FRANGIBLE TUBULAR KINETIC ENERGY PENETRATOR

Abstract: A projectile (1) of tubular form is disclosed which is formed of a brittle material whose elongation to fracture is not greater than 10 per cent, preferably not greater than 2 per cent. Such projectiles can be more effective than projectiles of conventional geometric form in generating heavier lethal fragments, with more consistent break-up on target impact. The effect is enhanced when the projectile is spin stabilized. The brittle material can be a metal, especially tungsten or a tungsten alloy, or other dense particles in a plastics matrix, tungsten carbide, cast iron, or alloys of depleted uranium. The material density is preferably not less than 7 gm/cc. A sub-calibre projectile assembly is disclosed, comprising a tubular projectile (1), a discarding sabot (8), a pusher (9) and driving and centring bands (10, 11). A full-bore projectile assembly is also disclosed.

Description: FRANGIBLE TUBULAR KINETIC ENERGY PENETRATOR This invention relates to ammunition for the attack of lightly or medium-protected targets where there is a requirement to provide a destructive effect over a wide area behind the target surface. When a target such as an aircraft or a light or medium armoured vehicle is attacked by a conventional kinetic energy projectile, the effect within the target is likely to be limited. The projectile will normally remain intact after passing through the target skin, and the area of damage will be limited to the line of travel of the projectile. In recent years a form of frangible kinetic energy projectile has been developed which is designed to break up on impact with a target, with the object of producing a number of fragments which can spread out within the target so as to produce damage over a wider area than the conventional solid, non-bursting projectile. The geometry of such projectiles is similar to that of conventional solid non-frangible projectiles and it has been found necessary to use a very brittle material in order to ensure that the projectile will break up on impact. This means that the fragments which are produced and which penetrate the target tend to be very small, and the damage they can produce is thus considerably less than would occur with the fragments of optimum size. Although such ammunition is relatively cheap and simple to produce, because it requires no fuze, its limited behind-ar our lethality potential is a weakness of this concept. A well known way of increasing the damage area, is to include a bursting charge within the body of the projectile. This charge is detonated at a predetermined time after target penetration, so as to disperse fragments of the projectile body over an area within the target. However a fuze is required to control such detonation, for example a mechanical, electronic or pyrotechnic fuze. This and the need for the bursting charge itself increases the cost of the projectile substantially. It also has the effect of reducing the useful payload in terms of the mass of material which is available to cause direct damage within the target. The Applicant has now appreciated that a considerably enhanced fragmentation effect can be obtained without the aforesaid technical or cost disadvantages. Accordingly the present Invention provides a projectile of tubular form, said projectile being of a brittle material whose elongation to fracture under static tensile test is not greater than 10 per cent. Preferably the material has an elongation to fracture not greater than 6 per cent, and in many cases the elongation to fracture should ideally be not greater than 2 per cent. Projectiles in accordance with the invention have been found by the Applicant to have surprising and considerable technical advantages over the prior art fragmenting projectiles hereinbefore mentioned. For example, as compared with the prior art frangible ammunition, although of a generally brittle nature, the material of the tubular projectile can be significantly more ductile than that of a known frangible projectile of conventional geometry, without losing the ability to break up on target penetration. This is possible ' because a tubular projectile of given mass has a wall section which is thin as compared to a conventional solid projectile of equivalent mass. Thus, heavier more lethal fragments, can be generated on target impact and more consistent fragment break-up can be obtained on a range of varying target structure and materials. Preferably, in order to promote the generation of fragments of a desired minimum size, the material of the projectile is one having an elongation to fracture under static tensile test which is not less than a certain minimum value which would normally be 0.5 per cent or possibly 1.0 per cent. Moreover, a tubular projectile will most usually be spin stabilized. Hence, because its mass Is concentrated in an annular section, when broken up at target strike, the fragments will have a strong tendency to separate radially due to their greater angular momentum as compared to the equivalent