FIRE MITIGATION DEVICE

Abstract: A device for initiating burning of an explosive or propellant material subjected to external heating prior to detonation is described, the device comprising at least two materials, at least a first one of which materials melts at a temperature below the temperature at which detonation of said explosive or propellant material is induced and reacts with said second material to produce an exothermic reaction to cause initiation of burning in said explosive or propellant material.

Description: FIRE MITIGATION DEVICE The present invention relates to a device to mitigate the response of solid fuel propellant motors when subjected to unwanted extraneous heating particularly, though not exclusively, under so-called "slow coo off" conditions. Explosive materials when exposed to heat can react in various ways. Depending upon the nature of the explosive and the rate of heating, the two most common exothermic reactions of explosive materials are ignition and detonation. Desirably, an explosive substance should not react under any conditions other than those of its specific initiation procedure. Furthermore, it should not react either by ignition or detonation at relatively low temperatures of less than about 160 β C. However, this is seldom the case. Explosive materials can be broadly divided into those which are intended to detonate and those which are intended to ignite or burn, albeit very rapidly. The present invention is mainly concerned with the latter type of material, an example of which is propellant material used for applications such as .rocket motors. Propellant materials, based for example on nitrocellulose (NC) or mixtures of nitrocellulose and nitroglycerine (NG), when subjected to extraneous heating at a relatively rapid rate will generally react by producing an ignition event where the material burns rather than detonates. The temperature at which this ignition event occurs s dependent on the heating rate of the material. A typical ignition temperature range for propellants based cn cast N'C/N'G matrices is acout 160 to 180 β C at, for example, a heating rate cf 5'C per minute. However, aithougn undesired ignition of a propellant motor is a serious and dangerous event, the potentially more dangerous event of explosive detonation may occur when the material is subjected to a very low heating rate. A heating rate of about 0.05°C per minute, for example, from ambient temperature may cause detonation at a temperature of about 120"C. During a detonation event, substantially all molecules of the material release their chemical energy simultaneously. A slow heating rate serves to bring the majority or all of the molecules in the motor matrix to an energy level where the next increment of heat input takes the material above an activation energy "barrier" to promote a simultaneous reaction causing a detonation event. Under conditions of low heating rate it is desirable to be able to stimulate an ignition at a lower energy level and cause the material to burn prior to detonation taking place. The object of the present invention is to provide a means of initiating ignition of a propellant or propellant igniter before a detonation activation energy level or a temperature at which detonation will occur is reached by the propellant material. BADORIGINAL According t o the present invention there is provided a device for in i tiating burning cf an explosive or propellant material prior to detona t ion, the device comprising at least two materials , at leas t a first one cf whic materials melts at a temperature below the detona t ion temperature of said explosive cr propellant ma t erial and reacts with sa d second material to produce an exothermic reaction to cause initiation by burning of said explosive cr propellant material. The at least two materials may be metallic materials and the exothermic reaction may be an alloying or chemical reaction be t ween said at least two metallic materials. In one embodiment of the present invention where initiation of NC/NG matrix materials, for example, is required one of said at least two metallic materials may be sodium metal which melts at approximately 98 β C. The second of said at least two materials may be an alloy of at least two of bismuth, lead, tin, cadmium, tellurium and antimony; the alloy having a melting temperature beginning at a lower temperature than that of sodium. In a preferred embodiment of the present invention, the more reac t ive material may be contained within a jacket of a lower melting temperature, more chemically stable material. This has been found to be advantageous in that, in the case of reactive group 1 metals such as sodium for example, an outer jacket of a bismuth-based alloy, for example, provides protection against reaction of the sodium with substances in the immediate PAD ORI IN ΆL environment (such as oxygen or moisture) during storage, and also promotes mixing of the two metallic components as the molten sodium reacts with the metal of the jacket. If the two components of the device are molten at the point of reaction, a homogeneous phase reaction can occur which promotes liberation of energy compared to a solid/liquid reaction. The device of the present invention provides a thermal output only, no spark or flame being produced. It has been found that external case temperatures up to 250"C may be achieved depending upon the proportions of sodium and bismuth- based alloy components. Temperatures of up to 500 β C have been attained with modified component materials employing additional reactive metal components such as lithium for example. The device of the present invention may be incorporated into an igniter which in normal usage would be used to initiate burning of the propellant; the device causing initiation of the material in the igniter which in turn ignites the propellant material only if a dangerously high external temperature is achieved. An advantage of including the device in an igniter is that the casing of the igniter serves to contain all the molten components of the device in a relatively small volume, even during the exothermic reaction. αwawitt. A lternatively, the device may be sited inside the propellant ma t erial itself. The device may, for example be attached to the inside surface of a motor casing and propellant cast about the device so as to encapsulate it therein on solidification. Although the present invention has been described with reference to propellant materials ased on C NG matrices, the invention is equally applicable to other explosive/propellant materials such as the so-called "composite" propellant materials comprising a matrix of a rubber material having fuel and oxidiser combined therein. Such materials have a temperature at detonation in the range of about 150 to 200 β C depending upon the actual composition and heating rate. Alloys based on bismuth and/or lead and/or tin are particularly advantageous owing to the wide range of temperatures at which melting may be initiated and the fact that substantially all these alloys react in an exothermic manner with metals from Group 1 of the Periodic Table. In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figure 1 shows a cross section of a device according to the present invention incorporated into an igniter; F ig ure 2 s h ows a schematic cross section of an alterna ti ve em b o di men t w h erein a device according to the present inven ti on i s incorporated into a propellant motor; and F i gure 3 w h ic h shews a schematic cross section of a secon d em b e d imen t of a device according to the present invention. R eferring now to the drawings and where Figure 1 shows a propellan t igniter 10 having a device according to the presen t i nven t ion incorporated therein. The igniter comprises an outer cas i ng tu b e 12 having a pyrogen propellant material 14 therein ; t h e pyrogen propellant material 14 being initiated, under normal con d i t ions , by an electro-explosive device (not shown ) and pyro t ec h nic composition 16. Throats 18 are provided around t h e perip h ery of the casing 12 to allow hot gases from the burning pyrogen propellant material 14 to impinge upon and ignite the surroun d ing propellant material of the rocket motor (not shown ) . T h e t h roats 18 are covered by a membrane 20 to prevent the i ngress of moisture when stored. The pyrogen propellant mater i al is provided with conduits and cross holes 22 so as to promo t e s i multaneous burning over a wide area in a short time once in i tia t ed. The igniter is also provided with a closed cap mem b er 24 a t th e en d remote from the pyrotechnic composition 16. The cap mem b er 24 contains a device 26 according to the present i nvention. T h e device 26 comprises an outer jacket of a bismuth - b ased alloy 28 of nominal composition 50wt% Bi; 27wt% Pb ; 13wt% Sn ; 10wt%Cd, the jacket having therein a pellet of sodium 30. At the open end of the cap member 24 is a shim member 32 of RIGINAL aluminium situated between the device 26 and the pyrogen propellant 14 in order to prevent the device itself from burning under normal initiation conditions of the igniter. Surrounding the device 26 s a thin layer of thermal insulation material and aluminium foil 34 indicated as a heavy dashed line. The igniter 10 cn being subjected to unwanted extraneous heating eventually causes the sodium pellet 20 to melt within the jacket 28 when the temperature of the sodium reaches about 98^. The composition of the jacket 28 is such that the onset of melting occurs at a temperature slightly below 98 β C. The molten sodium begins to react exothermically with the molten jacket material which accelerates the rate of reaction. The rate of temperature rise and rate of liberation of heat is sufficient to cause the pyrogen propellant material 14 to ignite thus initiating burning of the surrounding propellant material in the rocket motor (not shown), the operation of the igniter 10 being substantially the same as if it had been initiated by the pyrotechnic composition 16. Figure 2 shows a schematic cross section through a propellant motor having a device 26 according to the present invention installed therein. The motor 40 comprises a castable material 42 within a motor casing 44. The device 26 is attached to the inside of the casing 44 by, for example, adhesive means and the propellant material is cast into the casing to solidify around β * D OR IGI A T the device. The operation of the device per se is the same as described with reference to Figure 1, the heat of the exothermic reaction causing initiation of the material 42. Figure 3 snows an axial cross section through an alternative embodiment of the present invention wherein a device 50 comprises two concentric interfitted cylinders of lithium 52 and sodium 54, respectively, encapsulated within a jacket of a bismuth-based alloy 56. In this emoodiment, the sodium 54 again melts first and reacts rapidly with the lithium 52 m an exothermic manner and with the surrounding molten bismuth alloy jacket material 56. The overall reaction between the three components of the device is substantially more exothermic than the device described with reference to Figures 1 and 2. Devices constructed according to this embodiment have generated temperatures of up to 500"C. The mitigation device described in Figure 3 may either be incorporated into an igniter similar to that described with reference to Figure 1 or may be directly incorporated into a propellant motor as described with reference to Figure 2. 8 BAD 0RIG, NΛI ^

