WO2014048859A1

WO2014048859A1 - A charge for underwater use

Info

Publication number: WO2014048859A1
Application number: PCT/EP2013/069650
Authority: WO
Inventors: Sidney Alford; Roland Alford
Original assignee: Alford Research Limited
Priority date: 2012-09-26
Filing date: 2013-09-22
Publication date: 2014-04-03
Also published as: GB2506364A; GB201217187D0; GB2506364B

Abstract

An explosive charge for use under wafer is provided. The explosive charge comprises a charge and a void-defining region adjacent the charge. The region comprises one or more enclosed volumes (145). The volume/s is/are non-matricised.

Description

A CHARGE FOR UNDERWATER USE

Technical Field The present invention relates to a means of adaptation of explosive charges, such as shaped charges, for use under water.

Background

Whereas a spherical mass of high explosive, initiated at its centre point, exerts force equally in all directions, a significant directionality can be imparted to the blast exerted by the same mass of explosive by forming it into an elongate shape and initiating it at one end. The detonation wave then travels along the explosive charge and exerts greater pressure at the end distal to the point of initiation. Much greater directionality and focussing of the liberated energy can be obtained by providing, for example, a cylindrical mass of explosive with the means of initiation at the centre point of one end and forming a cavity, which is conveniently conical or hemispherical, at the other end. In such an arrangement, the otherwise divergent shock wave emanating from the distal end of the charge is focussed along the longitudinal axis of the charge and generates an intensely increased pressure along a narrow axial path.

This focussing of intense pressure, and greatly extended effective range of the damaging effect, can be still further augmented by the provision of a thin metal liner to the cavity formed in the distal end of the mass of explosive. Such charges are known as radially symmetrical shaped charges and constitute a common form of warhead for the attack of armour as well as a useful tool for various civil engineering applications.

The essential mechanism is that at least part of the metallic liner is squeezed towards the axis and formed into a highly elongate linear projectile of a very high velocity which exerts such pressure on the target as to induce radial plastic flow and consequent deep target penetration which, typically, extends to a depth corresponding to three of four charge diameters beyond the target surface. The charge is ideally fired at such a distance from the target surface as allows the jet to be largely formed before impact. This space between the front of the charge and the target is referred to as the stand-off space.

In the case of a shaped charge with a conical liner, part of the material initially adjacent the outer surface of the liner is squeezed into a somewhat elongate "slug" of high velocity while the metal constituting the inner surface is everted and squeezed into a very narrow and elongate "jet" of extremely high velocity. The greatest penetrative power is exerted on a target of which the proximal surface is at a distance corresponding to several charge diameters from the mouth of the conical liner since it is within this stand-off space that the elongation of the jet takes place and the penetrative ability of the charge is approximately proportional to the length of the jet.

Since the effective performance of such charges depends upon the very rapid and radially symmetrical collapse of the hollow charge liner, it follows that anything within the liner which prevents or impedes that collapse will interfere with the formation of the jet and its penetrative ability will be correspondingly diminished.

A common instance of suppression of such jet formation results if a shaped charge designed for use in air is placed under water. The incompressibility of the water which floods the inside of the cone prevents effective jet formation. In underwater applications it is therefore usual to provide the cone immediately in front of the charge with a watertight capsule and to place the distal end of the capsule as close to the target as possible. Such arrangements are easy to use at depths of, for example, tens of metres but, the deeper the charge is submerged, the more difficulty and onerous it becomes to prevent the stand-off capsule from leaking or collapsing. One known method of preventing the stand-off capsule from collapsing is to fill it completely with a mass of plastics foam which expands inside it before solidification. A more robust filling material is known as syntactic foam. This is a composite material in which the continuous plastics matrix is filled with micro-balloons of, most commonly, glass or plastics. Such filling material may be poured into the stand-off capsule and rendered solid by a polymerisation of the continuous phase material or it may be machined to the requisite shaped from a block.

Disadvantages attend the use of syntactic foam. These include the relatively high density of the material and its consequent and inevitable inhibiting effect on jet formation and also the amount of work required for its application to the adaptation of a shaped charge designed principally for use in air or in shallow water for deep waters tasks.

Summary of the Present Invention

According to an aspect of the present invention there is provided an explosive charge for use under water, comprising a charge and a void-defining region adjacent the charge, the region comprising one or more enclosed volumes, in which the volume/s is/are non-matricised.

According to an alternative aspect there is provided a projectile-forming charge for use under water, comprising an explosive charge and a space-defining region adjacent the charge for providing a stand-off zone into which a projectile can form upon detonation, in which the region comprises one or more unconstrained enclosed volumes.

