GB2276704A - Propellant charge system for a mortar bomb - Google Patents

Abstract

A mortar bomb comprising a projectile having a tail boom (6), a combustion chamber (12) within the tail boom, one or more vent holes (32) through the wall of the tail boom and communicating with the combustion chamber, a propellant container (14) located within the combustion chamber, a quantity of propellant material (13) within the container, and means (15) for igniting the said propellant material. The combustion chamber includes an expansion chamber (16) which extends beyond the portion thereof occupied by the propellant container, and communication between the propellant container (14) and the expansion chamber (16) is closed by a relatively weak portion (35) of the propellant container, which may be in the form of a metal foil lightly secured to the container (14) so as to close an aperture (34) between the container (14) and the expansion chamber (15). <IMAGE>

Description

Propellant Charge System This invention relates to mortar bombs, and especially mortar bombs having a novel propellant charge system.

When designing a primary charge system for a mortar bomb, It is necessary to undertake a detailed study in order to optimise a number of design parameters. The object is to design a system which has a high reliability over the envisaged range of operating temperature and consistent ballistic performance. The primary propellant is normally retained within a container which is located within the tail boom of the mortar bomb. The container is designed to rupture only after a high percentage of the propellant has burnt. The propellant gases then escape through ventholes in the tail boom of the body. The "all-burnt" position should ideally occur with the bomb as close as possible to the firing pin of the mortar weapon.

The percentage of propellant consumed at rupture is dependent upon the combustion volume, the container wall strength and the size of the vent holes. With a relatively small combustion volume, the pressure would reach a value high enough to rupture the container at stage when only a low proportion of propellant has burnt. While It is possible to design the combustion volume such that a high proportion of propellant is burnt at rupture merely by increasing the volume, the volume of propellant may then be insufficient to fill the combustion volume, particularly when the propellant has a high bulk density. Thus, especially when firing at low angles of elevation, the propellant can move within the combustion chamber, and may remain in close contact with the ignition means, resulting in unreliable ignition.

The primary propellant is normally contained within a chamber in the tail boom of the mortar bomb, which physically supports the propellant container. The peak pressure generated on firing may be of the order of 10 tons per square inch, and the tail boom must be capable of withstanding this pressure. It is not generally feasible to increase the percentage of propellant burnt at rupture by increasing the rupture pressure (eg by increasing the container wall thickness or reducing the vent area) as this will often result in tail failure.

Thus, especially where a propellant having a relatively high bulk density is used, there is a problem in that the combustion volume may need to be substantially greater than the volume occupied by the propellant, in order to ensure that sufficient propellant is burnt prior to rupture without generating pressure which the tail boom cannot withstand.

Also, it may not be possible to increase the strength of the tail boom, eg because of mass space or other design constraints. However, if the combustion volume were to be increased the propellant would become free to move within its container, and this would cause unreliable ignition. The present invention seeks to provide a solution to this difficulty.

Accordingly the present invention provides a mortar bomb comprising a projectile having a tail boom, a combustion chamber within the tail boom, one or more vent holes through the wall of the tail boom and communicating with the combustion chamber, a propellant container located within the combustion chamber, a quantity of propellant material within the container, and means for igniting the said propellant material, wherein the combustion chamber includes an expansion chamber which extends beyond the portion thereof occupied by the propellant container, and communication between the propellant container and the expansion chamber is closed by a relatively weak portion of the propellant container.

Conveniently the said weak portion comprises an aperture in the wall of the propellant container, said aperture being closed by a closure member which is lightly secured to the container.

Preferably the closure member is in the form of a metal foil, for example of tin/antimony alloy.

Advantageously the closure member is secured to the container by means of adhesive, eg by providing a self-adhesive coating on one side of the closure member.

Preferably the volume of the propellant is such as substantially to fill the propellant container.

The ignition means may include an ignition train comprising a percussion cap containing an initiating material, and an intermediate charge separated from the propellant charge by an easily ruptured membrane, such as a tin/antimony disc.

The primary propellant material may be single-based (nitrocellulose) material, for example in cut tubular form.

The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure 1 is a sectioned view of a mortar bomb in accordance with the invention, and Figures 2-4 are sectioned views of part of the tail boom of the mortar bomb shown In Figure 1, with fins and augmenting charge omitted, and showing the sequence of events on ignition of the primary charge.

As shown in Figure 1, a mortar bomb in accordance with the invention comprise a bomb body 1 containing a quantity of high explosive 2, a fuze 3, and a tail unit 4 which is screwed into the bomb body 1 at 5.

The tail unit comprises a tall boom 6, and a set of six equi-spaced radial fins. The fins are divided into fore and aft sections 7,8 and in the space between the fin sections 7,8 there is located an augmenting propellant charge 9 comprising propellant material 10 within a combustible casing 11.

As shown in more detail in Figure 2, the tail boom 6 is bored along its axis from the rear to form a combustion chamber 12. The bore is stepped, and the rear part of the bore contains the primary propellant 13 housed within a tubular aluminium container 14, as well as the initiation means 15. The foreward part 16 of the bore is of reduced diameter, and has a blind forward end.

