Propellants

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Propellants

Propellants, fuels, and oxidizers - their characteristics and the vehicles that use them

Subtopics

30% Beryllium+Pentaborane in 70% Hydrazine High performance fuel developed in Russia. Never flown due to toxicity. The concept of using beryllium and pentaborane in fuels for both rockets and advanced aircraft was popular in the early 1960's (so-called 'zip' fuels, as intended for the American B-70 bomber, for example). However while performance was improved by as much as 50%, the engine exhaust was toxic. No such engines ever reached production.

Aerozine-50 A 50-50 mixture of hydrazine and UDMH developed by Aerojet for use as the fuel in the Titan 2 missile. Copied in one Russian missile but otherwise straight UDMH was more commonly used. Higher boiling point than UDMH.

Air Air (78 % nitrogen, 21% oxygen, etc.) used as an inert cold gas, can be held under pressure and released by valves to create thrust. Ambient air can be scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines and burned with fuel as an oxidizer. It also can be mixed with the rocket exhaust to augment thrust.

Air/Kerosene Air/Kerosene propellant. The propellants used in a conventional jet engine. Ambient air (78 % nitrogen, 21% oxygen, etc.) is scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines. It is used to burn aviation-grade kerosene, commercial grade JP-4 or JP-5, their military equivalents, or special high-temperature blends such as those used in the SR-71.

Air/LH2 Air/LH2 propellant. The propellants tested and proposed for use in environmentally-friendly or high-speed jet engines. Ambient air (78 % nitrogen, 21% oxygen, etc.) is scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines as an oxidizer. Liquid hydrogen has not been used as a fuel for aircraft to date due to its big drawbacks - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. It is mainly proposed in air-breathing engines for high-speed scramjet aircraft, and mixed propulsion reusable single-stage-to-orbit designs, where use of hydrocarbon propellants creates coking and other issues. It is also proposed for, and has been tested as, the fuel for all commerical jet aircraft, as part of a post-petroleum 'hydrogen economy'.

Air/Lox/LH2 Air/Lox/LH2 tripropellant scheme. Ambient air (78 % nitrogen, 21% oxygen, etc.) is scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines as an oxidizer. In single-stage-to-orbit variants the air may be liquefied prior to use, and later the motor converts to pure rocket propulsion, using on-board liquid oxygen for the final push to orbit. In the air augmented rocket, air is collected by an intake surrounding the rocket body, and used to augment the rocket exhaust.

Air/Slush LH2 Air/Slush LH2 propellant. In this propellant scheme, ambient air (78 % nitrogen, 21% oxygen, etc.) is scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines as an oxidizer. Slush hydrogen is formed by taking liquid hydrogen down to nearly the melting point. This produces a partly-solidified but still mobile version of the fuel with 20% greater density than liquid hydrogen itself. Proposed for use from the 1980's in air-breathing and rocket-powered single-stage-to-orbit vehicles where maximization of fuel weight to empty weight was absolutely essential.

Alcohol Alcohol (ethyl alcohol or ethanol) (C2H5OH) was the fuel used for the German V-2 rocket, and the first derivative rocket engines in the United States, Soviet Union, and China. Better performance was achieved by increasing the alcohol concentration in the post-war engines. But after better-performance rocket-grade kerosene was developed by Rocketdyne in the REAP program of 1953, use of alcohol was abandoned.

Alumizine Alumizine was a mixture of 43% aluminum powder suspended in anhydrous hydrazine with a gelling agent. The idea was to increase the heat of combustion due to the high enthalpy of formation of aluminum oxide as a combustion product, similar to the metallized kerosene ("Kerosol") tested by Saenger in the 30's. Alumizine was never flown and was only tested in static ground tests. A drum of alumizine exploded in California when it was not disposed of safely. The fuel was proposed for some pressure-fed 'big dumb booster' designs of the late 1960's.

Amine Early storable rocket systems sought to improve ignition characteristics and performance by eliminating the kerosene portion of the fuel. An amine is an organic compound produced when one or more hydrogen atoms of ammonia is replaced with organic groups. Mixed amine fuels were first developed by the Germans in World War II. TONKA-250, developed for the Wasserfall rocket, was used by the Russians after the war in various engines under the specification TG-02.

Ammonia Ammonia (NH3) is a colorless gas and liquid with a strong irritating characteristic odor. Although ammonia itself is toxic, the exhaust gases from the combustion of ammonia and oxygen are not. Used as the fuel in the XLR-99 engine that powered the X-15 manned spaceplane; used as the propellant in some electric engine designs; developed as the propellant in Russian nuclear-powered ICBM designs of the 1950's.

Ammonia+Alcohol Proposed as the propellant to be heated by a thermal nuclear reactor in one Soviet design of 1960.

Aniline Aniline was used as a hypergolic fuel in several early rocket engines of the 1940's. It was used straight or with furfural alcohol to improve its cooling properties. It was quickly succeeded after the war by hydrazine in storable rocket applications. Aniline ignites spontaneously in the presence of red fuming nitric acid, and with sodium.

BrF5 Bromine Pentafluoride was another of the extremely reactive and toxic oxidizers tested in the United States in the late 1950's. As in the other cases, it was found that the handling problems and safety risks if the toxic chemical outweighed the performance benefits.

BrF5/Hydrazine BrF5/Hydrazine propellant. No engines reached the operational stage using this storable propellant combination. The handling problems and safety risks of the toxic bromine pentafluoride oxidizer outweighed the performance benefits. Hydrazine (N2H4) found early use as a rocket fuel, but it was quickly replaced by UDMH in most applications.

BrF5/Hydyne BrF5/Hydyne propellant. No engines reached the operational stage using this storable propellant combination. The handling problems and safety risks of the toxic bromine pentafluoride oxidizer outweighed the performance benefits. Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application.

