WO2019177500A1

WO2019177500A1 - Pre-fragmentation of a warhead

Info

Publication number: WO2019177500A1
Application number: PCT/SE2018/000007
Authority: WO
Inventors: Christer Thuman; Abraham Langlet; Ulf Heiche
Original assignee: Bae Systems Bofors Ab
Priority date: 2018-03-14
Filing date: 2018-03-14
Publication date: 2019-09-19

Abstract

The invention relates to method for pre-fragmentation of a warhead (1) comprising a warhead body (2), an explosive charge (6), and a warhead shell (4) with the density pSheii/ wherein the warhead shell (4) comprises pre-formed cavities (5), where each cavity (5) comprises at least one pre-formed projectile (7) with the density ppr0j and filler material or agent (8) with the density pfm, wherein the method comprises the following steps (in an arbitrary order) : - pre-formation of the cavities (5) in the warhead shell (4), - arrangement of at least one projectile (7) in each pre-formed cavities (5), - filling of filler material or agent (8) in the cavities (5) so that the cavities (5) are filled, - treatment of the filler material or agent (8) so that the filler material or agent (8) forms a connected structure with high adhesiveness to the projectiles (7) and to the walls of the cavity (5). The invention further relates to a pre-fragmented warhead.

Description

Pre-fragmentation of a warhead

The present invention relates to a method for pre fragmentation of a warhead. The invention further relates to a pre-fragmented warhead.

Weapon systems for combating targets in air, on sea or on ground comprises different types of ballistic warheads such as grenades fired from barrel, robots or missiles of different kinds or gliding bombs launched from aircrafts. Pre fragmentation of a warhead, such as a grenade, by applying pre-formed projectiles, also known as fragmentation units, with high density, in pre-formed cavities in the shell of the grenade is previously known to improve the effect in target. From the weapon effects perspective a pre-fragmented grenade or bomb is more effective and more predictable than a natural fragmented grenade or bomb. Despite this, weapon systems with natural fragmented warheads still exist.

For a lot of the known weapon systems a need exists to replace natural fragmented warheads with pre-fragmented warheads.

By upgrading a weapon system, according to above, it has been shown that the warheads have been too heavy or too light which have affected the moment of inertia of the warhead and the ballistic performance of the weapon system.

This have thus lead to high cost of integration due to the need of modification or exchange of the weapon systems control system due to the changed ballistic performance.

There thus exists a need for an improved pre-fragmentation method where the above mentioned drawbacks are reduced or completely eliminated.

After pre-fragmentation it is a requirement that the projectiles should not be broken or deformed during acceleration when the warhead burst. The velocity of the projectiles should initially be high when the projectiles leave the warhead. At the same time the projectiles should be designed to avoid that the projectiles velocity is not reduced too fast on the way to the target and thus could reach high weapon effect in the target.

For a weapon system where the warhead is a grenade fired from a barrel, the warhead must be able to withstand the high acceleration and centrifugal forces occurring during launch. The shell of the grenade should act as a pusher plate for the projectiles when the projectiles leave the grenade and contribute to that the projectiles are accelerated to a high and even velocity in predetermined directions.

GB 2107032 discloses an explosive device, which includes a number or pre-formed elements in the form of balls or cubes. The pre-formed elements are arranged in longitudinally extending concavities.

WO 2009/102254 discloses a shell with two or more splinter elements, which are arranged individually in preformed cavities, which preferably are cylindrical with a cone-shaped bottom. The size of the cavities corresponds to the size of the splinter elements.

By document US 4 644 867 it is known of a grenade where the shell of the grenade includes preformed fragmentation units with high density mixed with a carrier material, preferably a metal powder. The preformed fragmentation units comprise together with the metal powder a connected shell enclosing the explosives in the grenades body.

The shell of the grenade is manufactured by a powder metallurgy process where the carrier material is mixed with the fragmentation units and pressed under high pressure and high temperature to a dense shell. The shell of the grenade forms a connected structural member that withstands the axial and radial forces acting upon the shell upon launch.

The process as described where the carrier material and the fragmentation units are mixed under high pressure and high temperature results in that the position of the fragmentation units within the carrier material could vary between different grenades. The high pressure could also results in variations in the outer geometry of the shell which affects the ballistic performance of the grenade. Further, the high temperature could result in that the fragmentation elements material characteristics are changed.

