Pyrotechnic fastener

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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 by a remote command[citation needed]. Explosive charges embedded within the bolts are typically activated by an electric current, and the charge instantly breaks the bolt into two or three pieces[citation needed]. The bolt is typically scored around its circumference at the point(s) where the severance should occur[citation needed]. Upon detonation a select part of the fastener rapidly disassembles without producing any shrapnel or fragments, freeing the mated units from each other. They are often used in space applications to ensure separation between rocket stages, since they are lighter and far more reliable than mechanical latches[citation needed].

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 the safety and reliability reasons, exploding bridgewire detonators and slapper detonators are frequently used in aerospace technology in place of classical blasting caps[citation needed].

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 fasteners[citation needed]. They are used to generate large amounts of gas, eg. for turbopumps, to inflate balloons especially airbags, to eject parachutes, and for other such applications[citation needed].

[edit] Compositions used

Various pyrotechnic compositions can be used, depending on the desired burn rate and required amount of energy and volume of gas produced[citation needed]. Some materials, eg. RDX, sublimate in vacuum, which limits their usefulness in aerospace applications[citation needed]. Composition with the character of bipropellants and flash powders are often used[citation needed].

[edit] 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.
  • 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.

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