Patent Publication Number: US-2023132848-A1

Title: Casing for a fragmentation weapon, fragmentation weapon, and method of manufacture

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of casings for fragmentation weapons, particularly casings comprising preformed fragments. 
     BACKGROUND TO THE INVENTION 
     Explosive weapons such as warheads, projectiles, bombs, anti-tank mines, shells, mortars, rockets, missiles, grenades, etc. that shatter following detonation of an explosive charge are primarily effective owing to fragmentation. In general, fragmentation occurs when a large and energetic charge of high explosive detonates and produces a powerful shock wave which shatters the entire casing of an explosive weapon into many fragments that are projected in all directions. Generally, the ability of a weapon to cause a lethal effect on a target is probabilistic. This is reflected in the often quoted ‘lethal radius’ for a weapon-defined as the distance from the blast at which it is expected a given weapon still has a 90% chance of lethality. The probabilistic nature of weapon lethal effect arises from dependency on a number of factors, including the number, mass, shape, size, velocity and distance travelled of any resulting fragments following detonation. An optimal lethal effect can be achieved by maximising the number of fragments available after detonation, thereby increasing the probability of a fragment hitting a target, and ensuring each fragment has sufficient momentum (which is a factor of mass and velocity) to damage the target. The number of fragments per unit area decreases with the square of the distance from the detonation point of the weapon, and the fragment momentum required to kill/destroy a target is dependent on the target type. Therefore fragmentation is a significant factor in lethality. Furthermore, collateral damage can occur if the fragments are larger than expected (as can occur where homogeneous casing materials are allowed to breakup organically at detonation) and have sufficient momentum to travel beyond the intended lethal radius. It is therefore additionally desirable to have predictable fragmentation, in order to predict and optimise the lethal effect of the weapon whilst mitigating collateral damage. 
     One approach to this is to include preformed fragments, such as many metal ball bearings as part of a weapon. This allows some predictability in distribution, mass and velocity of fragmentation after detonation. For example DE2536308 discloses a warhead comprising an explosive surrounded by a layer of preformed fragments sandwiched between two tubular walls of wire mesh. This is then cast into metal forming a single solid tubular wall that fragments upon detonation. Whilst this provides a degree of predictability in fragmentation, the concept suffers in that if the cast casing is structurally robust, then the predictability of fragmentation is reduced, conversely, if the cast casing is weak the mass allocated to the preformed fragments cannot play a meaningful part in the structural integrity of a penetrating warhead. Prior art includes designs where the preformed fragments actually occupy part of the volume inside the warhead that could otherwise be usefully allocated to the explosive charge. This is particularly a problem for weapon designers who are often limited to a maximum mass and spatial size for a weapon, and therefore any mass or volume allocated to a fragmentation layer needs to be compensated for by reducing the mass or volume allocated to either the casing (further affecting the structural integrity) or the explosive charge (affecting blast energy). 
     It is therefore an object of the present invention to provide a casing for a fragmentation weapon that mitigates these issues. 
     STATEMENT OF INVENTION 
     According to a first aspect of the present invention there is provided a casing for a fragmentation weapon, comprising an inner casing layer, an outer casing layer, and a plurality of preformed fragments located between the layers, wherein the casing further comprises a preformed lattice defining a plurality of preformed cells shaped to fit the preformed fragments, each preformed fragment being slidably fitted within a respective preformed cell, the preformed lattice and fragments being sandwiched between the inner and outer casing layers, such that the preformed fragments are held within the preformed cells by confinement, and such that when a fragmentation weapon comprising the casing is detonated the preformed fragments are freely separable from the preformed lattice. 
     This has the advantage that upon detonation of the weapon, when the outer layer naturally fragments the preformed fragments can freely slide out of their cells within the lattice; typically in a radial direction. Each fragment can therefore be expected to remain unattached to both the lattice and adjacent fragments. With a known explosive charge, this results in an improved predictability of the mass, shape and velocity of each fragment after detonation and therefore the lethal effect can be optimised and collateral damage can be reduced. It is expected that the preformed lattice itself and the inner and outer casing layers will also break up on detonation, contributing further to fragmentation. 