BrF5/MMH BrF5/MMH propellant. No engines reached the operational stage using this storable propellant combination. The handling problems and safety risks of the toxic bromine pentafluoride oxidizer outweighed the performance benefits. Monomethylhydrazine (CH3NHNH2) is a storable liquid fuel that found favor in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance.

BrF5/UDMH BrF5/UDMH propellant. No engines reached the operational stage using this storable propellant combination. The handling problems and safety risks of the toxic bromine pentafluoride oxidizer outweighed the performance benefits. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's.

CF2 CF2 was a free radical considered as a rocket oxidizer in the 1950's. It proved too unstable for use.

CF2/Hydrazine CF2/Hydrazine propellant. No engines reached the operational stage using this propellant combination. CF2 was a free radical considered as a rocket oxidizer in the 1950's. It proved too unstable for use. Hydrazine (N2H4) found early use as a fuel, but it was quickly replaced by UDMH.

CF2/LH2 CF2/LH2 propellant. No engines reached the operational stage using this propellant combination. CF2 was a free radical considered as a rocket oxidizer in the 1950's. It proved too unstable for use. Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks.

ClF3 Chlorine trifluoride was another of the extremely reactive and toxic oxidizers tested in the United States in the late 1950's. As in the other cases, it was found that the handling problems and safety risks outweighed the performance benefits. However methods of storing and using it were developed, and it found application in small Rocketdyne engines for missiles and anti-ballistic missile interceptors in the 1990's.

ClF3/Hydrazine ClF3/Hydrazine propellant. Chlorine trifluoride was another of the extremely reactive and toxic oxidizers tested in the United States in the late 1950's. This was the highest performance propellant using ClF3. Methods of storing and using it were developed, and it found application in Rocketdyne engines for missiles and anti-ballistic missile interceptors in the 1990's. Hydrazine (N2H4) produced better specific impulse when used with ClF3 than the UDMH fuel commonly used in other applications.

ClF3/Hydyne ClF3/Hydyne propellant. No engines reached the operational stage using this propellant combination; chlorine trifluoride had better performance using hydrazine fuel. Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application.

ClF3/Kerosene ClF3/Kerosene propellant. No engines reached the operational stage using this propellant combination; chlorine trifluoride had better performance using hydrazine fuel. Rocket propellant kerosene RP-1 was a straight-run kerosene fraction.

ClF3/UDMH ClF3/UDMH propellant. No engines reached the operational stage using this propellant combination. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. However hydrazine provided better performance when used with chlorine trifluoride.

ClO3F Perchloryl fluoride was another of the extremely reactive and toxic oxidizers tested in the United States in the late 1950's. As in the other cases, it was found that the handling problems and safety risks outweighed the performance benefits.

ClO3F/Hydrazine ClO3F/Hydrazine propellant. Perchloryl fluoride was another of the extremely reactive and toxic oxidizers tested in the United States in the late 1950's. this was the highest performance propellant using ClO3F. Hydrazine (N2H4) produced better specific impulse when used with ClF3 than the UDMH fuel commonly used in other applications.

ClO3F/Hydyne ClO3F/Hydyne propellant. No engines reached the operational stage using this propellant combination; perchloryl fluoride had better performance using hydrazine fuel. Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application.

ClO3F/Kerosene ClO3F/Kerosene propellant. No engines reached the operational stage using this propellant combination; perchloryl fluoride had better performance using hydrazine fuel. Rocket propellant RP-1, or its foreign equivalents, is a straight-run kerosene fraction.

ClO3F/MMH ClO3F/MMH propellant. No engines reached the operational stage using this propellant combination; perchloryl fluoride had better performance using hydrazine fuel. Monomethylhydrazine (CH3NHNH2) is a storable liquid fuel that found favor in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance.

ClO3F/UDMH ClO3F/UDMH propellant.No engines reached the operational stage using this propellant combination; perchloryl fluoride had better performance using hydrazine fuel. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. It is used in virtually all storable liquid rocket engines except for some orbital maneuvering engines in the United States, where MMH has been preferred due to a slightly higher density and performance. No engines reached the operational stage using this propellant combination.

CO+Air+N2+C2H5OH Mix of propellants to be heated by a gas dynamic laser in one Russian prototype.

Cordite N Propellant used in the guns used to fire the Martlet gun-launched space vehicles.

CxHy Term used for an unspecified hydrocarbon fuel used in some Glushko engines of the 1970's - perhaps what was later known as 'Sintin' (synthetic kerosene).

Diborane Boron 'zip' fuels were much in fashion in the late 1950's. They seemed to offer a means of boosting the performance of both aircraft (the B-70 bomber) and rockets. However expense, toxicity, and fouling of engines led to them being abandoned.

EDA Ethylenediamine NH2(CH2)2NH2 was considered for use as a rocket fuel in the late 1950's but never found an actual production application.

Electric The many versions of electric engines use electric or magnetic fields to accelerate ionized elements to high velocity, creating thrust. The power source can be a nuclear reactor, thermal-electric generator, or solar panels. Electric engines can operate only in space, and provide very high specific impulse, but at very low thrust:weight ratios. Therefore electric spacecraft can reach very high velocities, but at long travel times.

Electric/Ammonia Electric/Ammonia propellant. Ammonia is used as the propellant in an electric arcjet motor, where it is heated rather than ionized. It was used as the propellant in the ESEX Arcjet 2.0 N engine, flown in space in 1999.

Electric/Cesium Electric/Cesium propellant. An engine were cesium propellant is ionized and electrically accelerated to produce thrust. No electric engines reached the operational stage using it.

