Patent Application
20110146523
1. Field of the Invention
This invention relates to fragmentation warheads.
2. Description of the Related Art
Fragmentation warheads expel metal fragments upon detonation of an explosive. Fragmentation warheads are used as offensive weapons or as countermeasures to anti-personnel or anti-property weapons such as rocket-propelled grenades. A typical warhead includes an explosive inside a steel case. A booster explosive and safe and arm device are positioned in an aft section of the case to detonate the explosive. A fragmentation assembly is placed in an opening in a fore section of the case against the flat leading surface of the explosive. The fragmentation assembly will typically include ‘scored’ metal or individual fragments such as spheres or cubes to control the size and shape of the fragments so that the fragments are expelled in a predictable pattern and speed. Scored metal produces about an 80% mass efficiency while individual fragments are expelled with mass efficiency approaching 100% where mass efficiency is defined as the ratio of fragment mass expelled (therefore effective against the intended target) to the total fragment mass. In other words, the mass efficiency is the ratio of the total mass less the interstitial mass that was consumed during the launch process (therefore ineffective against the intended target) to the total mass.
The steel case confines a portion of the radial energy of the pressure wave (albeit for a very short duration) caused by detonation of the explosive and redirects it along the body axis of the warhead to increase the force of the blast that propels the metal fragments forward with a lethality radius of, for example, 25-50 meters. The lethality radius is defined as the radius of a virtual circle composed of the sum of all lethal areas (zones) meeting a minimum lethal threshold. For example, the lethality threshold may occur when 1% of people at that radius are killed. These fragments are generally expelled in a forward cone towards the intended target. The density of fragments per unit area is maximum near zero degrees and falls off with increasing angle with tails that extend well beyond the desired cone. As a result, the warhead has a maximum lethality confined to a very narrow angle and expels a certain amount of lethal fragments outside the desired target area that may cause collateral damage. As a result, the aimpoint and detonation timing tolerances to engage and destroy the threat while minimizing collateral damage are tight.
Detonation of the high explosive produces a gas blast that has a much smaller lethality radius, maybe 3 meters in this example, in all directions caused by the pressure wave of the blast. The detonation also tears the steel case into metal fragments of various shapes and sizes that are thrown in all directions, beyond the lethality radius of the gas blast. In this example, the expelled metal fragments from the case may have a lethality radius of 5-8 meters. Detonation of the steel case increases the potential for collateral damage without improving the lethality of the warhead to destroy the threat.
The present invention provides a high-lethality low collateral damage fragmentation warhead.
This is accomplished by forming the case of a material that is pulverized upon detonation of the explosive. The lethality radius of the pulverized case fragments is no greater than that of the gas blast, thus reducing potential collateral damage. Warhead lethality is improved by placing a pattern shaper between the fragment assembly and the explosive. The explosive and pattern shaper have a conformal non-planar interface that shapes the front of the pressure wave as it propagates there through to expel metal fragments from the fragmentation assembly with a desired pattern density over a prescribed solid angle. In an exemplary embodiment, the pattern shaper provides a more uniform density over only the prescribed solid angle. This improves lethality and further reduces collateral damage. The expelled metal fragments exhibit a mass efficiency of at least 70% with typical values of approximately 80% for scored metal and near 100% for discrete fragments such as cubes or spheres. By comparison the pulverized case fragments exhibit a mass efficiency of no more than 1% with preferred values near 0%. A metal retaining ring around the periphery of and at least coextensive with the fragmentation assembly provides a measure of confinement that directs fragments at the edges in the desired direction to reduce any tails outside the prescribed solid angle. The warhead may be configured as forward or side-firing. Although the preferred embodiment includes both the case material that is pulverized upon detonation and the pattern shaper, the fragmentation warhead may be improved by employing either feature alone to reduce collateral damage or improve lethality.
In an exemplary embodiment of a forward firing warhead, the case is made of a material that is pulverized with a mass efficiency near 0% upon detonation. Detonation is initiated with a single-point booster positioned aft along the body axis aft of the explosive. The fore end of the explosive and the pattern shaper are designed to progressively slow the advancing pressure wave with increasing radius from the body axis to make the number of expelled fragments per unit area more uniform across a prescribed solid angle. This is achieved by providing the explosive with a convex conical shape about the body axis having radius R1 and slope S1. The explosive and pattern shaper are also designed (suitably in conjunction with the retaining ring) to gradually speed the advancing pressure wavefront at the periphery to direct expelled fragments along the body axis to reduce the tails outside the prescribed solid angle. This is achieved by providing the explosive with a convex annular shape from radius R2 to the other edge with slope S2. The two shaped regions are typically separated by a planar annular region of R2−R1. The interior surface of the pattern shaper conforms to the shape of the explosive. The exterior surface is typically planar and abuts the fragment assembly. The thickness of the pattern shaper is dictated by the shock impedance of the material from which it is formed. The pattern shaper can be an integral part of the fragmentation assembly. However, discrete parts simplify machining and allows for more flexibility in the selection of the pattern shaper material.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
a-7d are diagrams illustrating the propagation of the pressure wave through a conventional fragmentation assembly;
a-8d are diagrams illustrating the propagation of the pressure wave through the pattern shaper and fragmentation assembly in accordance with the present invention;
a and 9b and 10a and 10b are diagrams plotting the number of expelled fragments and number of expelled fragments per area over solid angle for a conventional fragmentation assembly and for a pattern shaped fragmentation assembly in accordance with the present invention; and
a-11c are diagrams of an alternative side-firing warhead.
