GB2206401A - Explosive switch for electromagnetic projectile launcher - Google Patents

Abstract

An explosive switch 18 for rapid opening of a high current circuit to launch a projectile has a reduced tendency to arc across the switch gap once opened. The switch consists of a busbar in the form of a pair of conducting plates 30 backed by a linear hollow charge 38 lying across the plates whose cavity 43 and liner 48 oppose one surface of the plates. Switch opening is effected by detonating the charge 38 which collapses the liner 48 and causes a high speed jet penetrator to rapldly sever the plates and break the electrical connection between their terminal ends. The plates 30 may have interlocking grooves (60) mating with the cavity 43. The charge 38 is contained in a nitrogen-filled closed-cell polymer casing 50. <IMAGE>

Description

EXPLOSIVE SWITCH FOR ELECTROMAGNETIC PROJECTILE LAUNCHER The invention relates to an explosive switch for an electromagnetic projectile launcher, and to an electromagnetic projectile launcher incorporating same.

Electromagnetic projectile launchers of the "railgun" type utilise high direct currents (DC) to launch projectiles. The basic construction of a railgun comprises a power supply circuit having two generally parallel rails bridged by a projectile armature. In operation the rails are short circuited by a switch connected in parallel with the circuit defined by the rails and armature, until the current level required for launch is achieved whereupon the current is allowed to flow through the projectile armature by opening the switch. The projectile armature is accelerated to launch speed as a result of the interaction of the current in the projectile armature with the magnetic field induced between the rail.

The typical requirements for the switch short circuiting the rails during the current build up are very low resistance (usually less than 10t5U; high current pulse carrying capability (usually of the order of 0.5-2MA for periods of up to 200 ms); rapid opening; and either capacity for repeated operation or capacity for easy replacement coupled with simplicity and relatively low cost.

In several practical embodiments of such a short-circuiting switch, the switch itself is a subsidiary railgun and is usually referred to as a "railswitch". The railswitch has its own set of rails and has an armature which is tethered during the current build up.

Once released, the switch armature is driven, similarly to the projectile armature, to a final position, in which position the current has been switched to flow through the projectile armature.

In one form of railswitch, the switch armature commutates the current across a gap in one of the rails and, in its final, arrested position, remains as a resistive element in the circuit thereby affecting the performance of the railgun.

Other forms of railswitches have been proposed in which the switch armature is eliminated from the circuit in the arrested positiion thereof. However, in those proposals the projectile armature is itself in the circuit, and thus subject to ohmic heating and electro-motive forces, during the current build-up.

All of these forms of railswitch suffer from arcing at the trailing edge of the switch armature causing damage to the switch armature and the switch rails. This problem arises because a reactance voltage, driven by the elemental inductance of the circuit, is generated during commutation of the current. Although solutions have been proposed to this problem (for example see US Patent No 4369692), none have been entirely successful.

Fast-acting, circuit-opening switches are known for railgun and other applications which consist of flat metal busbars, connected across terminals in the power supply circuit, which are shattered to break the circuit by the detonation of an explosive charge adjacent the sheet.

Although this type of switch has been used in the past in the basic railgun construction described above instead of a railswitch, the short-circuiting switch has to employ relatively thick and massive busbars to carry the very high currents required. Such busbars can only be shattered using large explosive charges which require a considerable amount of shielding to prevent damage to the rest of the railgun equipment and have only hitherto been regarded as useful in experimental railguns. Furthermore, the large amount of metal debris produced from the shattered busbar has caused problems of arcing across the terminals of the switch. This has lead to the design of complex and costly one-and two-stage explosive switches having many components which must be replaced or rebuilt between railgun firings.An example of this type of switch is disclosed by B M Rech & R C Zowark Jr in their article "Design and Construction of a two stage opening switch", IEEE Transactions on Magnetics, Vol. Mag. 22, No 6, November 1986.

It is an object of the present invention to provide an explosive switch for an electromagnetic projectile launcher, and an electromagnetic projectile launcher incorporating an explosive switch, in which at least some of the aforementioned disadvantages are reduced or obviated.

According to the present invention in a first aspect, there is provided an explosive switch for an electromagnetic projectile launcher, comprising a busbar consisting of at least one layer of conducting solid material having first and second terminal ends, and at least one hollow charge lying across and in engagement with a rearward face of the busbar between the two terminals ends, the at least one hollow charge including an explosive mass extending across the full width of the busbar and having a longitudinal cavity in its face opposing the rearward face of the busbar.