Electric/Krypton Electric/Krypton propellant. An engine were krypton propellant is ionized and electrically accelerated to produce thrust. No electric engines reached the operational stage using it.

Electric/LH2 Electric/LH2 propellant. Liquid hydrogen used as the propellant in an electric arcjet or resistojet motor, where it is heated rather than ionized. The power source can be a nuclear reactor, thermal-electric generator, or solar panels. Liquid hydrogen had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks, and making long-term storage difficult. Its use in electric engines, which by definition were low-thrust and long-duration, meant that no electric engines reached the operational stage using this propellant.

Electric/Mercury Electric/Mercury propellant. The many versions of electric engines use electric or magnetic fields to accelerate ionized elements to high velocity, creating thrust. The power source can be a nuclear reactor or thermal-electric generator, or solar panels. Mercury was used in several early electric engines tested in space, but Xenon was preferred and used in later applications.

Electric/Teflon Electric/Teflon propellant. The use of a pulsed plasma thruster using solid teflon as a propellant resulted in an all-solid state, high performance thruster with no moving parts. Developed by Aerojet Redmond but no production application to date.

Electric/Xenon Electric/Xenon propellant. The many versions of electric engines use electric or magnetic fields to accelerate ionized elements to high velocity, creating thrust. The power source can be a nuclear reactor or thermal-electric generator, or solar panels. Xenon became the propellant of choice for production electric engines in both Russia and the United States in the 1990's and 21st Century.

Flox Tests in the early 1960's showed mixtures of liquid fluorine and liquid oxygen (dubbed 'FLOx') to have higher performance with kerosene than fluorine alone and improved handling. FLOx 30 (30% LF2, 70% Lox) could be burned in Atlas rocket motors and improved performance by 5% compared to Lox and nearly identical to that of pure fluorine. FLOx 70 (70% LF2, 30% Lox) had the highest performance, with a specific impulse 8% better then LF2 alone and 15% better than Lox alone.

Flox/Kerosene Flox/Kerosene propellant. FLOx was a mixture of liquid fluorine and liquid oxygen. Formulations of 70% liquid fluorine and 30% liquid fluorine with liquid oxygen were tested in Atlas rocket engines in the 1950's and 1960's. It improved performance while avoiding the extreme handling problems of pure liquid fluorine. Although tested in Atlas booster engines with regular RP-1 kerosene, it did not find operational use.

Flox/UDMH Flox/UDMH propellant. FLOx was a mixture of liquid fluorine and liquid oxygen, and was test fired with UDMH engines in the 1950's. It improved performance while avoiding the extreme handling problems of pure liquid fluorine.

Free Radical Certain molecules,when are torn apart, give up large amounts of energy upon recombining. In the 1950's it was proposed that such 'free radicals' could be used as rocket propellants. However free radicals recombine as soon as they are formed, and despite research over the decades no method has been found to keep them stable long enough to use as a propellant. Atomic hydrogen was the most promising of these free radicals, which might yield a specific impulse of over 1,100 seconds, three times that of conventional chemical propellants.

Gas Dynamic Laser Laser propulsion involves using the power of a laser to heat or augment combustion of a mixture of gases.

Gas Dynamic Laser/CO+Air+N2+C2H5OH Gas Dynamic Laser/CO+Air+N2+C2H5OH propellant. Laser propulsion involves using the power of a laser to heat or augment combustion of a mixture of gases. The indicated mix of propellants were heated by a gas dynamic laser in one Russian prototype. Performance was not reported.

Gasoline Gasoline of various grades were used as fuel in the earliest rocket engines of Goddard and others. Once appropriate blends of kerosene were developed in the United States and Soviet Union, that became the hydrocarbon fuel of choice.

GOX Gaseous oxygen is used as an oxidizer in thrusters for orbital maneuvering and orientation. It can be shared from the environmental control system in manned spacecraft; or as a productive use of the liquid oxygen in a main engine, which may be slowly boiling off over time in an extended mission.

GOX/Alcohol GOX/Alcohol propellant. Gaseous oxygen used with alcohol as a non-toxic combination for manned spacecraft orientation. Glushko conducted tests of a small GOX/Alcohol thruster in the 1980's, and it was considered again for the CEV/Orion American capsule in the 2000's.

Gox/GCH4 Gox/GCH4 propellant. Gaseous oxygen has been proposed for use together with gaseous methane in future deep space manned spacecraft powered by liquid methane.

GOX/Kerosene GOX/Kerosene propellant. Gaseous oxygen was used together with the main engine propellant kerosene in the Buran spaceplane's maneuvering engine and orientation thrusters.

GOX/Sintin GOX/Sintin propellant. Gaseous oxygen was proposed for use together with the main engine propellant Sintin, synthetic kerosene, in Russian spacecraft maneuvering engine and orientation thrusters. However the collapse of the Soviet Union brought the production of Sintin, and any spacecraft that may have used it, to an end.

Guncotton Propellant imagined by Jules Verne for use in his gun-launched lunar mission. In practice it was too unstable for use in large guns.

H2O2 Hydrogen peroxide is used as both an oxidizer and a monopropellant. Relatively high density and non-toxic, it was abandoned after early use in British rockets, but revived as a propellant for the Black Horse spaceplane in the 1990's and USAF spaceplane concepts in the 21st Century.

H2O2/CxHy H2O2/CxHy propellant. Glushko developed a series of engines between 1965 and 1975 burning hydrogen peroxide and an unspecified hydrocarbon fuel dubbed CxHy (perhaps 'Sintin'). No flight engines resulted.

H2O2/Hydrazine H2O2/Hydrazine propellant. Hydrogen peroxide and hydrazine would have represented a storable propellant combination with higher density than the usual nitric acid or N2O4 and hydrazine. However no engines were ever developed using this combination.

