2), but is cheaper, i ...
, the specifications of which were finalized by 1954. A highly refined form of jet fuel, RP-1 burned much more cleanly than conventional petroleum fuels and also posed less of a danger to ground personnel from explosive vapours. It became the propellant for most of the early American rockets and ballistic missiles such as the Atlas, Titan I, and Thor. The Soviets quickly adopted RP-1 for their R-7 missile, but the majority of Soviet launch vehicles ultimately used storable hypergolic propellants. , it is used in the first stages of many orbital launchers.


Hydrogen

Many early rocket theorists believed that
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
would be a marvelous propellant, since it gives the highest
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 is ...
. It is also considered the cleanest when oxidized with
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 we ...
because the only by-product is water. Steam reforming of
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon ...
is the most common method of producing commercial bulk hydrogen at about 95% of the world production of 500 billion m3 in 1998. At high temperatures (700–1100 °C) and in the presence of a
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...
-based
catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
(
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...
), steam reacts with methane to yield
carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
and hydrogen. Hydrogen in any state is very bulky; it is typically stored as a deeply cryogenic liquid, a technique mastered in the early 1950s as part of the hydrogen bomb development program at Los Alamos.
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 l ...
is stored and transported without boil-off, because
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. ...
, which has a lower boiling point than hydrogen, acts as cooling refrigerant. Only when hydrogen is loaded on a launch vehicle, where no refrigeration exists, it vents to the atmosphere. In the late 1950s and early 1960s it was adopted for hydrogen-fuelled stages such as
Centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; ...
upper stages. Even as a liquid, hydrogen has low density, requiring large tanks and pumps, and the extreme cold requires tank insulation. This extra weight reduces the mass fraction of the stage or requires extraordinary measures such as pressure stabilization of the tanks to reduce weight. Pressure stabilized tanks support most of the loads with internal pressure rather than with solid structures, employing primarily the
tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials ...
of the tank material. The Soviet rocket programme, in part due to a lack of technical capabilities, did not use as a propellant until the 1980s when it was used for the
Energia Energia or Energiya may refer to: * Energia (corporation), or S. P. Korolev Rocket and Space Corporation Energia, a Russian design bureau and manufacturer ** Energia (rocket), a Soviet rocket designed by the company *Energia (company), a company th ...
core stage.


Upper stage use

The liquid-rocket engine propellant combination of
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 a ...
and hydrogen offers the highest specific impulse of currently used conventional rockets. This extra performance largely offsets the disadvantage of low density. Low density of a propellant leads to larger fuel tanks. However, a small increase in specific impulse in an upper stage application can have a significant increase in payload to orbit capability.


Comparison to kerosene

Launch pad fires due to spilled kerosene are more damaging than hydrogen fires, primarily for two reasons. First, kerosene burns about 20% hotter in absolute temperature than hydrogen. The second reason is its buoyancy. Since hydrogen is a deep cryogen it boils quickly and rises due to its very low density as a gas. Even when hydrogen burns, the gaseous that is formed has a molecular weight of only 18 u compared to 29.9 u for air, so it rises quickly as well. Kerosene on the other hand falls to the ground and burns for hours when spilled in large quantities, unavoidably causing extensive heat damage that requires time-consuming repairs and rebuilding. This is a lesson most frequently experienced by test stand crews involved with firings of large, unproven rocket engines. Hydrogen-fuelled engines have special design requirements such as running propellant lines horizontally, so traps do not form in the lines and cause ruptures due to boiling in confined spaces. These considerations apply to all cryogens, such as liquid oxygen and liquid natural gas (LNG) as well. Use of liquid hydrogen fuel has an excellent safety record and superb performance that is well above that of all other practical chemical rocket propellants.


Lithium and fluorine

The highest specific impulse chemistry ever test-fired in a rocket engine was
lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...
and fluorine, with hydrogen added to improve the exhaust thermodynamics (all propellants had to be kept in their own tanks, making this a tripropellant). The combination delivered 542 s specific impulse in a vacuum, equivalent to an exhaust velocity of 5320 m/s. The impracticality of this chemistry highlights why exotic propellants are not actually used: to make all three components liquids, the hydrogen must be kept below –252 °C (just 21 K) and the lithium must be kept above 180 °C (453 K). Lithium and fluorine are both extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with most fuels, including hydrogen. Fluorine and the hydrogen fluoride (HF) in the exhaust are very toxic, which makes working around the launch pad difficult, damages the environment, and makes getting a launch license that much more difficult. Both lithium and fluorine are expensive compared to most rocket propellants. This combination has therefore never flown. During the 1950s, the Department of Defense initially proposed lithium/fluorine as ballistic missile propellants. A 1954 accident at a chemical works where a cloud of fluorine was released into the atmosphere convinced them to instead use LOX/RP-1.


