GB2100395A - Pyrotechnic devices - Google Patents

Abstract

A pyrotechnic device, e.g. for an actuator producing motion by the action of the expanding gases on a piston in a cylinder, comprises a "minislapper" assembly including a barrel (4) and a thin plastic sheet (6) sandwiched between one end of the barrel and an electrically explodable metal foil (7). A pyrotechnic pellet (11) in a thin metal container (9) abuts the other end of the barrel. On exploding the foil, a plastic-sheet "flyer" is propelled up the barrel and ruptures the container, resulting in ignition of the pyrotechnic material. <IMAGE>

Description

SPECIFICATION Improvements in or relating to pyrotechnic devices This invention relates to pyrotechnic devices and provides an improved arrangement for igniting the pyrotechnic material thereof. Such devices are used, for example, in motion producers in which the gas from the ignited material moves a piston in a cylinder.

Conventional igniting arrangements for pyrotechnic devices are of the hot-wire or exploding-bridgewire kind, which have disadvantages from the safety aspect. One advantage of the present arrangement is improved safety.

An existing arrangement for initiating the detonation of high explosives is known as the "minislapper". Descriptions of the construction and performance of the minislapper have been given by J R Stroud in "A new kind of detonator - the slapper" (Proc 9th Symp on Explosives and Pyrotechnics, Franklin Institute Research Labs, Philadelphia, Pa, Sept 1 5-16, 1976, pp 22-1 to 22-6) and by D S Gilman in "Shock initiation of explosive pellets at low temperatures" (Proc 1 0th ditto, Franklin Research Center, San Francisco, Feb 14-16, pp 41-1 to 41-5).

Briefly, the minislapper comprises a thin sheet eg of 25,am plastic, sandwiched between a thin metal foil (eg 5ym Al) and one end of a short barrel. The latter is of the order of 1 mm diameter and similar length. The foil is exploded by passage of a high-energy electrical pulse, whereupon a disc of the plastic sheet (the "flyer' is propelled up the barrel and impacts upon a high-explosive pellet abutting its other end. The impact of the plastic flyer imparts a high-amplitude, short-duration shock to the pellet to initiate detonation.

As is known, the ignition characteristics of pyrotechnic materials differ from those of high explosives. In particular, pyrotechnic materials are not known to deflagrate by the application of such short-duration shock pulses but require relatively prolonged application of a high temperature.

Surprisingly, it has now been found that, notwithstanding the latter characteristic, the minislapper can be used to ignite pyrotechnic materials.

According to the present invention, a pyrotechnic device comprises: a minislapper assembly including a barrel and a thin electrically non-conducting sheet capable, in use, of forming a flyer, said sheet being sandwiched between one end of the barrel and an electrically explodable metal foil; and a volume of pyrotechnic material having a face which contacts or is closely adjacent to the other end of the barrel whereby explosion of the foil results in ignition of the pyrotechnic material.

The pyrotechnic material may be separated from said other end of the barrel by a thin wall which contacts said other end of the barrel and is adapted in use to be ruptured by the flyer following explosion of the foil. The wall may be of metal, eg aluminium, and may form part of a container for the material. The material may be in the form of a pellet.

To enable the nature of the present invention to be more readily understood, attention is directed by way of example to the accompanying drawing, which is a sectional eievation of a pyrotechnic device embodying the present invention.

In this drawing a minislapper assembly 1 comprises a cylindrical metal casing 2 of internal diameter 6mm containing a base 3 of a plastic or ceramic insulating material, eg glass-loaded nylon, into which are encapsulated two circuiar-section electrodes 8. Also within casing 2 is a barrel 4 made of eg glass-loaded nylon having a central bore 5 of diameter 1 mm and length 1 mm.

Sandwiched between the lower end of the barrel 4 and adhesively secured to the base 3 is a thin polyimide sheet 6 of thickness 25cm, on whose lower surface is deposited a 1 mm wide aluminium foil 7 of thickness Sum. The foil covers the end of bore 5 and extends beyond opposite sides thereof to contact the ends of the electrodes 8, which are flush with the surface of base 3.

Abutting the other end of barrel 4 is a pressed aluminium container 9 having a 75,um thick endwall 10 which contacts that end of the barrel.

Container 9 contains a pellet 11 of pyrotechnic material such as, for example, potassium picrate, barium styphonate, or aluminium/potassium perchlorate. Container 9 is completed by a lid 12 secured in place by the crimped end of casing 2.

The dimensions given are typical but not critical.

To ignite the pyrotechnic pellet 1 the foil 7 is exploded by applying a short high-energy electrical pulse to the electrodes 8, eg from a charged capacitor, in the same manner as is known for the initiation of high-explosive pellets.

Suitably the pulse is of about 4kV derived from a 0.2 yF capacitor.

As stated earlier, the resulting ignition of the pellet 11 is surprising since pyrotechnic compositions are not known to deflagrate by the application of such short-duration shock pulses as the minislapper produces. Indeed, without wishing to be bound by any particular explanation, it is believed that the role of the flyer in the present invention is quite different from that as a detonator of high explosive. In the present arrangement it is believed that the flyer simply ruptures the barrier constituted by the wall 10, allowing the hot plasma from the foil explosion to contact and ignite the pyrotechnic.Measurements of the output time at which pressure starts to rise from the pyrotechnic reaction (the onset of useful work) suggest such a thermal ignition mechanism, as the time is much longer (several milliseconds) than those associated with pure shock mechanisms (of the order of microseconds).

The present device has several attractive safety features for use with sensitive pyrotechnic materials where electrostatic and spurious pulse hazards may be present, in particular: (1) The pyrotechnic material can be enclosed in a metal container, the input end of which forms a metal barrier which is punctured by the flyer. The can will act as a Faraday shield to protect the material against electrostatic discharges.

(2) The explodable metal foil is fully insulated thermally and electrically by the plastic sheet which forms the flyer.

(3) The explodable metal foil is physically separated from the pyrotechnic material by the plastic film, an air gap (the barrel bore) and the metal barrier formed by the container wall.

Claims (7)

1. A pyrotechnic comprising: a minislapper assembly including a barrel and a thin electrically non-conducting sheet capable, in use, of forming a flyer, said sheet being sandwiched between one end of the barrel and an electrically explodable metal foil; and a volume of pyrotechnic material having a face which contacts or is closely adjacent the other end of the barrel whereby explosion of the foil results in ignition of the pyrotechnic material.

2. A device as claimed in claim 1 wherein the pyrotechnic material is separated from said other end of the barrel by a thin wall which contacts said other end of the barrel and is adapted in use to be ruptured by the flyer following explosion of the foil.

3. A device as claimed in claim 2 wherein the wall is made of metal.

4. A device as claimed in claim 3 wherein the metal is aluminium.

5. A device as claimed in any of claims 2, 3 or 4 wherein the wall forms part of a container for the pyrotechnic material.

6. A device as claimed in any preceding claim wherein the pyrotechnic material is in the form of a pellet.

7. A pyrotechnic device substantially as hereinbefore described with reference to the accompanying drawing.