H2O2/Hydyne H2O2/Hydyne propellant. Hydrogen peroxide and Hydyne would have represented a storable propellant combination with higher density than the usual nitric acid or N2O4 and hydrazine. Hydyne was a propellant blend of 60% UDMH and 40% diethyltrianine (DETA). It was pushed rather vigorously by the Redstone arsenal in the late 1950's, but found little application.

H2O2/Kerosene H2O2/Kerosene propellant. Hydrogen peroxide was used as an oxidizer with kerosene in the 1950's in British rockets. In combination with kerosene it represented a relatively high density propellant combination. Unlike other storable propellant combinations, it was non-toxic. However care was needed in storing and handling hydrogen peroxide, since it could react with trace elements. It was abandoned by the end of the 1960's with the cancellation of the British rocket programs. It was revived in the 1990's as a proposed propellant for the Black Horse spaceplane, and later other USAF proposed spaceplanes.

H2O2/Pentaborane H2O2/Pentaborane propellant. Hydrogen peroxide in combination with pentaborane was studied or developed by the Russians in 1965-1975 as a propellant. Although potentially high performance and high density, it did not find application in any production engines. Both hydrogen peroxide and pentaborane were 'bad actors' - reacting explosively with minor impurities in the propellant systems.

H2O2/Solid H2O2/Solid propellant. Hydrogen peroxide was proposed as the oxidizer in a hybrid rocket combination. No production engines resulted; safer nitrous oxide became the preferred oxidizer for such rockets.

H2O2/UDMH H2O2/UDMH propellant. Hydrogen peroxide is used as both an oxidizer and a monopropellant. Relatively high density and non-toxic, it was abandoned after early use in British rockets. It was proposed for use with Unsymmetrical Dimethylhydrazine, the storable liquid fuel of choice by the mid-1950's. Having no advantage over the usual oxidizer used with UDMH, N2O4, it did not find application in any production engines.

Hydrazine Hydrazine (N2H4) found early use as a fuel, but it was quickly replaced by UDMH. It is still used as a monopropellant for satellite station-keeping motors.

Hydyne Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application. Hydyne, which is also known as MAF-4, is a 60 per cent, by weight, mixture of UDMH and 40 weight percent diethyltrianine (DETA).

Isopropylnitrate Isopropylnitrate was tested as a monopropellant for missiles in the 1940's in the US (Lark) and 1950's in Russia (SAM's). However instability due to compression deflagration resulted to it being abandoned in both cases.

JP-X The addition of approximately 40 per cent of UDMH to JP-4 resulted in a formulation (JP-X) which solved both the combustion and the ignition difficulties experienced with WFNA/ JP-4 and IRFNA/JP-4.

Kerosene In January 1953 Rocketdyne commenced the REAP program to develop a number of improvements to the engines being developed for the Navaho and Atlas missiles. Among these was development of a special grade of kerosene suitable for rocket engines. Prior to that any number of rocket propellants derived from petroleum had been used. Goddard had begun with gasoline, and there were experimental engines powered by kerosene, diesel oil, paint thinner, or jet fuel kerosene JP-4 or JP-5. The wide variance in physical properties among fuels of the same class led to the identification of narrow-range petroleum fractions, embodied in 1954 in the standard US kerosene rocket fuel RP-1, covered by Military Specification MIL-R-25576. In Russia, similar specifications were developed for kerosene under the specifications T-1 and RG-1. The Russians also developed a compound of unknown formulation in the 1980's known as 'Sintin', or synthetic kerosene.

Kerosene/LH2 Tripropellant motors use high-density kerosene for the boost phase, then low-density, high-performance liquid hydrogen for the later stages of ascent. However the propellants are stored in separate tanks. The fuel density indicated is the average for the MAKS design, which burned 17,850 kg LH2 and 18,698 Kerosene to reach orbit using 175,758 kg of liquid oxygen oxidizer.

LCH4 Liquid methane has been proposed as a propellant by the Russians.

LF2 Liquid Fluorine is the highest performance oxidizer and in the early 1960's it seemed in both American and Russia that a new generation of higher performance engines would emerge. However although test engines were built, fluorine was found to be just too toxic and reactive to be safely used as a propellant.

LF2/Ammonia LF2/Ammonia propellant. In Russia this combination nearly made it into production in Glushko's RD-301 engines for use in a high-performance upper stage for the Proton booster in the 1970's. However although test engines were built, fluorine was just too toxic and reactive to be safely used as a propellant. Ammonia (NH3) is a colorless liquid with a strong irritating characteristic odor.

LF2/Hydrazine LF2/Hydrazine propellant. In the United States. In the 1960's the USAF sponsored development of engines by Bell and Rocketdyne using this propellant combination to power high-performance upper stages to replace the Agena and Transtage on the Atlas and Titan launch vehicles. However although test engines were built, fluorine was found to be just too toxic and reactive to be safely used as a propellant.

LF2/Kerosene LF2/Kerosene propellant. Use of fluorine with kerosene was studied in the 1950's, but performance was less than with other fuels, and no rocket engine reached test stage with this combination.

LF2/LH2 LF2/LH2 propellant. This was theoretically the highest-performance propellant combination. However although test engines were built in both the United States and Russia, fluorine was found to be just too toxic and reactive to be safely used as a propellant.

LF2/LLi LF2/LLi propellant. Liquid Lithium was a high energy fuel demonstrated with LF2 in the early 1960's. Lithium had to be heated to 179 deg C to be in a liquid state. However although test engines were built, this combination was found to be just too toxic and reactive to be safely used as a propellant.

LF2/UDMH LF2/UDMH propellant. No engines using this propellant combination reached the test stage.

LH2 Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was adopted for the core of the space shuttle, and Centaur stages still fly today.