PURPOSE OF THE INVENTION AND ITS DISTINCTIVE FEATURES

The purpose of the present invention is to improve a method for pre-fragmentation of a warhead comprising a warhead body, an explosive charge, a fin part, and a warhead shell with the density p_Sheii wherein the warhead shell comprises pre-formed cavities, where each cavity comprises at least one pre-formed projectile with the density p_pr0j and filler material or agent with the density p_fiii, wherein the method comprises the following steps:

- pre-formation of the cavities in the warhead shell,

- arrangement of at least one projectile in each pre-formed cavities,

- filling of filler material or agent in the cavities so that the cavities are filled,

- treatment of the filler material or agent, so that the filler material or agent forms a connected structure with high adhesiveness to the projectiles and to the walls of the cavity.

According to further aspects of the improved method of pre fragmentation of a warhead provision is made as follows:

the dimensions of the projectiles and the cavities and the density of the projectiles and the filler material or agent are selected so that the mass of the warhead before pre-fragmentation remains the same after the pre fragmentation .

the density of the filler material or agent, the warhead shell, and the projectiles are selected to fulfil the relation pfm Psheii Pproj ·

the cavities are mechanically pre-formed in the warhead shell by drilling or milling.

the filler material or the agent is treated to a strong and continuous structure by heat and pressure treatment. the filler material or the agent is treated to a strong and continuous structure by a curing treatment.

the positions of the cavities on the warhead shell are selected depending upon the mechanical strength of the warhead shell by utilizing a topological optimization method.

The invention also relates to an improved pre-fragmented warhead comprising a warhead body, a fin part, an explosive charge, and a warhead shell, with the thickness t _Sheii and the density p_sheii^ wherein the warhead shell comprises projectiles with the density p_{p ro}j and filler material or agent with the density p_fin, wherein the dimensions of the projectiles and the cavities and the density of the projectiles and the filler material or agent are selected so that the mass of the warhead before pre-fragmentation remains the same as after pre fragmentation .

According to further aspects of the improved pre-fragmented warhead provision is made as follows:

• the filler material or agent, the warhead shell, and the projectiles are selected to fulfil the relation P_fill Ps_hell ^ Pproj ·

• the projectiles are spherical with the diameter d_pr0j, and that the cavities are cylindrically shaped with the length l_cav and the diameter d_cav and where d_proj < dcav ·

• the projectiles comprise an alloy of wolfram.

• the filler material or agent comprises a magnesium powder .

• the filler material or agent comprises an aluminium powder .

• the filler material or agent comprises a zirconium powder .

• the filler material or agent comprises a thermosetting plastic.

• the warhead comprises a second warhead shell arranged on the first warhead shell.

• the warhead is a bomb.

• the warhead is a grenade.

BENEFITS AND EFFECTS OF THE INVENTION

The improved method of pre-fragmentation results in several advantages and effects of which the most important are:

A reduced cost of integration to existing weapon systems is obtained, since the pre-fragmented warhead according to the invention has the same mass, moment of inertia and outer geometry as the replaced, natural fragmented warhead, before cavities were formed therein. Existing warheads could be exchanged and replaced by an equivalent, high effect alternative without costly modification of the weapon system. An improved weapon effect is obtained without modification or replacement of the weapon systems other sub-systems, resulting in minimized or eliminated cost for integration. Thus a reduced number of warheads will be needed for the combatting of a target as compared to using the existing, naturally fragmenting warheads.

An improved weapon effect is obtained, since an existing warhead could be replaced by a warhead with heavy alloy projectiles .

An unchanged initial velocity of the fragments and projectiles of the warhead, compared to the original performance, will be attained since the accelerated mass of the warhead body is unchanged.

The invention provides a possibility to arrange a grenade or bomb with asymmetrically arranged projectiles, for example only on one side of the warhead without changing the centre of gravity of the warhead.

Shock wave reduction is obtained, since the projectiles achieve a certain protection against the initial shock wave due to the difference in mechanical impedance between material layers of different density.

The exact positioning of the cavities in the shell of the warhead allows the weapon effect of the warhead to be optimized with respect to the mechanical strength of the shell .

Additional benefits and effects according to the invention will be presented in this study and by the observance of the following detailed description of embodiments including a number of the most advantageous embodiments, patent claims and the attached drawings where:

Fig 1 shows a schematic view from the side of a warhead with projectiles arranged in the shell of the warhead;

Fig 2a-2d show schematic, partial enlargements of one projectile arranged in a cavity with a filling agent in the shell of the warhead according to figure 1;

Fig 3 shows a schematic, partial enlargement of three projectiles arranged in a cavity with the filling agent in a second embodiment of the shell of the warhead according to figure 1 ; and

Fig 4 shows a schematic, partial enlargement of several projectiles arranged in the filling agent in a third embodiment of the shell of the warhead according to figure 1.

