A hypergolic rocket propellant combination used in a rocket engine is one where the propellants spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. Although hypergolic propellants tend to be difficult to handle because of their extreme toxicity and/or corrosiveness, they can typically be stored as liquids at room temperature and hypergolic engines are easy to ignite reliably and repeatedly.
In contemporary usage, the terms "hypergol" or "hypergolic propellant" usually mean the most common such propellant combination, dinitrogen tetroxide plus hydrazine and/or its relatives monomethyl hydrazine and unsymmetrical dimethylhydrazine.
Soviet rocket engine researcher Valentin Glushko experimented with hypergolic fuel as early as 1931. It was initially used for "chemical ignition" of engines, starting kerosene/nitric acid engines with an initial charge of phosphorus dissolved in carbon disulfide.
Starting in 1935, Prof. O. Lutz of the German Aeronautical Institute experimented with over 1000 self-igniting propellants. He assisted the Walter Company with the development of C-Stoff which ignited with concentrated hydrogen peroxide. BMW developed engines burning a hypergolic mix of nitric acid with various combinations of amines, xylidines and anilines.
Hypergolic propellants were discovered independently, for the third time, in the US by GALCIT and Navy Annapolis researchers in 1940. They developed engines powered by aniline and nitric acid. Robert Goddard, Reaction Motors and Curtiss-Wright worked on aniline/nitric acid engines in the early 1940s, for small missiles and jet assisted take-off (JATO).
In Germany from the mid-1930s through World War II, rocket propellants were broadly classed as monergols, hypergols, non-hypergols and lithergols. The ending ergol is a combination of Greek ergon or work, and Latin oleum or oil, later influenced by the chemical suffix -ol from alcohol.[Note 1] Monergols were monopropellants, while non-hypergols were bipropellants which required external ignition, and Lithergols were solid/liquid hybrids. Hypergolic propellants (or at least hypergolic ignition) were far less prone to hard starts than electric or pyrotechnic ignition. The "hypergole" terminology was coined by Dr. Wolfgang Nöggerath, at the Technical University of Brunswick, Germany.
The earliest ballistic missiles, such as the Soviet R-7 that launched Sputnik 1 and the US Atlas and Titan-1, used kerosene and liquid oxygen. Although they are preferred in space launchers, the difficulties of storing a cryogen like 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 US Titan II and in most Soviet ICBMs such as the R-36. But the difficulties of such corrosive and toxic materials, including leaks and explosions in Titan-II silos, led to their near universal replacement with solid-fuel boosters, first in Western submarine-launched ballistic missiles and then in land-based US and Soviet ICBMs.
The trend among western space launch agencies is away from large hypergolic rocket engines and toward hydrogen/oxygen engines with higher performance. Ariane 1 through 4, with their hypergolic first 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. Hypergolic rockets are still widely used in upper stages when multiple burn-coast periods are required.
Hypergolic rockets do not need an ignition system, so they tend to be inherently simple and reliable. While the larger hypergolic engines used in some launch vehicles use turbopumps, most hypergolic engines are pressure fed. A gas, usually helium, is fed to the propellant tanks under pressure through a series of check and safety valves. In turn, the propellants flow through control valves into the combustion chamber. They ignite instantly on contact, without any risk that a mixture of unreacted propellants might build up and ignite in a potentially catastrophic hard start.
The most common hypergolic fuels, hydrazine, monomethylhydrazine and unsymmetrical dimethylhydrazine, and oxidizer, nitrogen tetroxide, are all liquid at ordinary temperatures and pressures. Thus they are sometimes referred to as storable liquid propellants. They are suitable for use in spacecraft missions lasting for years. In contrast, liquid hydrogen and liquid oxygen are both cryogens whose practical use is limited to space launch vehicles where they need be stored for only a short time.
Because hypergolic rockets do not need an ignition system, they can be fired any number of times by simply opening and closing the propellant valves until the propellants are exhausted. This makes them uniquely suited for spacecraft maneuvering. They are also well suited, though not uniquely so, as upper stages of space launchers such as the Delta II and Ariane 5 that must perform more than one burn. Restartable cryogenic (oxygen/hydrogen) rocket engines do exist, notably the RL-10 on the Centaur and the J-2 on the Saturn V.