Liquid Air Liquid air has no advantage as a stored propellant, but in a Liquid Air Cycle Engine (LACE) relatively freely available atmospheric air is scooped up, liquefied, and burned with a fuel in a conventional rocket engine.

Liquid Air/LH2 Liquid Air/LH2 propellant. Liquid air has no advantage as a stored propellant, but in a Liquid Air Cycle Engine (LACE) relatively freely available atmospheric air is scooped up, liquefied, and burned with a fuel in a conventional rocket engine. LACE engines were tested in Japan and the United States, but none reached flight status.

Liquid Air/Lox Liquid air has no advantage as a stored propellant, but in a Liquid Air Cycle Engine (LACE) relatively freely available atmospheric air is scooped up, liquefied, and burned with a fuel in a conventional rocket engine. In one variation this is replaced with stored liquid oxygen as the rocket ascends out of the atmosphere.

Liquid Air/Lox/LH2 Liquid Air/Lox/LH2 propellant. Liquid air has no advantage as a stored propellant, but in a Liquid Air Cycle Engine (LACE) relatively freely available atmospheric air is scooped up, liquefied, and burned with a fuel in a conventional rocket engine. In this variant, liquid oxygen is used to continue operation of the engine above the atmosphere, allowing the engine to be used in single-stage-to-orbit designs. This approach was used by Rolls Royce in the RB545 engine proposed for the HOTOL spaceplane, but later abandoned for the simpler Air/Lox/LH2 Sabre engine for the proposed Skylon SSTO.

LLi High energy fuel demonstrated in with LF2 in the early 1960's. Lithium had to be heated to 179 deg C to be in a liquid state.

LLi+30%LH2 Combination demonstrated in a tripropellant motor with LF2 in the early 1960's. Lithium had to be heated to 179 deg C to be in a liquid state.

LNG Liquefied natural gas - mainly methane, with traces of sulfur, etc.

LO3 Liquid ozone offered the possibility of higher performance than liquid oxygen. But it combined the dual drawbacks of very high toxicity and very shock sensitive and was found to be too difficult for practical use.

LOX Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidizer for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required.

Lox/Alcohol Lox/Alcohol propellant. This propellant combination was used for the German V-2 rocket, and the first derivative rocket engines in the United States, Soviet Union, and China. Improved specific impulse was achieved by increasing the alcohol concentration in the post-war engines. But after better-performance rocket-grade kerosene was developed by Rocketdyne in the REAP program of 1953, use of alcohol was abandoned. And liquid oxygen, being moderately cryogenic, was not suitable for military uses where storage of the fuelled missile and quick launch are required.

Lox/Ammonia Lox/Ammonia propellant. This relatively benign propellant combination was used in the XLR-99 rocket motor that powered the X-15 manned rocketplane in the 1960's.

Lox/Beryllium+Pentaborane in Hydrazine 30%/70% Lox/Beryllium+Pentaborane in Hydrazine 30%/70% propellant. The concept of using beryllium and pentaborane in fuels for both rockets and advanced aircraft (the American B-70 bomber, for example) was popular in the early 1960's. However while performance was improved by as much as 50%, the engine exhaust was toxic. No such engines ever reached production.

Lox/C3H8 Lox/C3H8 propellant. C3H8 liquid propane was proposed as a more 'environmentally friendly' rocket fuel in Russia in the 1990's, and was tested in place of kerosene in some test engines, but no such engine reached production.

Lox/Gasoline Lox/Gasoline propellant. Gasoline of various grades were used as fuel in the earliest rocket engines of Goddard and others. Once appropriate blends of kerosene were developed in the United States and Soviet Union, that became the hydrocarbon fuel of choice.

Lox/Hydrazine Lox/Hydrazine propellant. Motors were tested with this propellant combination in the 1950's, but hydrazine (N2H4) was quickly replaced by UDMH.

Lox/Hydyne Lox/Hydyne propellant. Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidizer for large space launchers.Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application. Hydyne was a 60 per cent, by weight, mixture of UDMH and 40 weight percent diethyltrianine (DETA).

Lox/Kerosene Lox/Kerosene propellant. Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidizer for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required. In January 1953 Rocketdyne commenced the REAP program to develop a number of improvements to the engines being developed for the Navaho and Atlas missiles. Among these was development of a special grade of kerosene suitable for rocket engines. Prior to that any number of rocket propellants derived from petroleum had been used. Goddard had begun with gasoline, and there were experimental engines powered by kerosene, diesel oil, paint thinner, or jet fuel kerosene JP-4 or JP-5. The wide variance in physical properties among fuels of the same class led to the identification of narrow-range petroleum fractions, embodied in 1954 in the standard US kerosene rocket fuel RP-1, covered by Military Specification MIL-R-25576. In Russia, similar specifications were developed for kerosene under the specifications T-1 and RG-1. The Russians also developed a compound of unknown formulation in the 1980's known as 'Sintin', or synthetic kerosene.

Lox/Kerosene/LH2 Lox/Kerosene/LH2 propellant. Tripropellant motors use high-density kerosene for the boost phase, then low-density, high-performance liquid hydrogen for the later stages of ascent. However the propellants are stored in separate tanks. The fuel density indicated is the average for the MAKS design, which burned 17,850 kg LH2 and 18,698 Kerosene to reach orbit using 175,758 kg of liquid oxygen oxidizer.