DETAILED DESCRIPTION OF EMBODIMENTS

Pre-fragmentation according to the invention means that cavities are pre-formed in the shell or cover of the body of a warhead such as a bomb. In the cavities one or more projectiles are arranged, such as spheres, cubes, rods or cylinders with a high density. The projectiles are surrounded with a filling agent or a material with a low density in a sufficient amount, so that the cavities are filled. The dimensions of the projectiles and the cavities as well as the densities of the projectiles and the filling agent, respectively, are selected so that the mass of the bomb is kept the same after the pre-fragmentation as before the prefragmentation .

To fulfil demands on weapon effect, the projectiles comprise a material of a high density, and preferably comprise a heavy alloy, such as an alloy comprising wolfram, also known as tungsten. Other materials and alloys with a high density could also be used, such as depleted uranium. The filling agent or material comprises a material of a low density, preferably a compressed and heat treated metal powder comprising, for example, manganese and/or aluminium or a mixture of manganese and aluminium. The filling agent or material could also be a thermosetting plastic, for example an isocyanate curing polyurethane plastic. Another option is glue of any suitable type. The filling agent or material could also comprise several layers of materials with different densities.

Figure 1 shows a bomb 1, or a warhead 1, arranged to be dropped from an airplane. With arrangements known in the art, the warhead 1 could also be made suitable for firing from a gun or cannon. The bomb 1 comprises a bomb body 2, or a warhead body 2, and a rear fin part 3. The bomb body 2 comprises an outer bombshell 4, or a warhead shell 4, enclosing an inner, explosive charge 6. Cavities 5 of defined size and form are arranged in the bombshell 4. The cavities 5 are pre-formed in the bombshell 4 at predefined positions in the bomb 1.

A filler agent or material 8 of a well-defined amount, with a specific density, is arranged surrounding the projectiles 7 in the cavities 5.

In a first embodiment, shown in figure 2a, each of the projectiles 7 is arranged in a cylindrical shaped cavity 5 with a circular cross section arranged on defined distances from each other, where the bottom of the cavity 5 is half spherical. As an alternative the cavities 5 could be arranged with a square or rectangular cross section, and the bottom of the cavity 5 could be arranged as a cone, as seen in fig 2b, where the cone formed bottom of the cavity 5 contributes to centering of the projectile 7 to the centre position of the cavity 5.

A cone shaped bottom of the cavity 5 also ensures that a filler agent or material 8 is well distributed around the projectile 7 during the filling operation.

Figures 2c and 2d show cavities 5 with a shape wherein the projectile 7 fits closely in one section of the cavity 5, and the filling agent 8 is arranged around the projectile 7 in a more shallow section of the cavity 5.

The projectile 7 may be completely countersunk in the bombshell 4, as shown in figures 2a and 2c. Another option is that the projectiles 7 extend a short distance outside the bombshell 4.

At least one projectile 7 of a defined form, size and density is arranged in each cavity 5. The projectiles 7 are preferably heavy alloy spheres but other types of projectiles 7 are also possible to use.

The method of pre-fragmentation comprises the following steps:

• Arranging or pre-forming cavities in the bombshell 4 of the bomb body 2.

• Applying projectiles 7 in the cavities 5.

• Arranging the filler agent or material 8 in the cavities 5, surrounding the projectiles 7, in an amount so that the filler agent or material completely fills the cavity

5.

• Treating the filler material or agent 8, so that the filler material or agent 8 forms a connected structure with high adhesiveness to the projectiles 7 and to the walls of the cavity 5.

The steps need not necessarily be performed in this order. One particular example is when the filler material 8 is preformed to fit into one of the cavities 5, but it is not arranged therein until a projectile 7 has been arranged in the filler material 8 itself. Thereafter, the unity of the filler material 8 and a projectile 7 is arranged in a cavity 5 in the shell 4. The filler material 8 may in this case be a deformable material, such as aluminium, which has been given a socket shape, wherein the projectile 7 fits, and which, in its turn, fits into one of the cavities 5 in the shell 4.

Pre-forming of the cavities 5 in the bombshell 4 is preferably done by mechanical machining such as drilling or milling. As alternatives the machining could also comprise laser ablation or etching. It is also possible to combine different types of machining .

In an alternative embodiment, the bomb 1 comprises a second, outer bombshell fixedly arranged on the outside of the first bombshell 4, where the function of the second bombshell is to ensure that the projectiles 7 are kept in the cavities 5 during rotation of the bomb 1. The second, outer bombshell is preferably a pre-formed metal shell of steel or plastic arranged to cover, in part or completely, the first, inner bombshell. It is arranged to be mountable directly on the first, inner bombshell by, for example, thermal expansion or alternative by other mounting means, such as a snap mount.