It is the purpose of the present invention to provide a means of making charges which are capable of resisting great hydraulic pressures or of imparting such pressure resistance to existing charges. It follows that the filling of a stand-off space with a single volume, or with a multiplicity of such volumes provides an effective means of preventing the collapse of the space thereby afforded under hydraulic pressure. By using one or more non-matricised, or unsupported/unconnected volumes, a very simple means of creating an air void can be provided. Furthermore, in the absence of a matrix material the resistance to the mechanism of the charge can thus be minimised. In some embodiments an arrangement of enclosed volumes at least partially filling a stand-off volume is used.

In some embodiments an incompressible air void is effectively formed under water. The present invention may be applicable to any suitable charge, for example a linear cutting or shaped charge.

The explosive charge may further comprise a capsule for receiving the volume/s. The charge may include a liner. In embodiments with a capsule the capsule may form part of the charge liner.

The capsule may be formed from a plastics material, for example acrylonitrile butadiene styrene (ABS). In some embodiments the capsule may be water-resistant and/or sealed to prevent ingress of water. In other embodiments the capsule may be water permeable and/or unsealed to allow for ingress of water.

The enclosed volume may be at least partly spherical and in some embodiments is generally spherical. The defining symmetry of spherical structures, providing their walls are of consistent thickness and composition, gives them the greatest possible resistance to uniform compressive force. The filling of a stand-off space with a single pressure- resisting hollow sphere, or with a multiplicity of such spheres provides an effective means of resisting/preventing the collapse of the space under hydraulic pressure.

Other shapes for volumes may include, for example, at least partly polyhedronal and/or at least partly prismatic. For example, cuboidal, tetrahedronal, dodecahedronal, ellipsoidal, hexahedronal. Structures including at least one flat face may be used, for example a shape with at least one triangular, square, pentagonal or hexagonal face (the remainder of the shape may, for example, be spherical).

Depending on the shape of the volumes and the form of the charge and an associated capsule, interstitial spaces may present between the volumes. In some embodiments the spaces may be deliberately allowed to flood in use. In other embodiments volumes are formed to, adapted to, or caused to pack together leaving substantially no space between itself and its fellows. Such shapes may result, for example, by compressing an arrangement of spheres or other shapes until interstitial air or water is expelled.

Various factors will influence the shape of the volumes, for example pressure resistance and packing density requirements. The enclosed volume may comprise a hollow envelope, for example a generally rigid, fhin-walled plastics sphere or ball.

Examples of materials from which hollow balls may be formed include: Polycarbonate, ABS, Cellulose, Acetate, Acetal, Nylon, Phenolic, Polypropylene, Polyethylene, PTFE, Thermoset Resins, Torlon®, Vespel®, Nitrile, Fluoroelastomer, Polyurethane, and EPDM,

One example of a hollow ball which may be used is a 3/8" diameter HDPE ball manufactured by The Precision Plastic Ball Co Ltd.

Example of ball diameters which may be used include ranges of approximately: 1 mm to 100mm; 5mm to 50mm; 10mm to 30mm.

A plurality of volumes may be provided. The volumes may be substantially the same. Alternatively volumes of two or more different sizes may be provided.

According to a further aspect there is provided an explosive charge for use under water, comprising a charge and a stand-off capsule for providing an air void adjacent the charge, in which the capsule houses one or more unconnected enclosed volumes.

According to a further aspect there is provided a void-defining region for an explosive charge intended for use under water, the region comprising one or more non- matricised enclosed volumes. The present invention may therefore provide a region which can be associated with a charge to adapt it for under water usage. All of the alternatives discussed herein in relation to void-defining regions may be applicable.

Different aspects of the invention may be used separately or together. Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with the features of the independent claims as appropriate, and in combination other than those explicitly set out in the claims.

The present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an explosive charge formed according to an embodiment of the present invention;

Figure 1 A is a schematic view of the charge of Figure 1 illustrated following detonation; Figure 2 is a schematic view of an explosive charge formed according to a further embodiment;

Figure 3 is a schematic view of an explosive charge formed according to a further embodiment;

Figure 4 is a schematic view of an explosive charge formed according to a further embodiment;

Figure 5 is a schematic view of an explosive charge formed according to a further embodiment; and

Figure 6 is a schematic view of an explosive charge formed according to a further embodiment. The present invention relates generally to underwater charges and to ideas for making them work at great depths.

Shaped charges require a stand-off, which is a space in front of the charge in which a jet or other projectile is able to form. This stand-off is required whether it is being used in air or underwater. If underwater, some form of means of keeping water away from the front of the charge is required. We have used, in the past, an underwater capsule, which is a sealed capsule held in place in front of the charge containing air. The problem is that at great depths, the capsules must be prevented from leaking, which can be hard, especially when using plastic or thin aluminium.