Lox/LCH4 Lox/LCH4 propellant. Liquid methane, or liquid natural gas, was originally proposed in the 1960's as an an alternate to hydrogen to power the spacecraft for long-duration manned Mars expeditions. It provided longer and easier storage and higher density than hydrogen. In the 1980's, it was proposed that spacecraft returning from Mars could extract methane fuel from the Martian atmosphere using Brayton-cycle processors. This made Lox/Methane a standard for NASA's deep space manned spacecraft shuttle follow-on concepts. Development began of engines intended for use in reaction control systems and satellite maneuvering systems after 2000. In the 1990's, liquid methane was proposed as a launch vehicle fuel by the Russians, to be applied to conversions of various existing launch vehicles, as well as the clean-sheet-of-paper Riksha design. NASA dropped Lox/Methane from its Orion manned capsule once Mars plans crumbled; and the Russian booster designs never found any funding. Development of the engines continued in the United States for proposed manned spacecraft and spaceplanes.

Lox/LH2 Lox/LH2 propellant.to be used on production space launch vehicles. Liquid oxygen was the earliest, cheapest, safest, and eventually the preferred oxidizer for large space launchers. Its main drawback is that it is moderately cryogenic, and therefore not suitable for military uses where storage of the fuelled missile and quick launch are required. Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks. The United States mastered hydrogen technology for the highly classified Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950's. The technology was transferred to the Centaur rocket stage program, and by the mid-1960's the United States was flying the Centaur and Saturn upper stages using the fuel. It was later adopted for the core of the US space shuttle, the European Ariane 5, and the Chinese CZ-5 launch vehicles. It is used in upper stages flown on American, European, Indian, and Chinese boosters. Although extensively developed in Russia, it never reached production for any Russian space launchers.

Lox/LNG Liquid oxygen and liquid natural gas have been proposed as a cleaner propellant combination than the standard liquid oxygen/kerosene.

Lox/Sintin Lox/Sintin propellant. Sintin - described as a 'synthetic kerosene' of unknown composition - was introduced in the Soviet Union in the 1980's. It increased specific impulse from 1-2% when used in engines using conventional kerosene, and was evidently denser. It use was discontinued after the breakup of the Soviet Union.

Lox/Solid Lox/Solid propellant. used in a hybrid rockets. Mixed liquid/solid propulsion systems offer the potential for the storability of a solid rocket, the safety and throttleability of a liquid rocket, and lower cost than either. Believers experimented throughout the last half of the 20th Century, but it only after the year 2000 that such systems went into production. Solid fuel for hybrids are in the form of a rubbery matrix. HTPB is most commonly used.

Lox/UDMH Lox/UDMH propellant. This propellant combination, with higher performance than either Lox/Kerosene or N2O4/UDMH, was developed by Glushko in Russia in the 1950's. However the use of toxic UDMH was unacceptable to rocket designer Korolev, while the use of cryogenic liquid oxygen was unacceptable to the Soviet military. Glushko's plans for use of the propellant in larger boosters had to be in abandoned, and it was used instead for in small upper stages for the Kosmos launch vehicle series.

Mercury (fuel) Elemental mercury, used as propellant for some early ion motors.

MMH Monomethylhydrazine (CH3NHNH2) is a storable liquid fuel that found favor in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance.

MON Mixed Oxides of Nitrogen - Nitric oxide (NO) is a low-boiling cryogenic gas. Both the liquid and the solid are blue. Solutions of NO in nitrogen tetroxide sharply depress the freezing point of the high-melting oxidizer. The mechanism of depression is believed to involve the formation of N2O3, which is soluble in nitrogen tetroxide. Solutions are called mixed oxides of nitrogen (MON), and have been used as oxidizers for liquid-rocket engines. Various concentrations have been considered. However, the high vapor pressure of MON limits the concentration of NO in N2O4 to about 30 per cent. Aside from the high vapor pressure of MON, the material is quite similar to nitrogen tetroxide.

MON/Hydrazine MON/Hydrazine propellant. Mixed Oxides of Hydrazine (N2H4) found early use as a fuel, but it was quickly replaced by UDMH.

MON/Hydyne MON/Hydyne propellant. Hydyne (60% UDMH/40% DETA) was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application. No rocket engines went into production using this propellant.

MON/MMH MON/MMH propellant. No rocket engines went into production using this propellant combination.

MON/UDMH MON/UDMH propellant. No rocket engines went into production using this propellant combination.

N2O Liquid nitrous oxide (N2O / dinitrogen monoxide / 'laughing gas') is the oxidizer of choice for hybrid rocket motors because it is benign, storable, and self-pressurizing to 48 atmospheres at 17 deg C.

N2O/C3H8 N2O/C3H8 propellant. Liquid nitrous oxide (N2O / dinitrogen monoxide / 'laughing gas') and C3H8, propane, a commonly-available natural gas, were proposed in the 21st Century as a propellant combination for manned spacecraft.

N2O/Solid N2O/Solid propellant. Liquid nitrous oxide (N2O / dinitrogen monoxide / 'laughing gas') is the oxidizer of choice for hybrid rocket motors because it is storable, and self-pressurizing to 48 atmospheres at 17 deg C. The combination of HTPB or PMMA solid fuel and N2O is benign, non-toxic, and non-explosive.

N2O4 Nitrogen tetroxide became the storable liquid propellant of choice from the late 1950's.

N2O4/Aerozine-50 N2O4/Aerozine-50 propellant. Nitrogen tetroxide became the storable liquid propellant of choice from the late 1950's. Aerozine was a 50-50 mixture of hydrazine and UDMH developed for use in the Titan missile family; it had a higher boiling point than UDMH. This propellant combination was copied in one Russian missile but otherwise straight UDMH was used in Russia.

N2O4/Alumizine N2O4/Alumizine propellant. Nitrogen tetroxide became the storable liquid propellant of choice from the late 1950's. Alumizine was a mixture of 43% aluminum powder suspended in anhydrous hydrazine with a gelling agent. The idea was to increase the heat of combustion due to the high enthalpy of formation of aluminum oxide as a combustion product, similar to the metallized kerosene ('Kerosol') tested by Saenger in the 30's. Alumizine was never flown and was only tested in static ground tests. A drum of alumizine exploded in California when it was not disposed of safely. The fuel was proposed for some pressure-fed 'big dumb booster' designs of the late 1960's.

