Ahypergolic propellant is arocket engine propellant, the components of whichspontaneously ignite upon contact with each other.
The two components usually consist of afuel and anoxidizer. The advantages of hypergolic propellants are that they can be stored at room temperature, and thatrocket engines can be ignited reliably and repeatedly. Common hypergolic propellants are difficult to handle due to hightoxicity orcorrosiveness.
In contemporary usage, the terms "hypergol" and "hypergolic propellant" usually refer to the most common combination:dinitrogen tetroxide plushydrazine.[1]
The fact thatturpentine may spontaneously combust when mixed withnitric acid was discovered in the late 17th century byFrederick Slare,[2][3] but it remained a scientific curiosity for centuries until it was proposed to use it forrocket-assisted take off during WWII.[4]
In 1935,Hellmuth Walter discovered thathydrazine hydrate was hypergolic withhigh-test peroxide of 80–83%. He was probably the first to discover this phenomenon, and set to work developing a fuel. Prof. Otto Lutz assisted theWalter Company with the development ofC-Stoff, which contained 30% hydrazine hydrate, 57%methanol, and 13% water, and spontaneously ignited with high-strengthhydrogen peroxide.[5]: 13 BMW developed engines burning a hypergolic mix of nitric acid with various combinations of amines, xylidines, andanilines.[6]
Hypergolic propellants were discovered independently, for the second time, in the U.S. byGALCIT and Navy Annapolis researchers in 1940. They developed engines powered by aniline andred fuming nitric acid.[7]Robert Goddard,Reaction Motors, andCurtiss-Wright worked on aniline/nitric acid engines in the early 1940s, for small missiles and jet assisted take-off (JATO). The project resulted in the successful JATO of severalMartin PBM and PBY bombers, but the project was disliked because of the toxic properties of both fuel and oxidizer, as well as the highfreezing point of aniline (−6.3 °C). The second problem was eventually solved by the addition of small quantities offurfuryl alcohol to the aniline.[5]: 22–23
In Germany from the mid-1930s throughWorld War II, rocket propellants were broadly classed asmonergols, hypergols, nonhypergols andlithergols. The endingergol is a combination ofGreekergon or work, and Latinoleum or oil, later influenced by the chemical suffix-ol fromalcohol.[Note 1] Monergols weremonopropellants, while nonhypergols werebipropellants that required external ignition, and lithergols were solid/liquid hybrids. Hypergolic propellants (or at least hypergolic ignition) were far less prone tohard starts than electric or pyrotechnic ignition. The "hypergole" terminology was coined by Dr. Wolfgang Nöggerath, at the Technical University ofBraunschweig (Brunswick), Germany.[8]
The only rocket-powered fighter ever deployed was theMesserschmitt Me 163BKomet, which had anHWK 109-509, a rocket motor which consumed methanol/hydrazine as fuel and high-test peroxideT-Stoff as oxidizer. The hypergolic rocket motor had the advantage of fast climb and quick-hitting tactics at the cost of being very volatile and capable of exploding with any degree of inattention. Other proposed combat rocket fighters such as theHeinkelJulia and reconnaissance aircraft like theDFS 228 were meant to use the Walter 509 series of rocket motors, but besides the Me 163, only theBachem Ba 349Natter vertical launch expendable fighter was ever flight-tested with the Walter rocket propulsion system as its primary sustaining thrust system for military-purpose aircraft.
The earliestballistic missiles, such as the SovietR-7 that launchedSputnik 1 and the U.S.Atlas andTitan-1, usedkerosene andliquid oxygen. Although they are preferred in space launchers, the difficulties of storing acryogen such as liquid oxygen in a missile that had to be kept launch ready for months or years at a time led to a switch to hypergolic propellants in the U.S.Titan II and in most Soviet ICBMs such as theR-36, but the difficulties of such corrosive and toxic materials, including injury-causing leaks and the explosion of a Titan-II in its silo,[9] led to their near universal replacement withsolid-fuel boosters, first in Westernsubmarine-launched ballistic missiles and then in land-based U.S. and Soviet ICBMs.[5]: 47
In the 1960s, late variants of FrenchVéroniquesounding rocket and theVesta rocket, as well as the first stage of the first orbital SLVDiamant used[10] the combination of nitric acid and turpentine discovered by Slare. It may also be used inamateur rocketry.[11]
TheApollo Lunar Module, used in theMoon landings, employed hypergolic fuels in both the descent and ascent rocket engines. TheApollo spacecraft used the same combination for theService Propulsion System. Those spacecraft and theSpace Shuttle (among others) used hypergolic propellants for theirreaction control systems.