mass solid projectile, whose mass is concentrated in a smaller diameter. Thus a tubular fragmenting projectile will provide a considerably greater fragment distribution, that is to say a much greater volume of the target is attacked and lethality is much increased. Another advantage of the use of a tubular shape is to enable relatively greater freedom in design. The geometry of the projectile can be designed in order to control and optimise the performance for specific terminal effects against specific types of target. Wall thickness, diameter, length and leading edge geometry are all parameters which can be taken. Into account In design and may have a critical effect on performance. For example by employing a toed-out leading edge design the bursting stress on the tube material can be maximised. Another technical advantage of a tubular projectile especially a spin stabilized tubular projectile is its ability to engage with a target at low incidence angles. A conventional projectile would normally present the cheek of an ogive to the target body and would thus graze the target and be deflected with little or no damage caused. The tubular projectile on the other hand even at low angles of attack will present a relatively sharp edge to the target, and especially if spinning, will thus have a cutting action which inhibits ricochet and actively promotes penetration into the target. Clearly the choice of material for the tubular projectile is of considerable importance, and suitable materials include certain tungsten alloys, tungsten carbide, cast iron, certain steels, and certain alloys of depleted uranium. Alternatively the said brittle material may comprise metal or other dense particles in a plastics matrix. In this event the dense particles can conveniently be of any suitable dense metal or other dense material, whose density is advantageously not less than 7gm/cc. Suitable dense materials include tungsten alloys, tungsten carbide, cast iron, steels, and alloys of depleted uranium. Suitable plastics matrix materials include resins such as polyester resin, which have sufficient mechanical strength when heavily loaded with the dense particles, to withstand the stresses of launch from a gun or other launching means. The loading of dense particles in the matrix should be as high as possible consistent with strength and other requirements, in order to achieve a high density and therefore a large mass, in the tubular projectile. The Applicant is aware that tubular projectiles are not novel per se. Such ammunition has found little practical application until the development by Flatau et al of their supersonic low drag tubular projectile as described and claimed in UK Patent No.1571010. The emphasis in this patent is upon the achievement of low drag, in order to increase the velocity of strike at the target, by adoption of novel and critical geometry. As to the terminal effect on striking the target, there is no suggestion of anything other than the normal, conventional kinetic energy effect of a projectile which penetrates the target as an integral whole. The teachings of Flatau et al, insofar as they relate to the reduction of drag in a tubular projectile, are applicable to the design of tubular projectiles in accordance with the present invention, especially to the extent that they may be consistent with the geometric requirements for optimum fragmentation. Clearly the latter will depend upon such factors as the material chosen, its heat treatment and mechanical working, but generally speaking the following geometric criteria are believed by the Applicant to provide an optimum effect. The ratio of average wall thickness to outside diameter of the projectile should preferably be not greater than 0.3, and most preferably not greater than 0.15, in each case excluding any leading end section in which the wall section tapers towards the leading edge. This latter factor expresses a simple arithmetic criterion for the control of wall thickness to promote the generation of optimum-sized fragments for maximum destructive effect within light or medium-protected targets. In use the projectile will form part of an assembly which may include other components such as driving band, sabot and/or pusher, and the calibre of the assembly is determined as the maximum diameter of any of these components. In order to promote the maximum dispersion of fragments especially in a spin stabilized projectile the tube diameter should preferably be relatively large within the calibre constraint, which is determined by the weapon to be used for firing the ammunition. For this reason the ratio of outside diameter projectile to calibre should be not less than 0.65 and not greater than unity. The combination of projectile material, its physical condition, and the geometry of the projectile are preferably chosen such that in use, when fired with an impact velocity of 950 metres per second against representative target of 6mm thick rolled homogeneous steel armour plate oriented normally with respect to the line of flight of