A hypergolic propellant is either of the two rocket propellants used in a hypergolic rocket engine, which spontaneously ignite when they come into contact. The two propellants are usually termed the "fuel" and the "oxidizer". Although hypergolic propellants tend to be difficult to handle, a hypergolic engine is easy to control and very reliable.

In common usage, the terms "hypergol" or "hypergolic propellant" are often used to mean the most common such propellant combination, hydrazine plus nitrogen tetroxide, or their relatives.

History

Soviet rocket engine researcher, Valentin Glushko experimented with hypergolic fuel as early as 1931. It was initially used to "chemically ignite" engines, starting kerosene/nitric-acid engines with an initial charge of carbolic acid. The Wac Corporal rocket developed by JPL in 1944 used nitric acid with aniline fuel.

During World War II, rocket propellants were broadly classed as monergols, hypergols and non-hypergols. 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. Monergols were monopropellants, while non-hypergols were bipropellants which required external ignition. As ignitionless combinations were seen as being more desirable than simple ergols, they were deemed 'hypergols.'

Advantages

A hypergolic engine can be precisely controlled with only two valves, one for each propellant. This simplifies the control system and eliminates points of failure. With no complex starting procedure the thrust is more predictable, i.e., the direction and velocity of the rocket will closely match calculations. Hypergolic propellants are also less likely to accumulate to dangerous quantities, then detonate when starting, a potentially catastrophic condition known as a hard start.

In addition, the two common hypergols, various hydrazines and certain oxides of nitrogen, can be stored at ordinary temperatures and pressures. This allows their use on spacecraft well after launch.

Use in ICBMs

Hypergolic propellants have been used for ballistic missiles, such as the Titan II, and most Soviet ICBMs in wide deployment. Switching to hydrazines and oxides of nitrogen eliminated cryogenic propellants, which boiled off during storage and needed constant replenishment. But because of difficulties in storing such corrosive and toxic hypergols, the trend in ICBMs has been to move toward solid-fuel boosters, first with Western submarine-launched ballistic missiles, then the next-generation land-based US ICBMs, then later Soviet ICBMs.^{citation needed}

Common hypergolic propellant combinations

• Unsymmetrical dimethylhydrazine (UDMH) + nitrogen tetroxide - frequently used by the Soviets, 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
• Aerozine 50 + nitrogen tetroxide - large engines, especially US: Titan first and second stages; Apollo Service Module Service Propulsion System; all Apollo Lunar Module engines
• UH 25 + nitrogen tetroxide - large engines: Ariane 1 through Ariane 4 first and second stages
• Monomethylhydrazine (MMH) + nitrogen tetroxide - smaller engines and thrusters: Apollo Service Module Reaction Control System (RCS); Space Shuttle OMS and RCS; Ariane 5 EPS; ISRO PSLV fourth stage

Less common and obsolete combinations

• Hydrazine + nitric acid (toxic but stable)
• Aniline + nitric acid (unstable, explosive), used in the Wac Corporal
• Aniline + hydrogen peroxide (dust-sensitive, explosive)
• UDMH + IRFNA - MGM-52 Lance missile system
• T-Stoff + C-Stoff - Messerschmitt Me 163 rocket fighter airplane
• 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.

Aerozine 50 is a mixture of 50% UDMH and 50% straight hydrazine (N₂H₄).

UH 25 is a mixture of 25% hydrazine hydrate and 75% UDMH.

The corrosiveness of nitrogen tetroxide can be reduced by adding several percent nitric oxide (NO), forming MON.

Sources

• "-ergol", Oxford English Dictionary.
• 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.