The trend among Western space-launch agencies is away from large hypergolic rocket engines and toward hydrogen/oxygen engines or methane/oxygen andRP-1/oxygen engines for variousadvantages and disadvantages.Arianes 1 through 4, with their hypergolicfirst and second stages (and optional hypergolic boosters on the Ariane 3 and 4) have been retired and replaced with the Ariane 5, which uses a first stage fueled by liquid hydrogen and liquid oxygen. The Titan II, III, and IV, with their hypergolic first and second stages, have also been retired for the Atlas V (RP-1/oxygen) and Delta IV (hydrogen/oxygen). Hypergolic propellants are still used in upper stages, when multiple burn-coast periods are required, and inlaunch escape systems.
Hypergolically fueled rocket engines are usually simple and reliable because they need no ignition system. Although larger hypergolic engines in some launch vehicles useturbopumps, most hypergolic engines are pressure fed. A gas, usuallyhelium, is fed to the propellant tanks under pressure through a series ofcheck andsafety valves. The propellants, in turn, flow through control valves into the combustion chamber; there, their instant contact ignition prevents a mixture of unreacted propellants from accumulating and then igniting in a potentially catastrophichard start.
As hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted, so are uniquely suited for spacecraft maneuvering and well-suited, though not uniquely so, as upper stages of such space launchers as theDelta II andAriane 5, which must perform more than one burn. Restartable nonhypergolic rocket engines nevertheless exist, notably the cryogenic (oxygen/hydrogen)RL-10 on theCentaur and theJ-2 on theSaturn V. TheRP-1/LOXMerlin on theFalcon 9 can also be restarted.[12]
The most common hypergolic fuels,hydrazine,monomethylhydrazine, andunsymmetrical dimethylhydrazine (UDMH), and oxidizer,nitrogen tetroxide, are all liquid at ordinary temperatures and pressures. They are therefore sometimes called "storable liquid propellants". They are suitable for use in spacecraft missions lasting many years. Thecryogenity ofliquid hydrogen andliquid oxygen has so far limited their practical use to space launch vehicles where they need to be stored only briefly.[13] The largest issue with the usage of cryogenic propellants in interplanetary space is boil-off, which is largely dependent onthe scale of spacecraft.
Another advantage of hypergolic propellants is their high density compared to cryogenic propellants.LO2 has a density of 1.14 g/ml, while hypergolic oxidizers such asnitric acid ornitrogen tetroxide have a density of 1.55 g/ml and 1.45 g/ml, respectively.LH2 fuel offers extremely high performance, yet its density only warrants its use in the largest of rocket stages, while mixtures of hydrazine and UDMH have a density at least 10 times greater.[14] This is of great importance inspace probes, as the higher propellant density allows the size of their propellant tanks to be reduced significantly, which in turn allows the probes to fit within a smallerpayload fairing.
Relative to their mass, traditional hypergolic propellants possess a lowercalorific value than cryogenic propellant combinations like LH2/LO2 orLCH4/LO2.[15] A launch vehicle that uses hypergolic propellant must therefore carry a greater mass of fuel than one that uses these cryogenic fuels.
Thecorrosivity,toxicity, andcarcinogenicity of traditional hypergolics necessitate expensive safety precautions.[16][17] Failure to follow adequate safety procedures with an exceptionally dangerous UDMH-nitric acid propellant mixture nicknamed"devil's venom", for example, resulted in the deadliest rocketry accident in history, theNedelin catastrophe.[18]
Common hypergolic propellant combinations include:[19]
Less-common or obsolete hypergolic propellants include:
Pyrophoric substances, which ignite spontaneously in the presence of air, are also sometimes used as rocket fuels themselves or to ignite other fuels. For example, a mixture oftriethylborane andtriethylaluminium (which are both separately and even more so together pyrophoric), was used for engine starts in theSR-71 Blackbird and in theF-1 engines on theNASASaturn V rocket, and is used in theMerlin engines on theSpaceXFalcon 9 rockets.
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