In the extreme case, the liner 10 of a shaped charge 15 might consist of part of the wall of a single water-resisting spherical capsule (Figure 1 ). A detonator 20 is used to detonate explosive 25. The stand-off distance would thus be approximately limited to the diameter of the capsule (Figure 1 A). In other words this would restrict the distance the charge 15 could be placed from a target 30 in order for a projectile 35 formed from the liner 10 to be effective. In the more general case, an outer capsule 140, which need possess no great resistance to external pressure, might be filled with a random array of hollow, pressure- resistant, spheres 145, the outer capsule serving only the function of defining and maintaining the stand-off space occupied by the pressure-resistant spheres (Figure 2). In other embodiments different patterns of packing may be used, for example random or regular, and with a higher or lower density of packing.

In some embodiments the principle used is to fill the space in front of the projectile with small, hollow plastic balls. These can have a high crush pressure and effectively prevent water from ingressing and also help to retain the structural integrity of the capsule. If fhe capsule is allowed†o leak, wafer fills fhe interstices between fhe balls, but the charge is still able to form properly.

By providing fhe outer capsule 140 with such a perforation, or perforations 150, as permit fhe ingress of wafer, neither fhe flooding process as fhe structure is immersed in water, nor fhe hydraulic pressure to which fhe container spheres are subjected, would cause significanf deformafion of fhe outer capsule which, in consequence, need nof be constructed so as to be capable of resisting great forces.

The spheres could, for example, be poured in loose or pre-glued.

In some embodiments the space is filled with a lower density substance - e.g. an oil or paraffin wax.

If different sized balls are used they would pack more closely, reducing fhe effective density of fhe stand-off medium (plastic, air, water).

Once fhe outer, shape defining, capsule is flooded, fhe mean density of its contents will be that provided by the gas-filled or vacuum-filled spheres and by whatsoever water has filled fhe interstitial space between fhem.

One advantageous arrangement of hollow, pressure resistant, spheres results from fhe stacking of a linear array of spheres of equal diameter. This may be readily accomplished by stacking fhe spheres in a straight tube of just enough internal diameter as can receive them. This tube, or whatsoever device as might replace it, such as fhree parallel and equally spaced rails, can extend from fhe liner of fhe shaped charge all or most of fhe way to fhe target surface (Figure 3). In this embodiment a linear array of hollow spheres 245 are arranged in a permeable capsule 240 comprising a generally cylindrical terminal section 242 and a generally conical proximal section 244 which is fitted around the charge liner 210. Further spheres (not shown) may be placed in the proximal section 244 to take up the vacant space.

Since the diameter of the linear array of spheres need not greatly exceed the diameter of the metal jet once it is formed, an advantageous arrangement may consist of a small number of the spheres adjacent to the charge liner which decrease successively in diameter over the zone proximal to the liner which leads into a column of smaller spheres of size enough to conduct the jet to the target (Figure 4). In this embodiment a largest sphere 346 is provided in the proximal section 344 of a capsule 340 adjacent the liner 310. Adjacent the sphere a smaller sphere 347 is provided in the mouth of the terminal section 342 and then a linear array of even smaller spheres 345 extend inside the terminal section 342.

If a succession of hollow spheres is fitted inside a tube of similar internal diameter, then the volume of a sphere of diameter 2r is given by V = ⁴/3TTr³ and the volume of that part of the cylinder which contains it is 2TTr³. It follows that that cylinder, when flooded, contains ⁴/3TTr³ of sphere and 2TTr³ - ⁴/3TTr³ = 2/3TTr³ of interstitial water. That is, the sphere occupies two thirds of the volume within the cylinder and interstitial water one third. In such an arrangement, the advancing jet would strike the wall of each sphere normal to the outer and inner surfaces respectively and the amount of water needing to be displaced would be minimal. Such water and sphere wall material as does need to be displaced can be accommodated in that part of each sphere as provides passage for the jet. Ablation of the jet by the amount of sphere material is thus considerably less than that which would be caused by water alone and considerably less than that caused by most frequently encountered target materials.

A practical way of achieving such linear packing of the spheres 445 would be to drop them through the narrow end of the cylindrical capsule and place a cap 441 over the end when the tubular part of a capsule 440 is full, as shown in Figure 5. In this embodiment the capsule 440 is not permeable and an O-ring seal 455 is provided at the mouth of the proximal capsule section 444 to allow it to be sealingly secured around the liner 410. In this embodiment the intention is that the capsule is not deliberately flooded. The spheres 445 help to prevent crushing of the capsule at greater depths.

In Figure 6 a capsule 540 formed according to an alternative embodiment is shown. The capsule is a similar shape to the capsule 440 of Figure 5 (although no end cap is provided) and in this embodiment the capsule is also not provided with perforations or the like (i.e. it is not floodable).