The initiation means 15 is in the form of an initiation train comprising an initiating cap and holder assembly 17, and a flash tube 18 containing an intermediate charge 19 of gunpowder. The initiating cap and holder assembly comprises a cap holder 20 which has a recess 21 in its rear face which receives a small quantity of a sensitive initiating composition 27, such as lead azide, covered on its forward face by a paper disc 23 and on its reward face by a copper cap 24. Flash holes 25 extend forwardly from the recess 21 into an axial bore 26 in the flash tube, which contains the intermediate charge 19.

The aluminium case 14 is crimped into a cannelure 27 on the exterior surface of the flash tube 18, and a thin self-adhesive disc 28 of tin/antimony alloy seals the forward end of the bore 26, to separate the propellant 13 from the gunpowder 19.

A base member 29 has a screw threaded connection 30 with the flash tube 18, and the cap holder 20 is firmly held in place between these components 18, 29 when they are assembled together/ The gunpowder charge 19 is thus also held in place by the cap holder 20.

The assembly comprising the initiating cap and holder assembly 17, flash tube 18, gunpowder 19, disc 28, container 14 and propellant 13 constitutes the primary cartridge 31 for the mortar bomb. The base 29 has a screw threaded connection with the boom 6, by which the primary cartridge is assembled to the boom.

The boom 6 is provided with a number of vent holes 32 through the wall of the boom which communicate with the rearward part of the combustion chamber 12, at an axial location which corresponds with the container 14 and which falls axially between the fin portions 7,8. An augmenting cartridge 9 may, if desired for extra range, occupy the position between fin portions 7,8 as shown in Figure 1.

The forward end of the tubular aluminium container 14 is swaged inwardly at 33, and the remaining aperture 34 in the forward wall thereof is closed by a thin disc 35 of tin/antimony allow which is secured thereon by a self-adhesive coating on the disc.

In use, the augmenting charge 9 is first removed If not required.

The mortar bomb is then dropped tail-first into the barrel of a mortar weapon (not shown) and the cap 24 impacts on the firing pin of the weapon. The initiating composition 22 is thus nipped between the firing pin and an anvil 36 formed in the cap holder 20.

The ignition of composition 22 causes a flame 37 to pass through the flash holes 25 into the bore 26 and to ignite the gunpowder 19 as shown in Figure 3.

The volume of the container 14 is chosen such as to be just sufficient for the quantity of primary propellant 13, and so the propellant is held closely against the disc 28, whatever the orientation of the bomb on firing. Thus the initiation of the gunpowder 19 causes the disc 28 to rupture easily, and the flash 38 (Figure 3) passing through the ruptured disc makes immediate and intimate contact with the propellant 13, thus ensuring the reliable ignition of this primary propellant.

As soon as the pressure in the container 14 begins to rise on ignition of the primary propellant 13, the foil disc 35 ruptures or blows off, as shown in Figure 3, and the volume of the combustion chamber 12 is thus immediately increased by the container 14 opening into the expansion chamber 16. The peak pressure in the combustion chamber is thus effectively reduced, without the associated disadvantage of loosely held propellant 13 which could inhibit its reliable initiation As the combustion of the propellant 13 continues, in the enlarged chamber 12, the pressure therein rises to a predetermined value at which the wall of the container 14 ruptures at the location of the vent holes 32, and fragments 39 of the container wall may fly out, and the propellant gas and flame 40 thus escapes through the vent holes 32.

The augmenting cartridge 9 if present is ignited by the flames 40, and the propellant pressure causes the mortar bomb to be propellant along the mortar tube (not shown) in the normal way. The combustion of the propellant 13 and of the augmenting charge 9 will continue during the early part of this movement. The present invention however allows the all-burnt condition of the propellant to be achieved while the bomb remains close to the firing pin, thus enabling optimum ballistics.

Claims (11)

Claims

1. A mortar bomb comprising a projectile having a tail boom, a combustion chamber within the tail boom, one or more vent holes through the wall of the tail boom and communicating with the combustion chamber, a propellant container located within the combustion chamber, a quantity of propellant material within the container, and means for igniting the said propellant material, wherein the combustion chamber includes an expansion chamber which extends beyond the portion thereof occupied by the propellant container, and communication between the propellant container and the expansion chamber is closed by a relatively weak portion of the propellant container.

2. A mortar bomb according to claim 1 wherein the said weak portion comprises an aperture in the wall of the propellant container, said aperture being closed by a closure member which is lightly secured to the container.

3. A mortar bomb according to claim 2 wherein the closure member is in the form of a metal foil.

4. A mortar bomb according to claim 3 wherein the metal foil is of tin/antimony alloy.

5. A mortar bomb according to any one of claims 2 to 4 wherein the closure member is secured to the container by means of adhesive.

6. A mortar bomb according to claim 5 wherein the closure member has a self-adhesive coating on one side thereof.

7. A mortar bomb according to any one preceding claim wherein the volume of the propellant is such as substantially to fill the propellant container.

8. A mortar bomb according to any one preceding claim wherein the ignition means includes an ignition train comprising a percusslon cap containing an initiating material, and an intermediate charge separated from the propellant charge by an easily ruptured membrane.

9. A mortar bomb according to any one preceding claim wherein the primary propellant material comprises nitrocellulose material in cut tubular form.

10. A mortar according to claim 1 and bomb substantially as hereinbefore described.

11. A mortar bomb substantially as hereinbefore described with reference to any one or more of the accompanying drawings.