The present invention provides a high-lethality low collateral damage fragmentation warhead. This is accomplished by forming the case of a material that is pulverized upon detonation of the explosive. As a result, the lethality radius of the pulverized case fragments is no greater than that of the gas blast, thus reducing potential collateral damage. Warhead lethality is improved by placing a pattern shaper between the fragment assembly and the explosive. The explosive and pattern shaper have a conformal non-planar interface that shapes the pressure wavefront caused by detonation of the explosive as it propagates there through to expel metal fragments from the fragmentation assembly with a desired pattern density over a prescribed solid angle.
The fragmentation warhead can be used in conjunction with a wide range of interceptors including projectiles and self-propelled missiles and spinning or non-spinning and various guidance systems. The aiming and detonation sequence may be computed and loaded into the interceptor prior to firing. For example, in a close-range countermeasure system, the guidance system will determine when to fire a sequence of motors on the interceptor and when to detonate the warhead. This sequence is loaded into the interceptor prior to launch. A more sophisticated longer range missile might fly to a target and compute its own aiming and detonation sequences or have those sequences downloaded during flight.
As shown in
The threat detection, guidance, navigation and control systems generate a firing solution to destroy the threat. That solution has a composite system error which means there is an aiming error that can be translated into an area or volume. The area or volume of the cone is typically 100 to 1,000 times larger than the presented area of the target. The fragmentation warhead must engage the entire area or volume with lethal force to destroy the threat. The area or volume and the lethality requirement per threat determine the number of fragments that must be expelled. Typically the threat can be in any place within the volume with equal probability. In this case, the fragmentation warhead is suitably designed to expel metal fragments having an approximately uniform pattern density (# fragments per unit area) over the prescribed solid angle of the volume and preferably no further. If the threat is not placed in the volume with equal probability but is skewed in some manner, the fragmentation warhead is suitably designed to match that distribution.
To accomplish the dual objectives of improved lethality and reduced collateral damage, the case 18 is formed of a material such as a fiber reinforced composite, engineered wood, thermoplastic (resin, polymer), or even foam that is pulverized into a cloud 23 of harmless fine particles 24 upon detonation of the explosive. The particles preferably have a mass efficiency near 0% and no greater than 1% so that the lethality radius of the expelled particles 24 is no greater than the lethality radius of the gas blast from the detonating explosives. Consequently, the threat to the soldiers on either side of the warhead is reduced to the threat posed by the gas blast. For typical countermeasure sized warheads this is a couple meters.
A pattern shaper is placed inside the case between the fragmentation assembly and the explosive. The explosive and pattern shaper have a conformal non-planar interface that shapes the pressure wavefront as it propagates there through to expel metal fragments 20 from the fragmentation assembly with a desired pattern density over the prescribed solid angle 22. In the typical scenario, the pattern shaper produces an approximately uniform density of fragments per unit area over the cone. The pattern shaper and explosive (suitably in conjunction with a metal retaining ring) are also designed to reduce or eliminate the tails of expelled fragments beyond the desired cone to further reduce collateral damage.
An exemplary embodiment of forward-firing fragmentation warhead 12 configured for use as a countermeasure to expel metal fragments with an approximately uniform density over only a prescribed solid angle is shown in
For a given design the space between the safe and arm device 36 and fragmentation assembly 38 defines a volume 44 for explosive. The conventional approach is to fill the entire volume 44 with explosive to maximize the force of the gas blast. Furthermore case 30 is formed from steel that at least partially confines the gas blast to expel fragments forward generally along body axis 34. This maximizes the lethality radius of the expelled fragments and presumably the overall lethality of the warhead.