Relative to their mass, traditional hypergolic propellants are less energetic than some cryogenic propellant combinations, such as liquid hydrogen / liquid oxygen or liquid methane / liquid oxygen. That is to say that for a given payload to be put into orbit, a launch vehicle using hypergolic propellant would be more massive than one using these cryogenic fuels due to the greater mass of hypergolic propellant required.
Traditional hypergolics are also corrosive, toxic and carcinogenic, requiring expensive safety precautions on the ground.
Hypergolic combinations 
- Aerozine 50 + nitrogen tetroxide (NTO) – widely used in historical American rockets, including the Titan 2; all engines in the Apollo Lunar Module; and the Service Propulsion System in the Apollo Service Module. Aerozine 50 is a mixture of 50% UDMH and 50% straight hydrazine (N2H4).
- Unsymmetrical dimethylhydrazine (UDMH) + nitrogen tetroxide (NTO) – frequently used by the Russians, such as in the Proton rocket and supplied by them to France for the Ariane 1 first and second stages (replaced with UH 25); ISRO PSLV second stage.
- UH 25 is a mixture of 25% hydrazine hydrate and 75% UDMH.
- Monomethylhydrazine (MMH) + nitrogen tetroxide (NTO) – smaller engines and reaction control thrusters: Apollo Command Module reaction control system; Space Shuttle OMS and RCS; Ariane 5 EPS; Draco thrusters used by the SpaceX Dragon spacecraft.
Less common and obsolete 
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- Hydrazine + nitric acid (toxic but stable), also known as "Devil's venom", as used in the Soviet R-16 rocket of the Nedelin catastrophe.
- Aniline + nitric acid (unstable, explosive), used in the WAC Corporal
- Aniline + hydrogen peroxide (dust-sensitive, explosive)
- Furfuryl alcohol + IRFNA (or white fuming nitric acid)
- UDMH + IRFNA – MGM-52 Lance missile system
- T-Stoff + C-Stoff – Messerschmitt Me 163 World War II German rocket fighter aircraft, for its Walter 109-509A engine
- Kerosene + hot hydrogen peroxide – Gamma, with the peroxide first decomposed by a catalyst. Because of the heat from H2O2 decomposition, this is arguably not a true hypergolic combination. Cold (undecomposed) hydrogen peroxide and kerosene are not hypergolic.
Related technology 
Although not hypergolic in the strict sense (but rather pyrophoric), triethylborane, which ignites spontaneously in the presence of air, was used for engine starts in the SR-71 Blackbird and the F-1 engines used in the Saturn V rocket.
- "-ergol", Oxford English Dictionary
- O. Lutz, in History of German Guided Missiles Development, 1957
- Sutton, George P., History of Liquid Propellant Rocket Engines
- The Papers of Robert H. Goddard
- Botho Stüwe, Peene Münde West, Weltbildverlag ISBN 3-8289-0294-4, 1998 page 220, German
- Clark (1972), p.214
- Clark (1972), p.45
- T.A. Heppenheimer, Development of the Shuttle, 1972–1981. Smithsonian Institution Press, 2002. ISBN 1-58834-009-0.
- "Space Launch Report: Ariane 5 Data Sheet".
- "SpaceX Updates — December 10, 2007". SpaceX. 2007-12-10. Retrieved 2010-02-03.
- Brown, Charles D. (2003). Elements of spacecraft design. AIAA. p. 211. ISBN 978-1-56347-524-5.
- Clark, John (1972). Ignition! An Informal History of Liquid Rocket Propellants. New Brunswick, New Jersey: Rutgers University Press. p. 14. ISBN 0-8135-0725-1.
- Modern Engineering for Design of Liquid-Propellant Rocket Engines, Huzel & Huang, pub. AIAA, 1992. ISBN 1-56347-013-6.
- History of Liquid Propellant Rocket Engines, G. Sutton, pub. AIAA 2005. ISBN 1-56347-649-5.