     In addition, it also has the further advantage that the mass and volume allocated to the preformed lattice and preformed fragments provides useful improvement to the structural strength/integrity of the casing. This is because, when the preformed lattice is combined with the preformed fragments (to form the fragmentation layer) and then sandwiched between the inner and outer layers of the casing (i.e. the faces of the inner and outer layers are pressed up against the faces of the fragmentation layer) the result is a substantially solid casing structure which provides improved structural strength to the casing. In this configuration, the preformed lattice provides tensile strength and the preformed fragments provide compressive strength which advantageously helps to distribute stress across the fragmentation layer thereby improving the overall structural strength of the casing. This is particularly advantageous for weapons or other projectiles that are exposed to excessive loading during impact with a solid structure such as a wall or vehicle. In this scenario it is often advantageous that the weapon does not buckle or tear as this can reduce the effectiveness of the weapon or prevent a successful engagement. 
     To ensure the preformed fragments contribute to the compressive strength and provide predictable fragmentation, the inner and outer casing layers need to sandwich the fragmentation layer such that the preformed fragments are confined to their cells throughout the engagement and are prevented from sliding out of their cells before the charge is detonated. 
     For example, in some embodiments the weapon is a cylindrical shaped weapon which provides an advantageous distribution of fragments and conforms to common weapon shapes. In these embodiments the inner and outer casing layers and fragmentation layers are tubular shaped having an annular cross section and an axial length. The inner casing layer cross section having a shorter diameter than that of the fragmentation layer and the outer casing later having a longer diameter than that of the fragmentation layer. The diameters of these layers are such that they sandwich against the fragmentation layer thereby preventing the preformed fragments from sliding out of their cells before detonation of the weapon. Generally, the inner and outer casing layers comprise a metal such as aluminium, titanium or steel, while fragments may be tungsten, steel or other hard materials. 
     Generally the fragmentation weapon is an explosive device wherein an explosive charge is contained within a casing such that when the charge is detonated the casing shatters into fragments which travel at high velocity over a wide area. In some embodiments, the casing may be provided with an end and/or nose cap as required by the weapon design/intended application. 
     Generally the casing is an external shell or wall that encloses the explosive charge primarily to provide structural strength to the weapon (as well as providing a vessel to transport the explosive throughout an engagement) but also to produce fragments upon detonation of the explosive charge that contribute to the lethal effect of the weapon. The casing is generally tubular shaped, depending on the shape of the overall weapon and comprises several layers: an inner casing layer, a fragmentation layer and an outer casing layer. The structural strength of the weapon is primarily defined by the structural strength of the combined layers of the casing. Each layer typically comprises a metal which could be any metal but typically the metal is chosen based on hardness and/or tensile strength to suit the intended application. 
     Generally, the inner casing layer is an interior layer of the casing, situated closer to the explosive charge than the other layers of the casing. In some embodiments, the inner casing layer is a pipe around some explosives. For instance the inner casing layer may be adjacent the explosive charge, or even substantially in abutment with the explosive charge. In some embodiments the inner casing layer defines a cavity into which an explosive fill is provided, for instance. If the casing is tubular, the inner casing layer has a shorter radius than the other layers. Generally, the inner layer comprises a homogeneous material in order to provide structural strength to the weapon when combined with the other casing layers. Typically the inner layer naturally fragments upon detonation of the explosive charge. 
     Generally, the outer casing layer is the exterior layer of the casing, situated further from the explosive charge than the other layers of the casing. The outer casing layer may define an exterior surface of a weapon for instance. Generally, the outer layer comprises a homogeneous material which provides structural strength to the casing when cooperating with the other casing layers. Typically the outer layer naturally fragments upon detonation of the charge. 
