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Hypergolic Propellants

Dateline: 08/09/99

By Alan Bruzel

Chemical energy from petroleum, cryogenic, solid propellant, and hypergolic fuel lifts a spacecraft from its launching pad. The Saturn V and the Space Shuttle orbiter provide examples. Five engines powered by petroleum and liquid oxygen comprised the first stage of the Saturn V. The cryogenic propellants liquid oxygen and liquid hydrogen were used by the second and third stages of this rocket. Solid propellants are used by fireworks as well as by the Space Shuttle (which also gets its lift from cryogenic fuel).

However, petroleum, cryogenic, and solid propellants must have an ignition system, thereby entailing additional engineering tasks of control, design, and troubleshooting. And, whereas petroleum and cryogenic propulsion systems can be switched on and off, solid fuel propellants continue to burn until their fuel is completely consumed.

Hypergols (hyper + erg + ol) offer propulsion engineers more flexibility. A hypergolic system needs no igniter, has no cryogenic equipment, and uses liquids whose flow can be regulated. The hypergolic system contains two principal ingredients: a fuel and an oxidizer.

Hypergolic fuels are members of the hydrazine family and include hydrazine, H₂N-NH₂, monomethylhydrazine, CH₃NH-NH₂, and unsymmetrical dimethylhydrazine, (CH₃)₂N-NH₂. The oxidizer is usually nitrogen tetroxide, O₂N-NO₂. As is the case with other spacecraft, the Space Shuttle uses hypergols for its orbital maneuvering. The immediate and controllable reaction initiated by mixing the fuel and the oxidizer allows the precise adjustments required for insertion into orbit and maintenance of orbit.

Because the two hypergolic components rapidly react on contact, monomethylhydrazine is stored at the southwest corner of NASA Kennedy Space Center's pad area, while nitrogen tetroxide is held at the southeast corner. Before a launch, they are carefully transferred into the Space Shuttle through the Hypergolic Umbilical System. The fate of the unmanned Mars Observer in August 1993, just three days before it would have entered orbit around Mars, acts as a reminder of the explosive potential of hypergols. An accidental mixing of monomethylhydrazine and nitrogen tetroxide is believed responsible for the uncontrollable spinning and subsequent loss of that spacecraft.

Although their reactivity is respected, there is one practical drawback to hypergols: they are quite poisonous. They not only react with each other, but with living tissue, as well. Consequently, hypergolic propellants are stored in keep-out zones and handled by technicians wearing full-body Self-Contained Atmospheric Protection Ensemble (SCAPE) suits. Also, launches propelled by hypergolic fuel, such as the Titan class rockets, are generally off-limits to the public.

Recommended Web resources for additional information:

Gases and Liquids Used on Space Shuttles
Dian Hardison describes hazards of spacecraft propellants. From Space Team Online.

Hyperventilation or Unseen Hazards
Article by Dan Kovalchik describes safety aspects of rocket fuels such as hypergols.

Liquid Propellants & Fluids Management Office
Overview of rocket fuels and other chemicals used in launch vehicles and the payloads they carry. From NASA Kennedy Space Center.

Mars Observer Investigation Report Released
An explanation of Aug. 21, 1993 loss of communication with the Mars Observer spacecraft.

Propulsion Capabilities Summary
Technical aspects of the White Sands Test Facility areas used for propellant research.

Propulsion Systems
Concepts and mechanisms used in spacecraft propulsion. From Lance K. Erickson, Embry-Riddle Aeronautical University.

Why Is It Preferable to Use Hydrazine as Rocket Fuel Instead of Ethane?
Chemistry of hydrazine and methylhydrazine. From The Mad Scientist Network.

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