The conducting busbar is preferably of metal, most preferably copper.

Once initiated, the hollow charge effectively focuses the explosive energy such that thick section busbar material can be cut very rapidly and with the minimum amount of explosive. Typically, hollow charges are lined with ductile liner material (such as copper or lead or a metal-loaded plastic matrix) which is compressed by the initiated charge and formed into a high velocity jet with velocities over 5 km/sec. This effect occurs for both cone and linear hollow charge systems, and the latter has been used for the present invention.

Typically most linear hollow charge systems also use a metallic outer case (eg lead). However, the metallic content is a disadvantage in the presence of very large currents as a large quantity of fragments and ionised particles are produced which can lead to arcing and possible toxicity problems. It is for this reason that the hollow charge is preferably either caseless or has a largely metal-free case, and is most preferably of a type which is disclosed and claimed in UK Patent specification GB-2176878-A, the contents of which are incorporated in the present specification by way of reference. This type of hollow charge employs a case which is metal free apart from the optional presence of minor amounts of metallic stiffening strips.

Two or more hollow charges may be provided to cut up the busbar into three or more discrete portions, the centre portion or portions being simultaneously blown away from the terminals ends. This arrangement may have the advantage of reduced arcing in high current, high voltage circuits because it significantly increases the arc length across the switch once cut over that produced by a single cut.

The laminar busbar preferably comprises a multiplicity of stacked layers of conducting material, that is to say it is preferably of laminar construction. It has been found that the use of multiple conducting layers both enhances the cutting effect (i.e. increases the total thickness of material which can be cut for a given charge size) and further reduces the tendency for arcing to occur. The charge cuts the multiple layers successively, the final layer being both cut and blown apart thereby rapidly increasing the arc length between the layers either side of the cut. The directed blast from the hollow charge also tends to increase the ionisation path through the air very rapidly which also reduces the chance of arcing.

In a further embodiment of this invention, the busbar may be provided with at least one indentation in its forward face across the busbar between the terminal ends, each indentation forming a corresponding protrusion in the rearward face of the busbar which engages the longitudinal cavity of the explosive material. This arrangement generally provides for even faster cutting of the busbar, the protrusion/indentation feature on the busbar forming in effect the liner of the hollow charge, but since there is now no standoff distance between hollow charge and target, penetration efficiency is likely to be reduced and so a larger mass of explosive material may be required.

According to a second aspect of the present invention, there is provided an electromagnetic projectile launcher comprising an electrical power source for supplying direct current, a pair of substantially parallel rails, a projectile armature locatable between the rails for movement relative thereto, and an explosive switch according to the first aspect of this invention providing a short circuit across the rails and connected in parallel with the armature.

Explosive switches and electromagnetic projectile launchers incorporating same will now be described to illustrate the invention by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram showing the basic principle of an electromagnetic projectile launcher incorporating the present explosive switch connected betweeen terminals in the circuit; Figure 2 is a perspective view of part of the projectile launcher shown schematically in Figure 1; Figure 3 is a perspective view of a first embodiment of the present explosive switch connected in the schematic circuit diagram of Figure 1; Figure 4 is a schematic sectional view taken through the explosive switch of Figure 3 between the terminals; and Figure 5 is a schematic sectional view similar to Figure 4 of a second embodiment of the present explosive switch.

Referring to Figures 1 and 2 an electromagnetic projectile launcher of the "railgun" type, is shown generally at 10. The railgun 10 has an electrical power supply consisting of a homopolar direct current (DC) generator 12; a closing switch 14; a storage inductor 16 (which may be integral with the generator); and a short-circuiting switch 18 connected between terminals 19. Two parallel conducting rails 20 are connected to the supply across the shorting switch 18. A projectile armature 22 is located between the rails 20 and is designed to propel a projectile 24. In general, the projectile armature 22 may be of metal or other conducting material, insulated at 26 from the projectile 24, or of plasma.

In operation, the switch 14 is closed to charge the inductor 16 and, once the required current level has been achieved, the short-circuiting switch 18 is opened to divert the current through the projectile armature 22. The armature 22 is then propelled by electromagnetic forces along the rails 20 to launch the projectile 24.