The warhead design of the present invention takes a different approach countering conventional design philosophy to improve overall lethality while reducing the risk of collateral damage. First, case 18 is formed of a material such as fiber reinforced composite, engineered wood, thermoplastic (resin, polymer), or even foam that is pulverized upon detonation of explosive 30. This eliminates the metal fragments thrown radially from the detonating warhead at the cost of losing the confinement provided by the steel case. Second, explosive material is removed from the fore surface 46 of explosive 30 and a pattern shaper 48 conformal with the shaped fore surface is placed in the case to fill the missing volume. The interface between the explosive and the pattern shaper changes the relative velocities of a propagating pressure wave across an aft surface of the fragmentation assembly 38 to shape the pattern density of expelled metal fragments. The conformal shape and thickness of the pattern shaper are determined by a number of design parameters including the detonation scheme, the material used for the pattern shaper, the design of the fragmentation assembly, the prescribed solid angle and the desired pattern density over the solid angle. A metal retaining ring 50 is preferably placed around the periphery and at least coextensive with fragmentation assembly 38. This ring provides a degree of confinement to direct fragments axially instead of radially. The ring contributes to reducing or eliminating the tails of the pattern density beyond the prescribed solid angle. Although some volume of explosive material and confinement are sacrificed, simulations and live-fire test data demonstrate that the capability to control or shape the pattern density of expelled metal fragments over the prescribed solid angle improves the overall lethality of the warhead and reduces collateral damage because the case is pulverized and the expelled metal fragments from the assembly are better confined to the prescribed solid angle.
An exemplary embodiment of the pattern shaper 48 and conformal interface between explosive 30 and the pattern shaper is illustrated in
Pressure wave 54 travels relatively faster in the convex center and peripheral regions 56 and 58, respectively, because explosive 30 continues to detonate. Once the wave reaches the pattern shaper it slows down. How much the wave slows down is dictated by the shock impedance of the shaper material which is a function of the material's density and the speed of sound in the material and the thickness of the pattern shaper. Lower density materials such as composites are generally preferred because they absorb less energy. However, higher density materials can have a smaller volume leaving more space for explosive. The range of materials suitable for the shaper includes fiber reinforced composites, thermoplastic (resin, polymer), nylon, rubber, stereolithographic (SL) materials, structural foams, and metals. The only qualification is that it be either castable or machinable.
Retaining ring 50 placed around the periphery and at least coextensive with fragmentation assembly 38 provides confinement albeit for a few milliseconds that emphasizes the expelled fragments axial velocity over their radial velocity. The design of the retaining ring and the other annular region 58 are jointly optimized to bring the tails of the distribution of the expelled fragments in to the prescribed solid angle. As shown in
The design of the pattern shaper depicted in
With this energy budget, we can select the right class of material that will meet not only the mass requirement but the right shock impedance. It is usually preferable to use a light density material, provided that the material meets the impedance and mass requirement. An advantage is that this class of material will not damage the fragments. It is conceivable to select a material with higher density, for example a light metal, again meeting the impedance and mass requirement. But because of its strength and ductility, it unfortunately changes the fragment fly-out characteristics. The shaper, then, becomes coupled to the fragment disk, making the shaper geometry design more complicated.
The radius and slope R1/S1 and R2/S2 of the convex conical region and the convex annular region are determined based on test data and/or computer simulation of the warhead without the pattern shaper and the desired distribution of the pattern fragment density (fragments per unit and number of fragments) at a certain target distance and solid angle. If test data is available, the computer model is calibrated to match it. Near one-to-one mapping can be made from the initial fragment position to the target location. These individual mappings are sorted and turned into the mapping between the fragment annulus and the on-target annulus. The required mapping yields the magnitude of the radial trajectory corrections that must be made from the baseline warhead. These trajectory corrections are essentially the fragment velocity vector corrections. The fragment velocity vector corrections can be realized by contouring of the explosive and fragment interface. But since we desire to have flat fragment disk surface (assembly, cost), we introduce an interface material in the form of the pattern shaper that will effectively act as a surrogate to change the wave front. (R1, S1) & (R2, S2) are determined based on the desired corrections (magnitude and direction), for each annulus. But because there is an immediate effect from the adjacent annuli, computer modeling must be used to arrive at the desired (R1, S1), (R2, S2), and, if needed; (R3, S3), etc.
The radial blast patterns from the detonation of the explosive and pulverized case and the forward axial blast pattern from the detonation of the fragmentation assembly are depicted in
Looking along the body axis 32 from above, detonation of the explosive expels metal fragments forward with a prescribed solid angle 80 about the body axis. The uniform pattern, resulting from a properly designed shaper, thus increases the probability of a hit in the prescribed volume. Each fragment is designed to be lethal such that given a hit, it will provide a kill. The probability of a kill Pk being greater than 99% (81) to a radius of approximately 40 meters over the prescribed angle, greater than 50% (82) to a radius of approximately 50 meters and greater than 1% (lethality threshold 83) to a radius of approximately 53 meters and beyond that less than 1%. Also, the Pk 84 outside the prescribed solid angle (except for within the gas blast radius 85) is less than 1%.
Propagation of pressure waves 90 and 92 at times T1, T2, T3 and T4 through two warheads one with and one without the pattern shaper 48 is illustrated in
Actual and simulated results of the pattern density produced by the two warheads one with and one without the pattern shaper are shown in
Although a forward-firing warhead configuration is the most typical, the principles of the invention, the pulverized case material and the pattern shaper can also be applied to a side-firing warhead 120 as illustrated in
While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
