Bridgewire Research Articles

Special Grade RDX for Exploding Bridge Wire Initiators

Special Grade RDX for Exploding Bridge Wire Initiators

Design concepts for an exploding bridge-wire electronic ignition system

An exploding bridge wire (EBW) is a fine wire conductor that is fused and vaporized by the discharge of a discrete package of electric energy in an extremely short time period. It can be used as a localized source of heat. Electrical and mechanical design features of the electronic firing unit used to actuate the various EBW devices on space vehicles are described and advantages of this ignition system over low-voltage squib iginition are pointed out

Squibs, Squib-Igniters, and Power-Packs

A NEW concept of electric squibs, igniters, and explosive ordnance has emerged because of the need for simplification in electrically initiated explosive systems. The development of a pressure tight squib (electric primer) has reduced the difficulties of conventional squib installation involving complicated wiring of electric leads through complex pressure seals into reaction chambers. The electric lead of the new type squib is hermetically sealed to the threaded case with high dielectric, pressure stressed ceramic (Fig. 1). The squib can be threaded directly into any high temperature pressure chamber. Advantages of this system include reduced costs, increased reliability, and improved performance and environmental storage characteristics. Because the squib is germane to all explosive systems, a natural outgrowth was the unification of the squib and the igniter or power-pack container into one part. Therefore, igniters and power-packs containing less than five grams of explosive are constructed integrally with the squib. Most units containing more than five grams can be more economically constructed by utilizing a separate main-charge case and a threaded-in squib. Power-packs are similar to igniters but differ in their function—igniters initiate the deflagration of propellants, while power-packs produce high energy gas for use in pressure actuated systems (Fig. 2). A hermetic and pressure seal is obtained by high temperature fusion of a pressure-stressed ceramic (3) to the steel contact pin (1) and case (2). The ceramic has an insulation resistance in excess of 10,000 megohms. The headed-type contact pin when placed under thermal shock and pressure exerts compressive, instead of shear, stresses on the ceramic. This system has supported 60,000 psi applied in 10 milliseconds. The unheaded contact pins of the M-30 fail at 8000 psi hydrostatic. To insure reliability, a welded, parallel circuit of nichrome or platinum-iridium bridge wire (4) is used and makes unnecessary the use of two single circuit squibs. On heating of the bridge wire (4) by application of d.c. current, the heat sensitive priming material (5) initiates the main charge explosives (6). Lead styphnate, diazodinitrophenol, and proprietary zirconium priming mixtures are used. Some are capable of high temperature storage (400°F-24 hr) and initiation at 90,000 foot altitudes. When used in an igniter, the main charge (6) contains ignition materials whose type and grain size are controlled by parameters of the propellant to be ignited. When used in an explosive powerpack the required interior ballistics of the actuator system (Fig. 2) determine the composition, and grain or pellet size of the main charge. Typical igniter materials are black powder, metal-oxidizer combinations, and single or double base propellants, while power-pack materials are gun propellants or black powder. Aluminum or nitrocellulose closure disks (7) are crimped and sealed with NRC or epoxy compounds. Hermetic ring welding of metal closures is currently being developed. Contact pins which can be attached to crimp or disconnect Stakon or AN connectors are illustrated and are available in 1, 2, or 3 pin styles. Products manufactured by McCormick Selph include squibs, igniters for liquid and solid propellants, high explosive bolt cartridges, explosive detents, gas generating canisters, and pressure power-packs. Power-packs like the M-48 and M-47 are used in the following applications: piston pressure (Fig. 2) for bolt and parachute release, cable and bolt cutters, liquid fuel valves, multiple stage missile separation, seat and canopy ejection, and rocket launcher, jato and wing tank jettisoning. Numerous squib, igniter, and power-pack units have been developed for, and tested by, engineering groups engaged in actual design and production of liquid and solid fuel motors, release mechanisms, gas generators, fracture bolts, and other explosive ordnance devices. The reliability and performance records obtained from these sources together with static testing conducted by McCormick Selph are compiled into Technical Data Sheets on each unit. Information in these regular bulletins includes: minimum and maximum static sensitivity, maximum current for zero probability of ignition, applied current vs. functioning time curves at ambient, — 65 °F, and +165°F, bomb chamber pressure and time data, and a detailed part description.

Electronic Shutter Photographs of Exploding Bridge Wires

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Societies and Academies

Societies and Academies

Kohlrausch's Meter-Bridge

MR. GLAZEBROOK, in commenting at the Physical Society on my use of Kohlrausch's meter-bridge with the telephone for the measurement of the resistance of the human body, suggested that the latter instrument was too sensitive, and that from self-induction perfect silence could not be obtained. Both these remarks are true; but if time and the chairman had permitted, I should have said that absolute silence is rarely got, but that the minimum of sound is so easy, after a little practice, to estimate, that onehundredth of a revolution on either side of it is instantly detected. The bridge wire takes ten turns on the barrel; consequently this amount is the thousandth part of a wire three metres long. Using a fixed resistance of 100ω, the possible error is quite unimportant, and even with 1000ω it is far within other instrumental accidents.