the projectile, the projectile disintegrates on impact therewith into fragments of which at least 85 per cent by weight are in the weight range 1 gramme to 10 grammes. The invention will now be described by way of example only with reference to the accompanying schematic drawings, of which Figure 1 is a sectional elevation of a simple form of tubular projectile in accordance with the invention, Figure 2 is a sectional elevation of a convergent-divergent form of tubular projectile in accordance with the invention, Figure 3 is a sectional elevation of a projectile assembly comprising a sub-calibre tubular projectile of the kind shown in Figure 1 together with a sabot and a pusher, Figure 4 is a sectional elevation of a projectile assembly comprising a substantially full-calibre tubular projectile of the kind shown in Figure 1 together with a nose cap and pusher, Figure 5 is a sectional elevation of a tubular projectile in accordance with the Invention which also Includes incendiary material Figure 6 is a sectional elevation of a modified form of the tubular projectile shown in Figure 1 which also includes tracer material, Figure 7, is a sectional elevation of a fin-stablilized tubular projectile in accordance with the invention, Figure 8 is a diagrammatic representation of a target comprising impact plate and witness plate, Figure 9 is a diagrammatic representation showing the spread of fragments passing through the impact plate of Figure 8, Figure 10 is a diagrammatic representation of the damage caused to a relatively light example of the witness plate of Figure 8, and Figure 11 is a diagrammatic representation of the damage caused to a heavier witness plate. As shown in Figure 1, a tubular projectile In accordance with the invention comprises a thin-walled cylindrical tube 1 having an outer diameter d and a constant wall thickness t. The projectile may be fired either as a sub-calibre projectile or as a full calibre projectile from a gun (not shown) whose calibre is d f . The geometry of the tube 1 is designed such that - /d is less than or equal to 0.3 and preferably /d is less than or equal to 0.15 - thus the tube wall should be relatively thin in relation to its diameter. In addition the tube geometry should also be such that ds Id. is not less than 0.65 and not greater than unity - in other words the projectile diameter should preferably be relatively large within the constraint set by the calibre of the gun from which it is to be fired. Full calibre or sub-calibre projectile assemblies in accordance with the invention are possible for firing from guns, or possibly for rocket launched applications also. The material of the tube is a matter of great importance. It should be selected as one which breaks into fragments of optimum size, neither too large nor too small, on impact with a target. Hence a brittle material is required, having an elongation to fracture of less than six per cent, usually less than two per cent, under static tensile test conditions. Excessively brittle material should be avoided however, as such materials are liable to form very small fragments or powder on target impact and their effectiveness against the target would be small. Materials having an elongation to fracture of less than 0.5 per cent, or perhaps less than one per cent should thus be avoided. In this class are certain materials which have normally been used for solid frangible APDS penetrators. A dense material is preferred as this will lead to reduced projectile drag and increased effectiveness at the target. Normally a density at least as great as that of steel, or not less than 7g/cc is required in order to achieve effective attack against a range of practical targets. With these factors in mind, the Applicant has found that one suitable material for the tube 1 is a tungsten alloy commonly used for non-frangible long rod penetrators of the type used in APFSDS ammunition, having a composition 92% tungsten, balance iron and nickel. The tube 1 is conveniently formed by machining from a cold worked, sintered isostatically pressed powder preform. Other materials are possible depending upon the performance required, for example certain steels, cast iron, certain alloys of depleted uranium, and certain other tungsten alloys. The selected material must exhibit the correct brittleness, and this will of course be governed by such factors as its physical state, degree of hot or cold working, heat treatment history, etc in addition to the material composition. The tube 1 shown in Figure 1 has a simple regular cross-section and a blunt leading edge 2. These characteristics are acceptable for some uses, but the tube 3 shown in Figure 2 can exhibit considerably less aerodynamic drag. Hence for a given launch velocity the projectile 3 will possess greater kinetic energy and thus greater potential for destruction on target impact. The tube 3 has a sharp annular leading edge 4, and a leading end section 5 in which the inner diameter reduces steadily towards the rear, and the outer diameter increases steadily so