Methane

In NASA's Design Reference Mission 5.0 documents (between 2009 and 2012), liquid methane/ LOX is the chosen propellant mixture for the lander module. ,
SpaceX Space Exploration Technologies Corp. (SpaceX) is an American spacecraft manufacturer, launcher, and a satellite communications corporation headquartered in Hawthorne, California. It was founded in 2002 by Elon Musk with the stated goal of ...
uses Raptor methalox bipropellant rocket engines in test flights for its
Starship A starship, starcraft, or interstellar spacecraft is a theoretical spacecraft designed for interstellar travel, traveling between planetary systems. The term is mostly found in science fiction. Reference to a "star-ship" appears as early as 188 ...
super-heavy-lift launch vehicle. In November 2012, CEO
Elon Musk Elon Reeve Musk ( ; born June 28, 1971) is a business magnate and investor. He is the founder, CEO and chief engineer of SpaceX; angel investor, CEO and product architect of Tesla, Inc.; owner and CEO of Twitter, Inc.; founder of The ...
announced plans to develop liquid methane/ LOX rocket engines. SpaceX had previously used only
RP-1 RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 provides a lower specific impulse than liquid hydrogen (LH2), but is cheaper, i ...
/LOX in SpaceX rocket engines. Although it has a lower specific impulse than liquid hydrogen, liquid methane can be produced on Mars via the Sabatier reaction and is easier to store than liquid hydrogen due to its higher boiling point and density, as well as its lack of
hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbe ...
. It also leaves less residue in the engines compared to kerosene, which is beneficial for reusability. In July 2014,
Firefly Space Systems Firefly Alpha (Firefly α) is a two-stage orbital expendable launch vehicle developed by the American company Firefly Aerospace to compete in the commercial small satellite launch market. Alpha is intended to provide launch options for ...
announced their plans to use methane fuel for their small satellite launch vehicle, Firefly Alpha with an
aerospike engine The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes. It belongs to the class of altitude compensating nozzle engines. Aerospike engines have been studied for several years an ...
design. In September 2014,
Blue Origin Blue Origin, LLC is an American privately funded aerospace manufacturer and sub-orbital spaceflight services company headquartered in Kent, Washington. Founded in 2000 by Jeff Bezos, the founder and executive chairman of Amazon, the company is ...
and
United Launch Alliance United Launch Alliance (ULA), legally United Launch Alliance, LLC, is an American spacecraft launch service provider that manufactures and operates a number of rocket vehicles that are capable of launching spacecraft into orbits around Earth, ...
announced the joint development of the BE-4 LOX/LNG engine. The BE-4 will provide of thrust.


Monopropellants

;
High-test peroxide High-test peroxide (HTP) is a highly concentrated (85 to 98%) solution of hydrogen peroxide, with the remainder consisting predominantly of water. In contact with a catalyst, it decomposes into a high-temperature mixture of steam and oxygen, with n ...
: High test peroxide is concentrated
Hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
, with around 2% to 30% water. It decomposes to steam and oxygen when passed over a catalyst. This was historically used for reaction control systems, due to being easily storable. It is often used to drive
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 purpo ...
s, being used on the V2 rocket, and modern
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 Социали ...
. ;
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, hydrazin ...
: decomposes energetically to nitrogen, hydrogen, and ammonia (2N2H4 → N2+H2+2NH3) and is the most widely used in space vehicles. (Non-oxidized ammonia decomposition is endothermic and would decrease performance). ;
Nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...
: decomposes to nitrogen and oxygen. ;
Steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizat ...
: when externally heated gives a reasonably modest Isp of up to 190 seconds, depending on material corrosion and thermal limits.