     The fragmentation layer comprises a preformed lattice and a plurality of preformed fragments. The fragmentation layer is located between the inner and outer casing layers in a sandwiched arrangement. The internal and external faces of the preformed lattice and preformed fragments are pressed up against the adjacent faces of the inner and outer casing layers. Consequently, before the explosive is detonated, the preformed lattice and preformed fragments are securely held between the inner and outer casing layers such their movement is substantially prevented. However, after the explosive charge is detonated, the inner and outer casing layers will typically shatter, exposing the preformed lattice and permitting movement of the preformed fragments from their respective cells of the preformed lattice. Slidably fitted means that the preformed fragments may occupy the volume of the preformed cells such that the preformed fragments touch the cell walls but are not physically attached to it. They therefore slide out of the cell when the charge is detonated. Owing to the fragments being slidably received into the lattice, they offer improved separation from the lattice when the fragmentation weapon is detonated. 
     Generally preformed fragments are individual pieces of a material (ideally of a high hardness metal) that have a predefined size, shape and mass. As the preformed fragments are physically unattached to the lattice and are solely held within the cells of the lattice by confinement, they will typically maintain the same size, shape and mass throughout the engagement (i.e. whilst secured in the casing and after the casing shatters upon detonation) therefore enabling improved predictability in respect of lethal effect and collateral damage. 
     Generally a lattice is a frame or structure comprising a homogeneous piece of material which is typically a solid sheet of metal with a plurality of preformed cells which are essentially holes formed through the sheet and arranged across the sheet. Preferably, the metal has a high hardness (such as high hardness/hardened steel) to improve the structural strength of the warhead against compressive forces. Preferably, the metal has a high tensile strength (such as high tensile steel) to improve the structural strength of the warhead against tension forces. Typically the preformed cells are not touching each other (which would form a larger hole affecting the structural integrity of the lattice) but are separated such that each preformed cell is surrounded by the sheet material (forming a boundary between the cells). The material/boundaries between the cells are known as the lattice struts. The lattice struts form the walls of the preformed cells. The arrangement of the cells across the lattice may be a regular or irregular pattern as required to optimise the lethal effect and/or reduce collateral damage. Preferably, the preformed cells are squares and are orientated such that the walls of the cell are parallel with the axial plane of the lattice; this combination of features is advantageous because it improves the tensile strength of the preformed lattice. 
     The preformed cells are shaped to fit the preformed fragments. This means that the shape and size of each preformed cell are such that a preformed fragment will fit inside it and the cell walls will hold the fragment in place generally preventing tangential or axial movement relative to the weapon. Furthermore, a fragment can easily be inserted into a cell and slide through it without significant force. In some embodiments, there are between 500 and 1500 separate preformed cells cut into the preformed lattice; however, clearly this is dependent on the size of the lattice and cells. 
     The preformed fragments should be able to freely separate from the cells following detonation of the explosive charge. Preferably, there are no adhesive or metallic bonds between the preformed fragments and the lattice and the inner and outer radial faces of the preformed fragments are exposed. Preferably, each fragment is loosely held within a cell purely by the friction between the surface of the fragment and the adjacent cell walls. As the preformed fragments are physically unattached (i.e. there are no metallic or adhesive bonds) to any other part of the casing or weapon they are therefore only held within their cells by confinement within the lattice and the inner and outer casing layers. This means that, as the preformed cells are shaped to fit the preformed fragments, the external surfaces on the preformed fragments will typically touch the cell walls (struts of the lattice) and the inner and outer casing layers. This prevents movement of the preformed fragments before detonation (which improves structural strength) and enables them to freely separate from their cells and the lattice once the inner and outer casing layers shatter after detonation of the explosive charge. 
     The lattice can be any thickness as required for the weapon design. In one embodiment, the thickness of the preformed lattice is the same thickness as the preformed fragments. Optionally the lattice layer thickness is between 5-15 mm. 
