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A pyrotechnic fastener (also called an explosive bolt, or pyro, within context) is a fastener, usually a nut or bolt, that incorporates a pyrotechnic charge that can be initiated remotely. One or more explosive charges embedded within the bolt are typically activated by an electric current, and the charge breaks the bolt into two or more pieces. The bolt is typically scored around its circumference at the point(s) where the severance should occur. Such bolts are often used in space applications to ensure separation between rocket stages, because they are lighter and much more reliable than mechanical latches.
Gas generators are close cousins of pyrotechnic fasteners. They are used to generate large amounts of gas, as for turbopumps, to inflate balloons, especially airbags; to eject parachutes; and for similar applications.
Various pyrotechnic compositions can be used, depending on the desired burn rate and required amount of energy and volume of gas produced. Some materials, such as RDX, sublimate in vacuum, which limits their usefulness in aerospace applications. Composition with the character of bipropellants and flash powders are often used.
Standard pyrotechnic mixtures used by NASA
- Manganese / barium chromate / lead chromate: Time-delay mix, used for sequencing. Gasless burning.
- RDX / nitrocellulose: Gas generator, unsuitable for deep space missions, burn rate dependent on pressure.
- Boron / potassium nitrate: Gas generator and rocket-motor igniter, thermally stable, stable in vacuum, burn rate independent of pressure.
- Zirconium / potassium perchlorate: NASA standard initiator (NSI). Rapid pressure rise, little gas but emits hot particles, thermally stable, vacuum stable, long shelf life. Sensitive to static electricity. Known to cause circuit damage during ground testing.
- Lead azide: Used in detonators. Sensitive to impact, friction, and static electricity. Thermally and vacuum stable, if dextrin is not used for desensitizing. Long shelf life.
- Hexanitrostilbene: Used in detonators, linear shaped charges, and bulk explosives. Insensitive to stimuli other than explosion. Thermally stable. Vacuum stable. Detonates at 22,000 feet per second (6,700 m/s).
- AMCP 706-179 pg 108
- Explosive Trains, Engineering Design Handbook Series, U.S. Army Materiel Command, March 1965, AMCP 706-179