A pyrotechnic fastener (also called an explosive bolt, or pyro, within context) is a

fastener A fastener (US English) or fastening (UK English) is a hardware device that mechanically joins or affixes two or more objects together. In general, fasteners are used to create non-permanent joints; that is, joints that can be removed or disman ...

, 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 Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

current, and the charge breaks the bolt into two or more pieces. The bolt is typically scored around its

circumference In geometry, the circumference () is the perimeter of a circle or ellipse. The circumference is the arc length of the circle, as if it were opened up and straightened out to a line segment. More generally, the perimeter is the curve length arou ...

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

latch A latch or catch (called sneck in Northern England and Scotland) is a type of mechanical fastener that joins two or more objects or surfaces while allowing for their regular separation. A latch typically engages another piece of hardware on the ...

es.

In applications that require safety, precision and reliability, such as the aerospace industry, pyrotechnic fasteners are triggered using exploding bridgewire detonators, which were themselves later succeeded by

slapper detonator A slapper detonator, also called exploding foil initiator (EFI), is a detonator developed by Lawrence Livermore National Laboratory, US Patent No. 4,788,913 (Filed 1971, Granted 1988). It is an improvement over the earlier exploding-bridgewire de ...

s. Classical

blasting cap A detonator is a device used to make an explosive or explosive device explode. Detonators come in a variety of types, depending on how they are initiated (chemically, mechanically, or electrically) and details of their inner working, which of ...

s are generally avoided for such usage. More recent developments have used pulsed

laser diode file:Laser diode chip.jpg, The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD or semiconductor laser or diode laser) is a semiconductor device similar to a light-emittin ...

s to detonate initiators through

fiber-optic cable A fiber-optic cable, also known as an optical-fiber cable, is an assembly similar to an electrical cable but containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with p ...

s, which subsequently fire the main charge.

Gas generator A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical. The term often refers to a device that uses a ...

s are similar to pyrotechnic fasteners. They are used to generate large amounts of gas, as for turbopumps, to inflate balloons, especially airbags, to eject parachutes and similar applications.

Compositions used

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 RDX (Research Department Explosive or Royal Demolition Explosive) or hexogen, among other names, is an organic compound with the formula (CH2N2O2)3. It is white, odorless, and tasteless, widely used as an explosive. Chemically, it is classified ...

, sublime in vacuum, which limits their usefulness in aerospace applications. Composition with the character of

bipropellant The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical (a monopropellant) or a mix of two chemicals, called bipropellants. Bipropellants can further be divided into ...

s and

flash powder Flash powder is a pyrotechnic composition, a mixture of an oxidizer and a metallic fuel, which burns quickly (deflagrates) and produces a loud noise, regardless of confinement in some formulations. It is widely used in theatrical and display pyro ...

s are often used.

Standard pyrotechnic mixtures used by NASA

Manganese Manganese is a chemical element; it has Symbol (chemistry), symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese was first isolated in the 1770s. It is a transition m ...

/ barium chromate / lead chromate: Time-delay mix, used for sequencing. Gasless burning. *

nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...

: Gas generator, unsuitable for deep space missions, burn rate dependent on pressure. *

Boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

potassium nitrate Potassium nitrate is a chemical compound with a sharp, salty, bitter taste and the chemical formula . It is a potassium salt of nitric acid. This salt consists of potassium cations and nitrate anions , and is therefore an alkali metal nit ...

: Gas generator and rocket-motor igniter, thermally stable, stable in vacuum, burn rate independent of pressure. *

Zirconium Zirconium is a chemical element; it has Symbol (chemistry), symbol Zr and atomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustrous transition metal with a greyis ...

potassium perchlorate Potassium perchlorate is the inorganic salt with the chemical formula K Cl O4. Like other perchlorates, this salt is a strong oxidizer when the solid is heated at high temperature, although it usually reacts very slowly in solution with reducin ...

: Used in the 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 Lead(II) azide is an inorganic compound. More so than other azides, it is explosive. It is used in detonators to initiate secondary explosives. In a commercially usable form, it is a white to buff powder. Preparation and handling Lead(II) azi ...

: Used as a

primary explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An exp ...

. Sensitive to impact, friction, and static electricity. Thermally and vacuum stable, if

dextrin Dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch and glycogen. Dextrins are mixtures of polymers of D-glucose units linked by α-(1→4) or α-(1→6) glycosidic bonds. Dextrins can be produced fro ...

is not used for desensitizing. Long shelf life. * Hexanitrostilbene: Used in detonators, linear

shaped charge A shaped charge, commonly also hollow charge if shaped with a cavity, is an explosive charge shaped to focus the effect of the explosive's energy. Different types of shaped charges are used for various purposes such as cutting and forming metal, ...

s, and bulk explosives. Insensitive to stimuli other than explosion. Thermally stable. Vacuum stable. Used in deep space applications where

can not be utilized, such as aboard the

Apollo Lunar Module The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed sp ...

Detonates at .

References

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External links

Explosive bolts discussion in a mailing list
Fasteners Pyrotechnics Spacecraft pyrotechnics

Compositions used

Standard pyrotechnic mixtures used by NASA

See also

References

External links