     The inner casing layer and outer casing layers can be the same or different thicknesses to each other and the fragmentation layer depending on the requirement of the weapon. Optionally the inner and outer casing layers thicknesses range between 120-260 mm. 
     Preferably, the preformed fragments have a square cross section and the walls of the cells are orientated parallel to the axial plane of the lattice i.e. for a cylinder, the axial plane is one which is parallel to the axis of symmetry/revolution and perpendicular to the radial, circumferential and tangential axes. This combination of features provides further improvement to load distribution across the fragmentation layer therefore further improving the structural integrity of the casing. The struts may be any shape or thickness such as required by the expected loading on the structure. 
     For a cylindrical shaped casing, the lattice may be a tubular shaped structure with a circular cross section and an axial length. Optionally, the axial length of the lattice is such that it runs the full axial length of the casing to maximise the number of preformed fragments; however this may reduce overall structural integrity. 
     The lattice may be of a different shape if required to conform to the shape or form factor of a different shaped weapon. The lattice may be a solid structure; preferably it is formed from a single piece of material and is homogeneous in order to improve structural strength. Optionally, the lattice comprises a metal with a high hardness such as high hardness steel or tungsten to improve the structural strength against compression forces. Optionally, the lattice comprises a material with a high tensile strength such as carbon fibre or high tensile steel to improve the structural strength against tensile forces. 
     Optionally, the axial length of the lattice is less than the full length of the weapon to reduce the mass, cost and complexity of the lattice or to improve the overall strength of the casing. Optionally the lattice is less than ½ the length of the weapon, optionally the lattice is less than ¾ the length of the weapon. 
     The preformed fragments may be any shape, regular or irregular. In addition, the fragments may or may not be of uniform size and design throughout the lattice. The size of the fragments also impacts the thickness of the lattice struts and therefore the required structural strength of the lattice should be taken into account. It is possible to design preformed fragments shape and size as required for a specific target type, collateral damage area and lattice structural strength. Optionally, the fragments are of non-uniform size and design throughout the lattice to optimise the weapon effect against different types of targets. 
     Generally the preformed fragment material is harder than or at least as hard as the intended target material. Preferably, the preformed fragments comprise a metal with a high density such as Tungsten or high hardness steel. Tungsten being a significantly higher density than steel. Higher density materials lead to a higher mass and higher momentum which advantageously provides resistance against impacts with other fragments and allows deeper penetration into the target. However, the preformed fragment material choice may be limited by the mass allocation for the warhead. 
     The preformed lattice and preformed fragments are physically separate components of the casing and therefore advantageously they can be manufactured separately and do not need to be physically attached together to form the casing. This reduces the manufacturing cost and complexity when compared to casings in the prior art. The lattice may be a solid piece of metal constructed using conventional manufacturing methods; which includes casting but also includes stamping the preformed cells into a flat sheet of steel before shaping the sheet into the desired casing shape and welding along the seam. A casing constructed using conventional manufacturing techniques will typically have an improved structural strength to that of a casing constructed using additive manufacturing. This allows the weapon designer to reduce the mass of any other layers in the casing without reducing the overall structural integrity of the weapon. 
     The improved predictability of the blast enables the skilled person to optimise the lattice structure and preformed fragment shape/size to meet the requirements of the weapon. Generally a single fragment may be confined within a single cell for simplicity but embodiments having multiple fragments held within a single cell are also possible. All cells within the lattice may be completely populated such that every cell is occupied by one or more preformed fragments which maximise the number of fragments to improve the lethal effect of the weapon. Optionally, a number of cells are empty which has the advantage of reducing the overall mass/weight of the weapon; this may negatively impact structural strength and overall fragmentation effect, but this may be acceptable in some circumstances. 