Claims: CLAIMS 1. A device for initiating burning of an explosive or propellant material prior to detonation, the device comprising at least two materials, at least a first one of which materials melts at a temperature below the detonation temperature of said explosive or propellant material and reacts with said second material to produce an exothermic reaction to cause initiation by burning of said explosive or propellant material. 2. A device according to claim 1 wherein the at least two materials are metallic materials and the exothermic reaction is a chemical reaction between said at least two metallic materials. 3. A device according to either claim 1 or claim 2 wherein one of said at least two metallic materials is sodium metal. 4. A device according to any one preceding claim wherein the second of said at least two materials is an alloy of at least two of bismuth, lead, tin, cadmium, tellurium and antimony. 5. A device according to either claim 3 or claim 4 wherein the second of said at least two metals has a melting temperature lower than that of said first metal. 6. A device according to any one preceding claim wherein the more reactive of said at least two materials is contained within a jacket of said second of said at least two materials. / 9 ι B D 0*, Gι " L. 7. A device according to any one preceding claim wherein only a thermal ou t put is produced by said exothermic reaction. 8. A device according to any one preceding claim wherein at leas t three materials are employed, on of which is lithium. g> An igniter including the device of any one of preceding claims 1 to 8. 10. A propellant motcr including the device of any one of claims 1 to 8. 11. A propellant motcr including the igniter of claim 9. 12. A device for initiating burning substantially as hereinbefore described with reference to the accompanying description and Figure I; or Figure 2; or Figure 3 of the drawings. 13. An igniter for initiating burning substantially as hereinbefore described with reference to the accompanying description and Figure 1; or Figure 3 of the drawings. 4. A propellant motcr having an initiation device or an igniter subs t antially as hereinbefore described with reference to the accompanying description and Figure 1; or Figure 2 ; or Figure 3 of the drawings. 10 £AD ORIGINAL- j_& ι____-_-__-_-_ • s**'

Inventors: WAGSTAFF DOUGLAS CLIVE, WAGSTAFF, DOUGLAS, CLIVE
Applicants: ROYAL ORDNANCE PLC, WAGSTAFF DOUGLAS CLIVE, WAGSTAFF, DOUGLAS, CLIVE
Application year: 1996
Main classification: F42B39
IPC codes: F02K9, F42B39