Present use

, liquid fuel combinations in common use: ;
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 reg ...
(RP-1) /
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 a ...
(LOX): Used for the lower stages 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 Социали ...
boosters, the first stages of
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 three stages, and powered with liquid fuel. It was flown from 1 ...
and the Atlas family, and both stages of Electron and
Falcon 9 Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX. The rocket has two stages. The first (booster) stage carries the second stage and pa ...
. Very similar to Robert Goddard's first rocket. ;
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 l ...
(LH) / LOX: Used in the stages of the
Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...
,
Space Launch System The Space Launch System (SLS) is an American super heavy-lift expendable launch vehicle developed by NASA. As of 2022, SLS has the highest payload capacity of any rocket in operational service, as well as the greatest liftoff thrust of any r ...
,
Ariane 5 Ariane 5 is a European heavy-lift space launch vehicle developed and operated by Arianespace for the European Space Agency (ESA). It is launched from the Centre Spatial Guyanais (CSG) in French Guiana. It has been used to deliver payloads in ...
,
Delta IV Delta IV is a group of five expendable launch systems in the Delta rocket family introduced in the early 2000s. Originally designed by Boeing's Defense, Space and Security division for the Evolved Expendable Launch Vehicle (EELV) program, t ...
, New Shepard,
H-IIB H-IIB (H2B) was an expendable space launch system jointly developed by the Japanese government's space agency JAXA and Mitsubishi Heavy Industries. It was used to launch the H-II Transfer Vehicle (HTV, or ''Kōnotori'') cargo spacecraft for ...
,
GSLV Geosynchronous Satellite Launch Vehicle (GSLV) is an expendable launch system operated by the Indian Space Research Organisation (ISRO). GSLV was used in fourteen launches from 2001 to 2021. Even though GSLV Mark III shares the name, it is a ...
and
Centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
. ;
Unsymmetrical dimethylhydrazine Unsymmetrical dimethylhydrazine (UDMH; 1,1-dimethylhydrazine, НДМГ or codenamed Geptil) is a chemical compound with the formula H2NN(CH3)2 that is used as a rocket propellant. It is a colorless liquid, with a sharp, fishy, ammonia-like smell ...
(UDMH) or monomethylhydrazine (MMH) /
dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russia rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an Chemical equi ...
(NTO or ): Used in three first stages of the Russian Proton booster, Indian Vikas engine for
PSLV The Polar Satellite Launch Vehicle (PSLV) is an expendable medium-lift launch vehicle designed and operated by the Indian Space Research Organisation (ISRO). It was developed to allow India to launch its Indian Remote Sensing (IRS) satellites ...
and
GSLV Geosynchronous Satellite Launch Vehicle (GSLV) is an expendable launch system operated by the Indian Space Research Organisation (ISRO). GSLV was used in fourteen launches from 2001 to 2021. Even though GSLV Mark III shares the name, it is a ...
rockets, most Chinese boosters, a number of military, orbital and deep space rockets, as this fuel combination is hypergolic and storable for long periods at reasonable temperatures and pressures. ;
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, hydrazin ...
(): Used in deep space missions because it is storable and hypergolic, and can be used as a monopropellant with a catalyst. ; Aerozine-50 (50/50 hydrazine and UDMH): Used in deep space missions because it is storable and hypergolic, and can be used as a monopropellant with a catalyst.


Table

The table uses data from the JANNAF thermochemical tables (Joint Army-Navy-NASA-Air Force (JANNAF) Interagency Propulsion Committee) throughout, with best-possible specific impulse calculated by Rocketdyne under the assumptions of adiabatic combustion, isentropic expansion, one-dimensional expansion and shifting equilibrium.Huzel, D. K.; Huang, D. H. (1971), NASA SP-125, "Modern Engineering for Design of Liquid-Propellant Rocket Engines", (2nd ed.), NASA Some units have been converted to metric, but pressures have not.


Definitions

;''V''e: Average exhaust velocity, m/s. The same measure as specific impulse in different units, numerically equal to specific impulse in N·s/kg. ;''r'': Mixture ratio: mass oxidizer / mass fuel ;''T''c: Chamber temperature, °C ;''d'':
Bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components ( soil, gravel), chemical substances, ( pharmaceutical) ingre ...
of fuel and oxidizer, g/cm3 ;''C*'': Characteristic velocity, m/s. Equal to chamber pressure multiplied by throat area, divided by
mass flow rate In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot ('' ...
. Used to check experimental rocket's combustion efficiency.


Bipropellants

Definitions of some of the mixtures: ; IRFNA IIIa: 83.4% HNO3, 14% NO2, 2% H2O, 0.6% HF ;IRFNA IV HDA: 54.3% HNO3, 44% NO2, 1% H2O, 0.7% HF ;
RP-1 RP-1 (alternatively, Rocket Propellant-1 or Refined Petroleum-1) is a highly refined form of kerosene outwardly similar to jet fuel, used as rocket fuel. RP-1 provides a lower specific impulse than liquid hydrogen (LH2), but is cheaper, i ...
: See MIL-P-25576C, basically kerosene (approximately ) ;MMH monomethylhydrazine: Has not all data for CO/O, purposed for NASA for Martian-based rockets, only a specific impulse about 250 s. ;''r'': Mixture ratio: mass oxidizer / mass fuel ;''V''e: Average exhaust velocity, m/s. The same measure as specific impulse in different units, numerically equal to specific impulse in N·s/kg. ;''C*'': Characteristic velocity, m/s. Equal to chamber pressure multiplied by throat area, divided by
mass flow rate In physics and engineering, mass flow rate is the mass of a substance which passes per unit of time. Its unit is kilogram per second in SI units, and slug per second or pound per second in US customary units. The common symbol is \dot ('' ...
. Used to check experimental rocket's combustion efficiency. ;''T''c: Chamber temperature, °C ;''d'':
Bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components ( soil, gravel), chemical substances, ( pharmaceutical) ingre ...
of fuel and oxidizer, g/cm3


Monopropellants


References


External links


Cpropep-Web
an online computer program to calculate propellant performance in rocket engines

is a computer program to predict the performance of the liquid-propellant rocket engines. * for a history of liquid rocket propellants in the US by a pioneering rocket propellant developer. {{spacecraft propulsion Rocket propulsion Rocket propellants ja:ロケットエンジンの推進剤#液体燃料ロケット