     According to a second aspect of the present invention, there is provided a fragmentation weapon comprising a casing according to the first aspect of the invention. Optionally, the fragmentation weapon is a projectile, but alternatively may comprise a warhead sub-section of a larger weapon system. 
     According to a third aspect of the present invention, there is provided a method of manufacturing a casing for a fragmentation weapon comprising the steps of:
         Providing a casing for a fragmentation weapon, comprising an inner casing layer and an outer casing layer;   Providing a plurality of preformed fragments;   Providing a preformed lattice defining a plurality of preformed cells shaped to fit the preformed fragments, each preformed fragment being slidably fitted within a respective preformed cell;   Arranging the inner and outer casing layers to sandwich the preformed lattice such that the preformed fragments are held within the preformed cells by confinement, and such that when a fragmentation weapon comprising the casing is detonated the preformed fragments are freely separable from the preformed lattice.       

     Optionally, the preformed lattice comprises layers of 1 mm thick sheets of high tensile or high hardness steel or other metal forming a 5 mm thick lattice. Each layer having preformed cells stamped into it before being formed into a cylinder with the appropriate diameter (each layer will require different diameters) and welded. This will require ensuring the preformed cells in each layer align by adjusting the strut thickness in each layer to compensate for the different layer diameters. 
     Optionally, the preformed lattice comprises a single layer of 5 mm thick sheet of high tensile or high hardness steel or other metal; the preformed cells stamped into it before being formed into a cylinder. 
     Optionally, the preformed cells comprise square holes of 5 mm×5 mm that have been stamped into a 5 mm thick sheet of high tensile or high hardness steel or other metal in a regular square pattern. The sheet is then rolled into a tubular shape and welded along the seam forming the lattice. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Preferred embodiments of the invention will now be described by way of example only, with reference to the figures, in which: 
         FIG.  1    provides an illustration of an embodiment of a tubular fragmentation layer in perspective view. 
         FIG.  2    provides an illustration of the tubular fragmentation layer of  FIG.  1    in cross section view. 
         FIG.  3   a    provides an illustration of an embodiment of a preformed lattice comprising square cells arranged in a square pattern with and without cubic preformed fragments. 
         FIG.  3   b    provides an illustration of an embodiment of a preformed lattice comprising circular cells arranged in a hexagonal pattern with and without spherical preformed fragments. 
         FIG.  4    provides an illustration of a cylindrical warhead embodiment having a tubular fragmentation layer. 
     
    
    
     DETAILED DESCRIPTION 
     Turning to  FIG.  1   , one embodiment of a tubular shaped fragmentation layer  1  is shown which would be suitable for a cylindrical shaped warhead. The fragmentation layer  1  comprises a tubular lattice  2  with preformed square cells  6  each containing a cubic preformed fragment  3 . The fragmentation layer  1  and lattice  2  have an axial length z of 800 mm. The lattice  2  is formed from a single flat sheet of steel having a thickness of 5 mm shaped to provide the tubular lattice  2 . 
       FIG.  2    shows the cross-sectional view of an embodiment of a tubular shaped casing having the fragmentation layer  1  of  FIG.  1   . The figure illustrates that the casing comprises 3 layers: an inner casing layer  4 , a fragmentation layer  1  and an outer casing layer  5 . In this embodiment, the inner casing layer  4  has an internal diameter k of 139.87 mm and an external diameter m of 149.6 mm giving an inner casing layer  4  thickness of 4.87 mm. The fragmentation layer  1  has an internal diameter p of 150 mm and an external diameter q of 160 mm giving a fragmentation layer  1  thickness of 5 mm. The outer casing layer  5  has an internal diameter x of 160 mm and an external diameter y of 165.1 mm giving an outer casing layer  5  thickness of 2.55 mm. 