In other embodiments (not shown) a floodable capsule is used. The capsule 540 includes a plurality of longitudinal flutes 560 which mean that in use when the capsule is subjected to pressure it can partially collapse i.e. "tighten".

In this embodiment raw material 565 for making spheres is loaded into the capsule 540 and then expanded once in the capsule (e.g. by heating or irradiation).

In subsequent use, when the capsule starts to collapse the spheres are squeezed closer together.

Another more general type of sphere arrangement could be achieved by filling a stand-off capsule with a non-linear array. In this case the ratio of the volume of the stand-off capsule to the volume occupied by the spheres will depend upon the packing patterns of the spheres. Since the density of the spheres would be lower than that of the interstitial and surrounding water, the least resistance to the progress of the metal shaped charge jet would be afforded by the arrangement in which as many spheres were packed into the capsule as possible.

The process of simply pouring the spheres into one end of the capsule through a hole made in its side would lead to an internal arrangement of spheres which would be dependent upon the manner in which they were added and the ratio of the sphere diameter and diameter and shape of the outer capsule. Provided the outer capsule were sufficiently voluminous compared with the diameter of the spheres, random close packing would give a filling density in which about 64% of the total void is occupied by spheres [Jaeger and Nagel, 1992], significantly smaller than the optimal packing density for cubic or hexagonal close packing of about 74%.

The concept of "random close packing" was shown by Torquato et al. (2000) to be a mathematically ill-defined idea that is better replaced by the notion of "maximally random jammed." Donev et al. (2004) showed that a maximally random jammed state of M&M's chocolate candies has a packing density of about 68%, or 4% greater than spheres. Furthermore, Donev et al. (2004) also showed by computer simulations other ellipsoid packings resulted in random packing densities approaching that of the densest sphere packings, i.e., filling nearly 74% of space.

The ideal close packing arrangement for the present invention using spheres would therefore be limited to a filling density of about 74% of the total void.

The greatest reduction in interstitial voids may be achieved by mixing spheres of more than two diameters so that the smaller species occupies the interstitial spaces left by the larger species.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiments shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCES:

Donev, A.; Cisse, I.; Sachs, D.; Variano, E. A.; Stillinger, F. H.; Connelly, R.; Torquato, S.; and Chaikin, P. M. "Improving the Density of Jammed Disordered Packings using Ellipsoids." Science, 303, 990-993, 2004.

Jaeger, H. M. and Nagel, S. R. "Physics of Granular States." Science 255, 1524, 1992.

Torquato, S.; Truskett, T. M.; and Debenedetti, P. G. "Is Random Close Packing of Spheres Well Defined?" Phys. Lev. Lett. 84, 2064-2067, 2000.

Claims

1 . An explosive charge for use under wafer, comprising a charge and a void- defining region adjacent the charge, the region comprising one or more enclosed volumes, in which the volume/s is/are non-mafricised.

2. A projecfile-forming charge for use under water, comprising an explosive charge and a space-defining region adjacent the charge for providing a standoff zone into which a projectile can form upon detonation, in which the region comprises one or more unconstrained enclosed volumes.

3. An explosive charge as claimed in claim 1 or claim 2, further comprising a capsule for receiving the volume/s.

4. An explosive charge as claimed in claim 3, in which the charge includes a liner and in which the capsule forms part of the charge liner.

5. An explosive charge as claim in claim 3 or claim 4, in which the capsule is formed from a plastics material.

6. An explosive charge as claimed in any of claim 3 to 5, in which the capsule is water permeable.

7. An explosive charge as claimed in any preceding claim, in which the enclosed volume is at least partly spherical.

8. An explosive charge as claimed in any preceding claim, in which the enclosed volume is generally spherical.

9. An explosive charge as claimed in any preceding claim, in which the enclosed volume comprises a hollow envelope.

10. An explosive charge as claimed in any preceding claim, in which a plurality of volumes is provided.

1 1 . An explosive charge, as claimed in claim 10, in which the volumes are substantially the same.

12. An explosive charge as claimed in claim 10, in which the volumes of two or more different sizes are provided.

13. An explosive charge as claimed in any of claims 10 to 12, in which interstitial spaces are present between the volumes.

14. An explosive charge as claimed in claim 13, in which the spaces are deliberately flooded in use.

15. An explosive charge as claimed in any of claims 10 to 14, in which the volumes are arranged in a random array.

16. An explosive charge as claimed in any of claims 10 to 14, in which the volumes are arranged in a regular array.

17. An explosive charge for use under water, comprising a charge and a stand-off capsule for providing an air void adjacent the charge, in which the capsule houses one or more unconnected enclosed volumes.

18. A void-defining region for an explosive charge intended for use under water, the region comprising one or more non-matricised enclosed volumes.

A charge substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

A void-defining region substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.