that the wall thickness increases steadily in this region. Next behind the leading section 5 is a parallel-sided section 6, of constant wall thickness. In the rear section 7 the inner surface of the tube diverges conically towards the rear end 7A, which can be blunt, rounded or sharp. The outer surface Is boat-tailed, le tapers convergently, so that the tube wall thickness becomes less towards the rear end 7A. Tubular convergent-divergent projectiles of this form having specific geometric characteristics are described and claimed in UK Patent No 1571010, the disclosure of which is incorporated herein by reference. Such projectiles exhibit very low drag under appropriate flight conditions, and projectiles in accordance with the present invention may desirably take the same form, subject to a preference for the geometric characteristics described in relation to Figure 1. In applying the latter characteristics to non-parallel tubes such as the tube 3, the value of the tube thickness t is to be taken as the average thickness of the tube excluding any leading section such as 5, ie in relation to the tube 3 t is the average wall thickness over the total length of sections 6 and 7. The value of d is the mean outer diameter, in this region. Tubular projectiles in accordance with the invention can be fired from a gun as either full-calibre or sub-calibre projectiles. As shown In Figure 3, the tubular projectile 1 of Figure 1 forms part of a projectile assembly which also includes a discarding sabot 8 and a discarding pusher 9 which provides obturation and bears on the rear end of the tube 3. The assembly may conveniently be of the kind described and claimed in European Patent Application No 87311236, and is suitable for firing the tube 1 as a sub-calibre projectile from a gun of calibre d. equal to the outer diameter of the sabot and pusher including any driving and/or centring band 10,11. Figure 4 shows an alternative projectile assembly for firing from a gun with the tube 1 as a substantially full-bore projectile. The assembly in this case includes a discarding pusher 12 which provides obturation and bears on the rear end of the tube 1 and a discarding ballistic nose cap 13. As with the sabot 8 and the pusher 9, the pusher 12 and nose cap 13 are designed to separate from the projectile 1 shortly after exit from the gun barrel. The pusher and nose cap may include driving and/or centring bands 14,15. Figure 5 shows a tubular projectile 6 which is of modified form as compared to the tube 1. The projectile 16 has a flared leading end 17 which is provided with a sharp leading edge 18. The inner surface of the leading end is conical, and the outer surface is cylindrical. Behind the leading end 17 the projectile 16 has a parallel-sided cylindrical tubular section 19, the inner diameter of the tube is stepped so as to accommodate carried material 20, for example incendiary material, which can cause additional damage or disruption at the target. The flared leading end section can assist in the initiation of projectile break-up on impact with the target, by raising hoop stresses tending to cause bursting of the projectile. The leading end section 17 can be integral with the tubular section 19, or can optionally be formed of an incendiary material such as zirconium, in which case the section 17 is a separate section which has for example a screw threaded connection to the tubular section 19 (as shown). Figure 6 shows another modified form of tubular projectile comprising a leading principal tube section 21 in accordance with the invention which is cylindrical with constant wall thickness, and is formed of brittle material. The rear end of the section 21 receives a tracer module 23 which is attached thereto by a suitable design of joint, for example by corresponding screw threaded portions 22, 24. The module 23 has a body of mild steel or other convenient material compatible with a conventional engineering stress and thermal and manufacturing considerations, which could be a plastics material, and has a recess in its rear end containing a tracer material 25. Other designs of tubular projectile assembly having tracer material in the rear end thereof, and having provision for igniting the tracer material on firing from a gun, are described and claimed in the Applicant's co-pending European Patent Applications Nos 87907785 and 90112207, and such designs may conveniently be adopted in conjunction with the present invention. The embodiments of the invention described with reference to Figures 1-6 will normally be launched as spin stabilized projectiles, although designs which are stable in flight without spin stabilization are possible. One example of a fin stabilized projectile 26 which is in accordance with the invention is shown in Figure 7. The projectile 26 comprises a cylindrical tube of constant wall section which is provided with four stabilizing fins 27 and a ballistic cap 28 of light-weight material, and may be sabot-launched from a gun barrel in conventional manner. In use projectiles in accordance