N2O4/Hydrazine N2O4/Hydrazine propellant. Hydrazine (N2H4) found early use as a fuel, but it was quickly replaced by UDMH, and this combination was not used in any production motors.

N2O4/Hydyne N2O4/Hydyne propellant. Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application. This propellant combination was not used in any production rocket motors.

N2O4/Kerosene N2O4/Kerosene propellant. This low-cost propellant combination was used in the Otrag low-cost modular rocket system, flight-tested 1977-1983.

N2O4/MMH N2O4/MMH propellant. Monomethylhydrazine (MMH) is a storable liquid fuel that found favor in the United States for use in orbital spacecraft engines. Its advantages in comparison to UDMH are higher density and slightly higher performance.

N2O4/Pentaborane N2O4/Pentaborane propellant. Pentaborane (B5H9) was considered as a high performance fuel in the US in the 1950's. Its development was pursued with some vigor by Glushko in Russia during the 1960's. But like the other fluorine and boron motors of the time, it presented too many handling and safety problems to be adopted as a flight engine.

N2O4/UDMH N2O4/UDMH propellant. Nitrogen tetroxide became the storable liquid propellant of choice from the late 1950's. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital maneuvering engines in the United States, where MMH has been preferred due to a slightly higher density and performance.

Nitric Acid Drawing on the German World War II Wasserfall rocket, nitric acid (HNO3) became the early storable oxidizer of choice for missiles and upper stages of the 1950's. To overcome various problems with its use, it was necessary to combine the nitric acid with N2O4 and passivation compounds. These formulae were considered extremely secret at the time. By the late 1950's it was apparent that N2O4 by itself was a better oxidizer. Therefore nitric acid was almost entirely replaced by pure N2O4 in storable liquid fuel rocket engines developed after 1960.

Nitric acid/Amine Nitric acid/Amine propellant. Drawing on the German World War II Wasserfall rocket, nitric acid (HNO3) became the early storable oxidizer of choice for missiles and upper stages of the 1950's. To overcome various problems with its use, it was necessary to combine the nitric acid with N2O4 and passivation compounds. These formulae were considered extremely secret at the time. By the late 1950's it was apparent that N2O4 by itself was a better oxidizer. Therefore nitric acid was almost entirely replaced by pure N2O4 in storable liquid fuel rocket engines developed after 1960. Early storable rocket systems sought to improve ignition characteristics and performance by eliminating the kerosene portion of the fuel. An amine is an organic compound produced when one or more hydrogen atoms of ammonia is replaced with organic groups. Mixed amine fuels were first developed by the Germans in World War II. TONKA-250, developed for the Wasserfall rocket, was used by the Russians after the war in various engines under the specification TG-02.

Nitric acid/Ammonia Nitric acid/Ammonia propellant. No rocket engines with this propellant combination entered production.

Nitric acid/Gasoline Nitric acid/Gasoline propellant. Gasoline of various grades were used as fuel in the earliest rocket engines of Goddard and others. Once appropriate blends of kerosene were developed in the United States and Soviet Union, that became the hydrocarbon fuel of choice.

Nitric acid/Hydrazine Nitric acid/Hydrazine propellant. Problems which caused the abandoning of this propellant combination were the absence of reliable hypergolic ignition and unstable combustion. IRFNA (inhibited red fuming nitric acid)/UDMH and IRFNA/JP-X finally did prove satisfactory. No rocket engines with this propellant combination entered production.

Nitric acid/Hydyne Nitric acid/Hydyne propellant. Hydyne was a propellant blend pushed rather vigorously by the Redstone arsenal in the late 1950's, but it found little application. No rocket engines with this propellant combination entered production.

Nitric acid/JP-X Nitric acid/JP-X propellant. The addition of approximately 40 per cent of UDMH to JP-4 resulted in a formulation (JP-X) which solved both the combustion and the ignition difficulties experienced with WFNA/ JP-4 and IRFNA/JP-4.

Nitric acid/Kerosene Nitric acid/Kerosene propellant. This propellant combination, theoreticaly a very low-cost solution, proved to have ignition and stability problems. Finally the addition of approximately 40 per cent of UDMH to JP-4 resulted in a formulation (JP-X) which solved both the combustion and the ignition difficulties experienced with WFNA/ JP-4 and IRFNA/JP-4. However by then N2O4/UDMH was settled on as the optimum storable propellant combination.

Nitric acid/MMH Nitric acid/MMH propellant. No engines using this combination were developed.

Nitric acid/Solid Nitric acid/Solid propellant. Potential hybrid rocket combination, but less corrosive oxidizers (liquid oxygen, nitrous oxide) have been preferred for safety reasons.

Nitric acid/Turpentine Nitric acid/Turpentine propellant. Combination used in early test rockets; no production engines resulted.

Nitric acid/UDMH Nitric acid/UDMH propellant. Drawing on the German World War II Wasserfall rocket, nitric acid (HNO3) became the early storable oxidizer of choice for missiles and upper stages of the 1950's. To overcome various problems with its use, it was necessary to combine the nitric acid with N2O4 and passivation compounds. These formulae were considered extremely secret at the time. By the late 1950's it was apparent that N2O4 by itself was a better oxidizer. Therefore nitric acid was almost entirely replaced by pure N2O4 in storable liquid fuel rocket engines developed after 1960. Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines, normally in combination with N2O4 rather than nitric acid.

Nitrogen Inert cold gas held under pressure and released by valves to create thrust.