In figures 3 and 4, two alternative embodiments of a bombshell 4 according to the invention are shown. Figure 3 shows an embodiment with a bigger cavity 10 shaped to comprise three projectiles 7, instead of one projectile, and a surrounding filler agent or material 8. The bigger cavity 10 implies a more compact shell and a simplified method of pre fragmentation compared to the embodiment as shown in figure 2.

Figure 4 shows an embodiment with mass neutral in-built projectiles 7 in the bomb shell 4 without the pre-formed cavities. The mass neutrality is obtained by careful selection of the sizes, amounts, and densities of the projectiles 7 and the surrounding filler agent 8, so that the combined mass of the projectiles 7 and the filler material 8 in one area of the shell 4 is equal to the mass of a corresponding area on another side of the shell, where projectiles 7 and filler material 8 have not been arranged. By a corresponding area is understood an area which has a similar size and shape, and which is arranged at a similar distance from a central axis of the warhead 1.

With a mass neutral arrangement of in-built projectiles 7, it is possible to arrange the projectiles 7 on one side of the bomb body 4 without affecting the moment of inertia or centre of gravity of the bomb 1. Since the bomb 1 is able to turn a predetermined side of itself towards the target, with help from the control system, and at the same time has the capacity to detonate at the right moment in time, this can be used to turn the right side, provided with the projectiles 7, at the right moment in time, towards the target.

An additional benefit of mass neutral built-in projectiles 7, as shown in figure 4, is that naturally occurring splinter fragments, generated from the side of the bomb 1 that lacks projectiles 7, are reduced in speed faster than the projectiles 7, due to the higher aerodynamic drag of the splinter fragments.

This is advantageous from an MCD perspective (MCD - Minimum Collateral Damage) , and it is also an advantage that the area of risk is reduced. Another advantage with this embodiment is that the bomb 1 needs projectiles 7 only on one side of the bomb 1, which is economically advantageous.

In additional embodiments, not shown, more than three projectiles 7 are arranged in the same cavity.

The projectiles could be of the same or different sizes and/or shapes. The cavities 5 could also be shaped as short or long grooves in the bombshell 4. Other geometrical shapes of the cavities 5 are also possible.

The positions of the cavities 5 on the bombshell 4 are selectable in dependence of the desired strength of the bombshell 4. In areas with high mechanical strain or stress, the distances between the cavities 5 are increased, and in areas with low mechanical strain or stress the distances between the cavities are reduced 5. Topological optimization methods are used in at least one embodiment of the invention to position the cavities 5 on the bombshell 4 in such a way as to increase the overall structural strength of the bombshell 4. With the aid of mathematical methods, and based on the restrictions regarding size, weight, weight distribution, minimum and maximum values for the desired strength in various areas, etc., an ideal positioning of each of the cavities 5 is calculated .

Projectiles 7 with the density p_pr0j are placed and fixed, or bound, to the preformed cavities 5. The projectiles 7 are shaped to only partly fill the cavities 5. The remaining space or volume of the cavities 5, not filled by the projectiles 7, is filled with a filling material or agent 8 with the density, P_fin . The density of the projectiles 7 and the density of the filling material or agent 8 are selected so that the weight of the bombshell material, which was removed when pre-forming the cavities 5 in the bombshell 4, is egual to the total weight of the added projectiles 7 and the added filler material or agent 8, i.e. the weight of the bomb is identical before and after pre-fragmentation is performed on the bomb 4. The density of the filling material or agent 8, P_fm, the bombshell p_sheii_/ and the projectiles p_pr0j are selected so that the relation p_fiu < Psheii < Pproj is fulfilled. In other words, the weight of heavy projectiles 7 is compensated by the use of a filler agent 8 with a density that is low, even lower than that of the material of the bombshell 4.

By adjusting the form, size and orientation of the cavities 5 and projectiles 7 when pre-fragmentation is performed, it is possible to affect the weapons effect and the direction of the weapons effect. In a first embodiment the cavities 5 are of a cylindrical shape and perpendicularly oriented relative the longitudinal axis of the bomb 1 to achieve maximal lateral weapons effect.

In yet another embodiment of the invention, the cavities 5 and the projectiles 7 in the front portion of the bomb body 2 are arranged askew or aslant relative to the longitudinal axis of the bomb 1 to increase the weapons effect in the front direction of the bomb 1. The cavities 5 and the projectiles 7 on the rear portion of the bomb body 2 are arranged perpendicularly oriented relative the longitudinal axis of the bomb 1 to achieve maximal lateral weapons effect. Inspection of the orientation of the cavities 5 and projectiles 7 could be performed, for example, with X-ray analysis.