     In this embodiment, the inner casing layer  4  and outer casing layer  5  are pressed up against the fragmentation layer  1  such that they are sandwiching the preformed lattice  2  and the preformed fragments  3  (i.e. there isn&#39;t a gap between them). Note in  FIG.  2    there is a small gap between the layers however this is for illustrative purposes only. The sandwiching of the fragmentation layer  1  by the inner casing layer  4  and outer casing layer  5  ensures that the preformed fragments  3  are held within the preformed cells  6  in the preformed lattice  2  by confinement.  FIG.  2    also shows some preformed cells  6  are empty. 
     Each cell  6  is separated from tangentially adjacent cells  6  by a distance of 2.08 mm on the external surface and 1.95 mm on the internal surface of the fragmentation layer  1 . Each cell  6  is separated from axially adjacent cells  6  by a distance of 2.08 mm on both the internal and external surfaces of the fragmentation layer  1 . 
     In this embodiment, the preformed fragments  3  are 5 mm×5 mm×5 mm cubes made from solid high hardness steel having a mass of approximately 1 g each.  FIG.  3   a    shows the preformed lattice  2  having square preformed cells  6  arranged in a regular square pattern that are shaped to fit the cubic preformed fragments  3 . Each square preformed cell  6  loosely holds and confines a single preformed fragment  3  using friction such that when the charge is detonated, the preformed fragments  3  can freely separate from the lattice  2  and consequently have an improved predictable lethal effect and reduced collateral damage when compared with fragmentation weapons in the prior art. 
     In another embodiment, as shown in  FIG.  3   b   , the preformed cells  7  comprise circular holes of 5 mm diameter that have been stamped into the sheet in a regular hexagonal pattern. In this embodiment, the preformed fragments  8  are solid high hardness steel spheres having a diameter of 5 mm. 
       FIG.  4    shows an embodiment wherein the fragmentation layer  1  has been integrated into a tubular warhead  9 . A single homogeneous tubular piece of steel of with an axial length d of 900 mm, a circular cross section having an internal diameter of 139.87 mm and an external diameter of 165.1 mm forms the tubular casing of the warhead  9 . A cavity  10  has been carved into the casing such that the fragmentation layer  1  (comprising the preformed lattice  2  with the preformed fragments  3  inserted into their preformed cells  6 ) can be inserted into the cavity  10 . The walls of the cavity  10  therefore form the homogeneous inner and outer casing layers;  4  and  5  respectively. The inner casing layer  4  internal diameter is 139.87 mm and external diameter is 149.6 mm. The outer casing layer  5  internal diameter is 160 mm and external diameter is 165.1 mm. This provides an inner casing layer thickness of 4.87 mm, an outer casing layer thickness of 2.55 mm. 
     In this embodiment, the warhead  9  is provided with an end and/or nose cap  11  as required by the warhead  9  design 
     Whilst the embodiments described relate to a casings for fragmentation weapons, and warheads comprising the casings, they are not intended to be limiting. For instance, the other aspects of fragmentation weapon and warhead design are implicitly disclosed. For instance, the casing may be provided enclosing an energetic material or explosive, and may be provided with an end and/or nose cap as required by the warhead or bomb design. The shape, size and mass of the casings will be tailored to the particular application. 
     In use, the tubular warhead  9  contains an explosive charge within the casing (i.e. encircled by the inner casing layer  4 ). The tubular warhead  9  also contains a guidance system connected to a flight control system to control and guide its trajectory towards the target and a fuse to detonate the warhead when the fuse condition is met. The base of the tubular warhead  9  is attached to a propellant engine to provide propulsion throughout the engagement. On detonation, the resulting shock wave shatters the inner casing layer  4 , preformed lattice  2  and outer casing layer  5  (which naturally fragments) and propels the cubic preformed fragments  3  out of their preformed cells  6  in a substantially radially outwards direction. Being physically unattached to any part of the preformed lattice  2  or any other part of the casing, the preformed fragments  6  freely separate from their cells  6  unattached to anything and thus having a known mass. The momentum of the preformed fragments can therefore be predicted when a known explosive charge is used.