Nitrogen gas Inert cold gases held under pressure and released by valves to create thrust. Inert cold gases held under pressure and released by valves to create thrust.

Nitrogen+Helium Inert cold gases held under pressure and released by valves to create thrust.

Nitrous oxide/Alcohol Nitrous oxide/Alcohol propellant. Nitrous oxide has advantages as a rocket engine oxidizer in that it is non-toxic, stable at room temperature, easy to store and relatively safe to carry on a flight. Its disadvantage is that it must be stored as a gas, which make it more bulky than liquid oxidizers. Alcohol (C2H5OH) was the fuel used for the German V-2 rocket, and the first derivative rocket engines in the United States, Soviet Union, and China. After better-performance rocket-grade kerosene was developed by Rocketdyne in the REAP program of 1953, use of alcohol was abandoned. Interest was renewed in the 21st Century as part of this non-toxic, storable propellant combination.

Nitrous oxide/Amines Nitrous oxide/Amines propellant. Nitrous oxide has advantages as a rocket engine oxidizer in that it is non-toxic, stable at room temperature, easy to store and relatively safe to carry on a flight. Its disadvantage is that it must be stored as a gas, which make it more bulky than liquid oxidizers. Early storable rocket systems sought to improve ignition characteristics and performance by eliminating the kerosene portion of the fuel. An amine is an organic compound produced when one or more hydrogen atoms of ammonia is replaced with organic groups. Mixed amine fuels were first developed by the Germans in World War II.

Nuclear Thermal Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. Although early Russian designs used ammonia or alcohol as propellant, the ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Nuclear engines would have twice the performance of conventional chemical rocket engines. Although successfully ground-tested in both Russia and America, they have never been flown due primarily to environmental and safety concerns. For operations in the atmosphere, some aircraft and missile designs of the 1950's would use the heat of the reactor to directly warm ambient air, resulting in an unlimited source of fuel and virtually unlimited range for the aircraft.

Nuclear/Air Nuclear/Air propellant. Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. For operations in the atmosphere, some aircraft and missile designs of the 1950's would use the heat of the reactor to directly warm ambient air, resulting in virtually unlimited range for the aircraft. Environmental contamination problems could not be solved and these projects were abandoned in both the USA and USSR in the 1960's.

Nuclear/Ammonia Nuclear/Ammonia propellant. Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. Although early Russian designs used ammonia or an ammonia/alcohol mixture as propellant, the ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Although successfully ground-tested in both Russia, they have never been flown due primarily to environmental and safety concerns.

Nuclear/Ammonia+Alcohol Nuclear/Ammonia+Alcohol propellant. Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. Although early Russian designs used ammonia or an ammonia/alcohol mixture as propellant, the ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Although successfully ground-tested in both Russia, they have never been flown due primarily to environmental and safety concerns.

Nuclear/LH2 Nuclear/LH2 propellant. Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. The ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Nuclear engines would have twice the performance of conventional chemical rocket engines. Although successfully ground-tested in both Russia and America, they have never been flown due primarily to environmental and safety concerns. Liquid hydrogen's drawbacks, especially for long-duration missions to Mars, for which nuclear thermal engines were mainly considered: it was highly cryogenic, and it had a very low density, making for large tanks and the need for solar shielding and reliquefaction systems to ensure the hydrogen would stay liquid during the long trip to Mars and back. There were also many operational issues regarding getting the engine into operation and shutting it down - it was a long process. The Russians solved some of these issues by using the engine as a power source during the cruise to Mars and back.

OF2 Oxygen difluoride was a candidate high performance propellant of the late 1950's that was less cryogenic then fluorine. It is also not so corrosive or reactive as fluorine; however, it will react with most substances under proper conditions. Due to safety concerns it was never adopted in a production engine.

Pentaborane Pentaborane (B5H9) was considered as a high performance fuel in the US in the 1950's. Its development was pursued with some vigor by Glushko in Russia during the 1960's. But like the other fluorine and boron motors of the time, it presented too many handling and safety problems to be adopted as a flight engine.

Slush LH2 Slush hydrogen is formed by taking liquid hydrogen down to nearly the melting point. This produces a partly-solidified but still mobile version of the fuel with 20% greater density than liquid hydrogen itself. Proposed for use from the 1980's in air-breathing and rocket-powered single-stage-to-orbit vehicles where maximization of fuel weight to empty weight is absolutely essential.

Solar thermal By use of concentrating mirrors, solar power can be used to heat a propellant (usually hydrogen) to produce thrust in space.

Solar/LH2 Solar/LH2 propellant. By use of concentrating mirrors, solar power can be used to heat a propellant (usually hydrogen) to produce thrust in space. Liquid hydrogen was identified by all the leading rocket visionaries as the theoretically ideal rocket fuel. It had big drawbacks, however - it was highly cryogenic, and it had a very low density, making for large tanks.

Solid Solid propellants have the fuel and oxidizer embedded in a rubbery matrix. They were developed to a high degree of perfection in the United States in the 1950's and 1960's. In Russia, development was slower, due to a lack of technical leadership in the area and rail handling problems

Steam Steam rockets used water, heated by an external source prior to launch and stored under pressure, to provide thrust. The heavy pressure vessel means use is usually confined to ground-based reusable applications, such as launch sleds.

Teflon Teflon was introduced in the late 1990's as the solid fuel heated electrically to provide a completely solid-state rocket system for spacecraft orientation with no moving parts.

UDMH Unsymmetrical Dimethylhydrazine ((CH3)2NNH2) became the storable liquid fuel of choice by the mid-1950's. Development of UDMH in the Soviet Union began in 1949. It is used in virtually all storable liquid rocket engines except for some orbital maneuvering engines in the United States, where MMH has been preferred due to a slightly higher density and performance.

Xenon Proposed as propellant for some ion motors.

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