This embodiment may be combined with any of the previously described embodiments. Hence cavities 5 with one single projectile 7 each, with a few projectiles 7, or with a greater number of projectiles 7, arranged at an angle with the longitudinal axis of the bomb 1, all result in further embodiments of the invention.

Claims

1. Method for pre-fragmentation of a warhead (1) comprising a warhead body (2), an explosive charge (6), and a warhead shell (4) with the density p_shei: wherein cavities (5) are formed in the warhead shell (4), and at least one pre-formed projectile ( 7 ) with the density p_proj and filler material or agent (8) with the density p_tm, are arranged in each cavity (5), characterized in that the method comprises the following steps:

- pre-formation of the cavities (5) in the warhead shell (4) ,

- arrangement of at least one projectile (7) in each pre formed cavity (5),

- arrangement of filler material or agent (8) in the cavities (5) so that the cavities (5) are filled,

- treatment of the filler material or agent (8) so that the filler material or agent (8) forms a connected structure with high adhesiveness to the projectiles (7) and to the walls of the cavity (5) .

2. Method according to claim 1 wherein the dimensions of the projectiles (7) and the cavities (5) and the density of the projectiles (7) and the filler material or agent (8) are selected so that the mass of the pre-fragmented warhead (1) is the same as that of the warhead (1) with the warhead shell (4) before the cavities (5) were formed therein.

3. Method according to claim 1 wherein the densities of the filler material or agent (8), the warhead shell (4), and the projectiles (7) are selected to fulfil the relation Pfill ^ Pshell ^ Pproj ·

4. Method according to claim 1, wherein the cavities (5) are mechanically pre-formed in the warhead shell (4) by drilling or milling.

5. Method according to claim 1, wherein the filler material or agent (8) are treated to a strong and

continuous structure by heat and pressure treatment.

6. Method according to claim 1, wherein the filler material or agent (8) are treated to a strong and

continuous structure by a curing treatment.

7. Method according to claim 1, wherein the positions of the cavities (5) on the warhead shell (4) are selected depending upon the mechanical strength of the warhead shell (4) by utilizing a topological optimization method.

8. Pre-fragmented warhead (1) comprising a warhead body (2), an explosive charge (6), and a warhead shell (4), with the thickness t_Sheii and the density p_shei: wherein the warhead shell (4) comprises projectiles (7) with the density p_proj and filler material or agent (8) with the density p_fin, characterized in that the dimensions of the projectiles (7) and the cavities (5) and the density of the projectiles (7) and the filler material or agent (8) are selected so that the mass of the warhead (1) before pre fragmentation remains the same as after pre-fragmentation.

9. Pre-fragmented warhead (1) according to claim 8, wherein the filler material or agent (8), the warhead shell (4), and the projectiles (7) are selected to fulfil the relation P_fm P_sheii ^ Pp_roj ·

10. Pre-fragmented warhead (1) according to claim 8 or claim 9, wherein the projectiles (7) are spherical with the diameter d_pr0j, and that the cavities (5) are cylindrically shaped with the length l_cav and the diameter d_cav, and where dproj ^ d_cav ·

11. Pre-fragmented warhead (1) according to any of claims 8-10, wherein the projectile (7) comprises an alloy of wolfram.

12. Pre-fragmented warhead (1) according to any of claims 8-10, wherein the projectile (7) comprises depleted

uranium.

13. Pre-fragmented warhead (1) according to any of claims

8-12, wherein the filler material or agent (8) comprises a magnesium powder.

14. Pre-fragmented warhead (1) according to any of claims 8-12, wherein the filler material or agent (8) comprises an aluminium powder.

15. Pre-fragmented warhead (1) according to any of claims

8-12, wherein the filler material or agent (8) comprises a zirconium powder.

16. Pre-fragmented warhead (1) according to any of claims

8-12, wherein the filler material or agent (8) comprises a thermosetting plastic.

17. Pre-fragmented warhead (1) according to any of claims 8-12, wherein the filler material or agent (8) comprises a glue

18. Pre-fragmented warhead (1) according to any of claims 8-17, wherein the warhead (1) comprises a second warhead shell arranged on the first warhead shell (4).

19. Pre-fragmented warhead (1) according to any of claims 8-18, wherein the warhead (1) is a bomb.

20. Pre-fragmented warhead (1) according to any of claims

8-18, wherein the warhead (1) is a grenade.

21. Pre-fragmented warhead (1) according to any of claims

8-18, wherein the warhead (1) is an artillery projectile.