Hypergolic propellant

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 are difficult to handle, a hypergolic engine is easy to control and very reliable.

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Derivation of the term

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 German chemical suffix -ol from alcohol.

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 predictable, i.e., the direction and velocity of the rocket will closely match calculations. For this reason, they were used by the returning LEM on the Apollo moon missions.

Hypergolic propellants are also less likely to detonate when starting, a potentially catastrophic condition known as a hard start.

Use in ICBMs

Hypergolic propellants have been used for intercontinental ballistic missiles, especially the Titan II, but because of difficulties in storing fuel, the trend in ICBMs has been to move toward solid-fuel boosters.^{[citation needed]}

Common hypergolic propellants combinations

• Hydrazine-nitric acid (toxic but stable)
• Aniline-nitric acid (unstable, explosive)
• Hydrogen peroxide-aniline (dust-sensitive, explosive)
• UDMH-nitrogen tetroxide (by far the most common hypergolic fuel, less reactive than others, but by no means inert)
• UDMH-IRFNA used in the MGM-52 Lance missile system (toxic and flammable but safe for long-term fueling of rockets)
• MMH-nitrogen tetroxide used in the Space Shuttle program OMS and the EPS Upper Stage of the European Ariane 5 rocket
• T-Stoff and C-Stoff used in the Messerschmitt Me 163 rocket fighter airplane
• Hydrazine and nitrogen tetroxide used in the Lunar Module of the Apollo missions as well as the reaction control system (RCS) of the US Space Shuttle program
• The Indian Space Research Organisation's PSLV rocket uses hypergolic propellants. Its stage 2 uses UDMH and N₂O₄, while its stage 4 uses MMH and MON as propellants.

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.