The short-circuiting switch 18 is shown in more detail in Figures 2, 3 and 4. It consists of two 200mm wide, 3mm thick stacked copper sheets 30 which are affixed at either terminal end 31,31A to railgun pulse power supply busbars 32 by clamps 34. Each busbar 32 is connected to a power supply terminal 36. Alinear cutting charge 38 is affixed to the underside of the sheets 30 by a layer of adhesive 40.

The linear cutting charge 38 has an explosive bar 42 of oblong cross section having a groove 43 in the one face of the oblong opposing the sheets 30. The groove 43 is defined by the intersecting surfaces 44 and 46 which 0 are inclined one to another at an angle of 120 C.

The bar 42 is formed by extrusion from a mixture of 88% by weight of RDX (Cyclotrimethylenetrinitramine) 8.4% PIB (Polyisobutylene), 2.4% DEHS (2(Diethylhexyl)sebacate), and 1.2% PTFE (polytetrafluoroethylene) and is itself plastic.

-1 The bar 42 has a mass of 63 g.m . A V-section liner 48 of 0.635mm thickness formed by extrusion of a plastic mixture of 85Z by weight of 300 mesh copper powder and 5.6Z PIB, 1.6% DEHS and 7.8% PTFE is bonded to the surfaces 44 and 46 by pressure.

A flanged casing 50 of a nitrogen-filled closed cell polymer completely surrounds the bar 42 and liner 48 and provides a stand-off distance between the edges 52 of the liner 48 and the copper sheets 30 of about 4mm.

The bar 42 may conveniently be initiated by an electrically operated detonator (not shown) which may be affixed externally to the casing 50 at one end of the linear cutting charge 38.

Opening of the switch 18 is achieved by detonating the explosive bar 42 which collapses the liner 48 into a fast-moving jet penetrator which both cuts and rapidly blows apart the plates 30 giving negligible plasma production and arcing. Experiments have shown that the linear cutting charge 38 cuts through the sheets 30 in less than 50y leaving a 3mm gap between remaining sheet portions. The low mass of material removed from the plates during the cutting procedure means that only relatively thin barrier material is required to stop the particles blown out of the cut. Little or no arcing across the cut was observed when the switch was tested up to a pulsed current magnitude of 320 kA.

Replacing the used switch 18 with a fresh switch was found to take about 40 minutes.

In a second embodiment of the short-circuiting switch 18 illustrated in Figure 5 the plates 30 include a pair of interlocking Vshaped grooves 60. Each groove is defined at the underside of the plates 30 by a pair of intersecting surfaces 62 and 64 which are 0 inclined to one another at an angle of 120 Explosive bar 42 is directly affixed to each of the grooves 60 by mating the grooves 44 of the bar with the inclined surfaces 62 and 64 and pressure bonding the grooves and surfaces together. A casing 66 of nitrogen filled closed cell polymer surrounds three sides of the bar 42 except the groove 44.

Claims (8)

Claims

1. An explosive switch for an electromagnetic projectile launcher, comprising a busbar consisting of at least one layer of solid conducting material having first and second terminal ends, and at least one hollow charge lying across and in engagement with a rearward face of the busbar between the two terminal ends, the at least one hollow charge including an explosive mass extending. across the full width of the bus bar and having a longitudinal cavity in its face opposing the rearward face of the busbar.

2. An explosive switch according to claim 1 wherein the hollow charge is either caseless or is housed in a case which is largely metal-free.

3. An explosive switch according to either claim 1 or claim 2, wherein the explosive masS is formed from a composite of explosive material and a first plastics material, and the hollow charge includes a liner formed from a composite of metal and a second plastics material located within the longitudinal cavity.

4. An explosive switch according to any one of the preceding claims wherein the bus bar comprises a multiplicity of stacked layers of conducting material.

5. An explosive charge according to any one of claims 1,2 and 4 wherein the longitudinal cavity engages a correspondingly shaped protrusion in the rearward face of the busbar which protrusion is itself paired with a corresponding cavity in the forward face of the bus bar.

6. An explosive switch for an electromagnetic projectile launcher substantially as hereinbefore described with reference to Figures 3 and 4 or to Figure 5.

7. An electromagnetic projectile launcher comprising an electrical power source for supplying direct current, a pair of substantially parallel rails, a projectile armature locatable between the rails for movement relative thereto, and an explosive switch according to any one of the preceding claims providing a short circuit across the rails and connected in parallel with the armature.

8. An electromagnetic projectile launcher according to claim 7 substantially as hereinbefore described with reference to Figures 1 and 2.