Hypergolic fuel

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Hypergolic rocket fuels spontaneously ignite when their two components come into contact with each other. Although the fuel is difficult to handle, the hypergolic engine is easy to control and very reliable. During World War II, rocket fuels 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.

The engine can be precisely controlled with only two valves, one for each fuel component. 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.

Hypergolic fuels have been used for intercontinental ballistic missiles, especially the Titan II missile, but because of difficulties in storing fuel, the trend in ICBMs has been to move toward solid fuel boosters.

They are less likely to explode when starting. Should a rocket explode upon starting, it is called a "hard start."

Some common hypergolic fuel combinations are:

• 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)
• 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 plane
• 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

Indian PSLV rocket uses Hypergolic propellants. PSLV stage 2 uses UDMH and N2O4 and PSLV stage 4 uses MMH and MON as propellants.

[edit] 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]