with the invention have been found to show superior terminal ballistic performance as compared to equivalent frangible APDS ammunition projectiles of conventional geometry. Trials have been conducted by the Applicant in which projectiles similar to that shown in Figure 2 (which were in accordance with the invention), and frangible APDS projectiles of a form which is in current use, were fired against the target illustrated schematically in Figure 8. The target comprised an impact plate 29 and a witness plate 30 parallel to and behind the target plate. In each firing, with each type of projectile, the projectile was arranged to strike the target plate normally, as Indicated by the arrow P, and with a velocity at impact of about 950 metres per second. In each firing, the tubular projectile in accordance with the invention was of tungsten alloy (92% by weight tungsten, balance nickel and iron), weighing 187 grammes and suitable for firing as a sub-calibre projectile from a Rarden 30mm gun. The APDS projectile was of a brittle tungsten alloy showing less than 0.5% elongation to fracture under static conditions, weighing 241 grammes and also suitable for firing from a Rarden 30mm gun. In the first series of tests, a variety of impact plates were used, as follows A - 1.6mm thick aluminium alloy B - 6.35mm thick aluminium alloy C - 6mm thick rolled homogeneous steel armour plate D - 10mm thick rolled homogeneous steel armour plate The witness plate was of 1.6mm thick aluminium alloy placed at a distance X from the impact plate. The resulting pattern of damage to the impact plate is illustrated schematically in Figure 10, where reference numeral 31 indicates the central hole caused by the projectile and numeral 32 indicates the holes caused by individual fragments of the projectile. Dimensions D. and D„ are respectively the greatest and least width of the central hole 31 and the dimensions F. and F„ are respectively the greatest and least width of the area affected by the fragment holes 32. The central hole 31 is taken as equivalent to a circular hole of diameter D= D, ♦ D„ 2 The fragment damage area affected by the holes 32 is taken as equivalent to a circular hole of diameter F = (F- ♦ F«) and area A = IT Λ.F 2 (A τ for the tubular projectile, A for the solid frangible APDS projectile). As illustrated in Figure 9, the nominal spread of fragments is taken as θ, the angle (expressed in degrees) subtended by a diameter F at a distance X. The results of a series of trials for each of the impact plates A,B, C and D for values of X as indicated, and for each of the two projectiles are summarised in Table 1 below, wherein N represents the number of fragment holes (including the central hole 31 ) and the dimensions of X,D and F are cm. Table 1 IMPACT PLAT As can be seen from Table 1, the damage area A_ for the tubular projectile according to the invention exceeds that for the conventional frangible APDS projectile in all cases by a factor between 1.35 and 2.85. The area density of fragments (A_/N; 2 A /N) varies between one fragment in 4 to 11 cm for the tubular 9 projectile and 1 fragment in one to 25 cm for the APDS. The tubular has a more consistent, and in most cases greater area density, and is relatively independent of impact plate type. Table 2 below shows the results of attack by the same projectiles against a similar target but wherein the impact plate is in each case 1.6mm thick aluminium alloy, and the witness plate is either: A - 6.35mm thick aluminium alloy, or B - 6mm thick rolled homogeneous steel armour plate With these heavier witness plates, there is no central hole corresponding to 31, but as shown in Figure 11 the damage comprises only fragment-generated holes 32. F. and F 2 again represent the maximum and minimum widths of the fragment-damaged area, and the results are summarised in Table 2 below wherein other symbols represent the same variables as in Table 1. As can be seen from Table 2, the frangible tubular projectile in accordance with the invention produced about two to three times the angular spread of fragments, five to ten times the damage area and 70% more perforating fragments in total, as compared to the conventional frangible APDS -projectile. From these test results, bearing in mind particularly that the frangible APDS projectile is about 30% heavier than the frangible tubular projectile, it is quite clear that the tubular projectile in accordance with the invention is very substantially more effective against light-and medium armoured target than the equivalent conventional solid frangible APDS projectile. While the invention has been described with reference to projectiles suitable for launch from a conventional gun barrel, it will be appreciated by those skilled in the art that other methods of launch are possible, for example by rocket propulsion, or by electromagnetic means. Other possible modifications will also be apparent to those skilled in the art, and these too are to be regarded as within the scope of the invention, which is defined by the appended claims.

