Pyrotechnic fastener

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. Explosive charges embedded within the bolts are typically activated by an electric current, and the charge breaks the bolt into two or more piecesFact|date=April 2008. The bolt is typically scored around its circumference at the point(s) where the severance should occurFact|date=April 2008. They are often used in space applications to ensure separation between rocket stages, since they are lighter and far more reliable than mechanical latchesFact|date=April 2008.

Often the term "explosive bolt" is used to refer to pyrotechnic fasteners in general. For example it is often stated that the Space Shuttle is held to the launch pad with explosive bolts but it is actually the large "frangible nuts" that each contain two pyrotechnic charges that split the nut into two pieces.

For safety and reliability reasons, exploding bridgewire detonators and slapper detonators are frequently used in aerospace technology in place of classical blasting capsFact|date=April 2008.

More recent developments have utilized pulsed laser diodes to detonate initiators through fiber-optic cables which subsequently fire the main charge.

Gas generators are close cousins of pyrotechnic fastenersFact|date=April 2008. They are used to generate large amounts of gas, eg. for turbopumps, to inflate balloons especially airbags, to eject parachutes, and for other such applicationsFact|date=April 2008.

Compositions used

Various pyrotechnic compositions can be used, depending on the desired burn rate and required amount of energy and volume of gas producedFact|date=April 2008. Some materials, eg. RDX, sublimate in vacuum, which limits their usefulness in aerospace applicationsFact|date=April 2008. Composition with the character of bipropellants and flash powders are often usedFact|date=April 2008.

tandard 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.
* Lead azide: used in detonators. Sensitive to impact, friction, and static electricity. Thermally and vacuum stable, if dextrin 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 ft/s.

External links

* [http://media.nasaexplores.com/lessons/01-032/fullarticle.pdf Article detailing the use of explosive bolts by NASA]
* [http://www.nasaexplores.com/show2_5_8a.php?id=03-032&gl=58 Explosive bolts being used to power rescue cutting equipment]
* [http://yarchive.net/space/spacecraft/explosive_bolts.html]