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Amonopropellant rocket (or "monochemical rocket") is arocket that uses a singlechemical as itspropellant.[contradictory][1] Monopropellant rockets are commonly used as small altitude and trajectory control rockets in satellites, rocket upper stages, crewed spacecraft, and spaceplanes.[2]
The simplest monopropellant rockets depend on thechemical decomposition of a storable propellant after passing it over a catalyst bed.[3] The power for the thruster comes from the high pressure gas created during the decomposition reaction that allows arocket nozzle to speed up the gas to create thrust.
The most commonly used monopropellant ishydrazine (N2H4, or H2N−NH2), a compound unstable in the presence of acatalyst and which is also a strongreducing agent. The most common catalyst is granularalumina (aluminum oxide,Al2O3) coated withiridium. These coated granules are usually under the commercial labels Aerojet S-405 (previously made byShell)[4] orW.C. Heraeus H-KC 12 GA (previously made by Kali Chemie).[5] There is noigniter with hydrazine. Aerojet S-405 is a spontaneous catalyst, that is, hydrazine decomposes on contact with the catalyst. Thedecomposition is highlyexothermic and produces a 1,000 °C (1,830 °F) gas that is a mixture ofnitrogen,hydrogen andammonia. The main limiting factor of the monopropellant rocket is its life, which mainly depends on the life of the catalyst. The catalyst may be subject to catalytic poisoning and catalytic attrition which results in the catalyst failure. Another monopropellant ishydrogen peroxide, which, when purified to 90% or higher concentration, is self-decomposing at high temperatures or when a catalyst is present.
Most chemical-reaction monopropellant rocket systems consist of afuel tank, usually atitanium oraluminium sphere, with anethylene-propylene rubber container or asurface tensionpropellant management device filled with the fuel. The tank is then pressurized withhelium ornitrogen, which pushes the fuel out to the motors. Apipe leads from the tank to apoppet valve, and then to the decomposition chamber of the rocket motor. Typically, asatellite will have not just one motor, but two to twelve, each with its own valve.
Theattitude control rocket motors for satellites andspace probes are often very small, 25 mm (0.98 in) or so indiameter, and mounted in groups that point in four directions (within a plane).
The rocket is fired when thecomputer sendsdirect current through a smallelectromagnet that opens the poppet valve. The firing is often very brief, a fewmilliseconds, and — if operated in air — would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss.
Chemical-reaction monopropellants are not as efficient as some other propulsion technologies. Engineers choose monopropellant systems when the need for simplicity and reliability outweigh the need for high delivered impulse. If the propulsion system must produce large amounts of thrust, or have a highspecific impulse, as on the main motor of an interplanetary spacecraft, other technologies are used.
A concept to providelow Earth orbit (LEO)propellant depots that could be used as way-stations for other spacecraft to stop and refuel on the way to beyond-LEO missions has proposed that waste gaseoushydrogen—an inevitable byproduct of long-termliquid hydrogen storage in theradiative heat environment ofspace—would be usable as a monopropellant in asolar-thermal propulsion system. The waste hydrogen would be productively utilized for bothorbital station-keeping and attitude control, as well as providing limited propellant and thrust to use fororbital maneuvers to betterrendezvous with other spacecraft that would be inbound to receive fuel from the depot.[6]
Solar-thermal monopropellant thrusters are also integral to the design of a next-generation cryogenicupper stagerocket proposed by U.S. companyUnited Launch Alliance (ULA). TheAdvanced Common Evolved Stage (ACES) is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULACentaur and ULADelta Cryogenic Second Stage (DCSS) upper stage vehicles. The ACESIntegrated Vehicle Fluids option eliminates allhydrazine andhelium from the space vehicle—normally used for attitude control and station keeping—and depends instead on solar-thermal monopropellant thrusters using waste hydrogen.[7]
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Soviet designers had begun experimenting with monopropellant rockets as early as 1933.[8] They believed their monopropellant mixes ofnitrogen tetroxide with gasoline, or toluene, and kerosene would lead to an overall simpler system; however, they ran into problems with violent explosions with pre-mixed fuel and oxidizer serving as a monopropellant that led the designers to abandon this approach.[8]
German engineerHelmuth Walter was an early pioneer of monopropellant rockets using hydrogen peroxide as fuel.[9] Initially developing a peroxide gas turbine for use in submarines, monopropellant motors of his design were used to power theHeinkel He 176 (the first aircraft solely powered by a liquid rocket motor) and earlyMesserschmitt Me 163 Komet prototypes (the only operational rocket-powered fighter).[9]
AfterWorld War II the British continued to experiment with hydrogen peroxide monopropellants.[9] They developed thede Havilland Sprite, a motor that could produce 5000lbf (22.2 kN) of thrust over 16 seconds. This rocket was tested on 30 flights on thede Havilland Comet 1 (the first commercial jet airliner) in an effort to boosthot and high takeoff performance, but was never used operationally.[10]
In the United States, when NASA began studying monopropellants at the Jet Propulsion Laboratory (JPL) the properties of the existing propellants demanded that the thrusters be impractically large.[11] The addition of a catalyst and pre-heating propellant made them more efficient, but raised concerns over safety and handling of hazardous propellants like anhydroushydrazine.[11] However the simplicity of the thrusters designed around early monopropellants offered many simplicities and were first tested in 1959 on theAble-4 mission.[12] This test allowed for theRanger andMariner missions to use a similar thruster for correction maneuvers[12] and in the orbital insertion ofTelstar, considered by the National Air and Space Museum to be the most significant communications satellite in the beginning of the space race.[13]
In 1964, NASA began use of theLunar Landing Research Vehicle to train Apollo astronauts in piloting theLunar Excursion Module (LEM) using an attitude control system consisting of 16 hydrogen peroxide monopropellant thrusters to steer the LEM to the lunar surface.[14]
Upper stage vehicles began using monopropellant thrusters as a convenient control device in the early 1960s when General Dynamics proposed the Centaur upper stage to the United States Airforce[15] of which versions are still in use inUnited Launch Alliance'sAtlas andVulcan rockets.[16]
NASA is developing a new monopropellant propulsion system for small, cost-driven spacecraft withdelta-v requirements in the range of 10–150 m/s. This system is based on ahydroxylammonium nitrate (HAN)/water/fuel monopropellant blend which is extremely dense, environmentally benign, and promises good performance and simplicity.[17]
The EURENCO Bofors company produced LMP-103S as a 1-to-1 substitute for hydrazine by dissolving 65%ammonium dinitramide, NH4N(NO2)2, in 35% water solution ofmethanol and ammonia. LMP-103S has 6% higher specific impulse and 30% higher impulse density than hydrazine monopropellant. Additionally, hydrazine is highly toxic and carcinogenic, while LMP-103S is only moderately toxic. LMP-103S is UN Class 1.4S allowing for transport on commercial aircraft, and was demonstrated on thePrisma satellite in 2010. Special handling is not required. LMP-103S could replace hydrazine as the most commonly used monopropellant.[18][19]
the waste hydrogen that has boiled off happens to be the best known propellant (as a monopropellant in a basic solar-thermal propulsion system) for this task. A practical depot must evolve hydrogen at a minimum rate that matches the station keeping demands.
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