Claims: Claims 1. A projectile of tubular form, said projectile being of a brittle material whose elongation to fracture is not greater than 10 per cent. 2. A projectile according to claim 1 wherein the elongation to fracture of the said brittle material is not greater than 6 per cent. 3. A projectile according to claim 1 wherein the elongation to fracture of said brittle material is not greater than 2 per cent. 4. A projectile according to any one of claims 1 to 3 wherein the elongation to fracture of the said brittle material is not less than 0.5 per cent. 5. A projectile according to claim 4 wherein the elongation to fracture of the said brittle material is not less than 1 per cent. 6. A projectile according to any one preceding claim wherein the said brittle material is a tungsten alloy, tungsten carbide, cast iron, steel or an alloy of depleted uranium. 7. A projectile according to claim 6 wherein the said brittle material is a tungsten alloy comprising 92% by weight tungsten, balance iron and nickel. 8. A projectile according to any one preceding claim wherein the said brittle material comprises particles of dense metal or other dense material in a plastics matrix. 9. A projectile according to claim 8 wherein the dense metal or other dense material is selected from the group comprising tungsten alloys, tungsten carbide, cast iron, steels and alloys of depleted uranium. 10. . A projectile according to claim 8 or claim 9 wherein the material of the plastics matrix Is a resin. 11. A projectile according to any one preceding claim wherein the ratio of average wall thickness to outside diameter of the projectile is not greater than 0.3. 12. A projectile according to claim 7 wherein the said ratio is not greater than 0.15. 13. A projectile according to any one preceding claim and forming part of a projectile assembly wherein the ratio of the outside diameter of the projectile to the calibre of the projectile assembly is not less than 0.65 and not greater than unity. 14. A projectile according to any one preceding claim wherein in use when fired with an impact velocity of 950 metres per second against a representative target of 6mm thick rolled homogeneous steel armour plate oriented normally with respect to the line of flight of the projectile, the projectile disintegrates on impact therewith into fragments of which at least 85 per cent by weight are in the weight range 1 gramme to 10 grammes. 15. A projectile according to any one preceding claim which comprises a cylindrical tube having a constant diameter and wall thickness over at least a substantial part of its length. 16. A projectile according to any one preceding claim having a sharp annular leading edge. 17. A projectile according to one preceding claim and having a flared leading end portion. 18. A projectile according to claim 17 wherein the projectile has a leading end section in which the inner diameter reduces steadily towards the rear and the outer diameter increases steadily; an intermediate section of substantially constant cross-section and wall thickness; and a rear section in which the inner surface of the tube diverges steadily towards the rear end of the projectile. 19. A projectile according to any one preceding claim and including carried material. 20. A projectile according to claim 19 wherein the carried material is an incendiary material. 21. A projectile according to claim 19 wherein the carried material is a tracer material. 22. A projectile according to claim 21 wherein the tracer material is carried in a separate module forming part of the projectile. 23. A projectile according to any one preceding claim in combination with an assembly comprising an obturating pusher. 24. A projectile according to claim 23 wherein the assembly also includes a sabot and the projectile is a sub-calibre projectile. 25. A projectile according to claim 23 or claim 24 wherein the assembly includes a driving band for imparting spin to the projectile on launch from a gun barrel. 26. A projectile according to any one of claims 1 to 24 which is provided with stabilizing fins towards its rear end. 27. A projectile according to claim 1 and substantially as hereinbefore described. 28. A projectile substantially as hereinbefore described with reference to any one or more of Figures 1 to 8 of the accompanying drawings.

Inventors: BAXTER JAMES EDWARD, BRADDOCK RICHARD ALLAN, IZOD DAVID COLLIN ASKET, JERVIS MICHAEL THOMAS, BAXTER, JAMES, EDWARD, BRADDOCK, RICHARD, ALLAN, IZOD, DAVID, COLLIN, ASKET, JERVIS, MICHAEL, THOMAS
Applicants: ROYAL ORDNANCE PLC
Application year: 1991
Main classification: F42B12
IPC codes: F42B10, F42B12