Patent Grant
8006623
This patent is related to a co-pending application U.S. Ser. No. 12/123,158, filed May 19, 2008, entitled “High-Lethality Low Collateral Damage Fragmentation Warhead”.
1. Field of the Invention
This invention relates to fragmentation warheads and in particular to a dual-mass fragmentation warhead that expels a mass of fragments in a forward-firing pattern and a mass of fragments in a side-firing pattern.
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. The warheads may be launched from ground, sea or airborne platforms. A typical warhead includes an explosive inside a steel case. A booster explosive and safe and arm device are positioned in the case to detonate the explosive.
A radial blast fragmentation warhead includes a steel case that has been pre-cut or scored along the length of the explosive. The booster explosive is positioned in a center section of the case. Detonation of the explosive produces a gas blast that emanates radially from the center point pulverizing the case and expelling the pre-cut metal fragments in all directions in a generally spherical pattern. Although lethal, the radial distribution of the fragments also presents the potential for collateral damage to friendly troops and the launch platform.
A forward blast fragmentation warhead includes a fragmentation assembly placed in an opening in a fore section of the steel case against the flat leading surface of the explosive. The fragmentation assembly will typically include ‘scored’ metal or individual pre-formed fragments such as spheres or cubes to control the size and shape of the fragments so that the fragments are expelled in a somewhat 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.
In the forward blast warhead the booster explosive is positioned in an aft section of the case. 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. 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 a specified threat. 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 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. Detonation of the steel case increases the potential for collateral damage to friendly troops and the launch platform.
The present invention provides a high lethality fragmentation warhead with reduced risk of collateral damage to the warhead launch platform.
In an embodiment, an explosive containment structure that contains the explosive is placed inside a case, the containment structure and case being formed of materials that are pulverized upon detonation of the explosive by an initiator. An aft section of the containment structure defines a void space between the case and the containment structure. A side-firing fragmentation assembly in the void space expels metal fragments in a side-firing pattern upon detonation of the explosive. A forward-firing fragment assembly positioned in front of the explosive expels metal fragments in a forward-firing pattern upon detonation. The combination of forward and side-firing patterns provides a high lethality warhead. The substantial elimination of metal fragments expelled radially in all directions, particularly backwards, reduces the risk of collateral damage to the warhead launch platform. The forward and side-firing fragmentation assemblies may be configured to control the respective firing patterns (e.g. fragment velocity, half-angle and uniformity of fragments).
In another embodiment, an explosive containment structure is placed inside a case, the containment structure and case being formed of materials that are pulverized with a mass efficiency no greater than 1% upon detonation of the explosive. A tapered aft section of the containment structure defines a tapered void space between the case and the containment structure. An explosive having a fore section with a diameter conformal with the case and a dome-shape end and a tapered aft section is fit inside the containment structure. An initiator aft of the explosive initiates detonation of the explosive at the end of the taper. A side-firing fragmentation assembly in the tapered void space expels pre-formed metal fragments in a side-firing pattern with a mass efficiency of at least 70% upon detonation of the explosive. A forward-firing fragmentation assembly positioned in the opening fore of the explosive includes a dome-shaped layer of pre-formed metal fragments that expels metal fragments in a forward-firing pattern with a mass efficiency of at least 70% upon detonation of the explosive. Detonation of the explosive produces a pressure wave that propagates forward through the tapered explosive. The taper is suitably optimized to maximize the void space without reducing the total explosive energy imparted to the forward-firing fragmentation assembly. The dome-shaped layer is approximately matched to the shape of the front of the pressure wave incident on the layer of pre-formed metal fragments to increase fragment velocity and uniformity over the pattern.
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 and 2b are side section and bottom views of an embodiment of a dual-mass forward and side firing fragmentation warhead;
a through 3e are plots of the gas blast propagation to expel the fragments in the forward-firing and side-firing patterns;
a through 5c are diagrams of embodiments of the forward-firing fragmentation assembly to control the half-angle of the forward-firing pattern;
a and 7b are side section and bottom views of an alternate embodiment of a dual-mass forward and side firing fragmentation warhead; and
The present invention provides a high-lethality fragmentation warhead with reduced risk of collateral damage to the warhead launch platform. High lethality is achieved with a forward-firing fragmentation assembly placed in front of the explosive and a side-firing fragmentation assembly placed in a void space in the aft section of the explosive. The risk of collateral damage to the launch platform is reduced by forming the case and explosive containment structures of materials that are pulverized upon detonation of the explosive. This substantially eliminates radial fragments and in particular fragments thrown back towards the platform. Performance may be enhanced by tapering the aft section of the containment structure and explosive to eliminate explosive that does not contribute to the total energy imparted to the forward-firing fragmentation assembly by the pressure wave to create the void space for the side-firing fragmentation assembly. Performance may be further enhanced by forming the end of the explosive and forward-firing fragmentation assembly with largely conformal dome shapes that approximately match the shape of the front of the pressure wave. This both increases the amount of explosive energy delivered to those fragments to increase their velocity and serves to expel them in a desirable pattern (e.g. half-angle and uniformity of fragment density over the half-angle).
The dual-mass fragmentation warhead was developed as a short-range, low-speed countermeasure for airborne launch platforms (e.g. helicopters) to intercept and destroy threats such as rock-propelled grenades (RPGs), unguided rockets or ManPADS while minimizing the risk of collateral damage to the platform. Due to limited armor protection, airborne launch platforms are typically more susceptible to damage from stray fragments than land or sea-based system. The dual-mass fragmentation warhead is however adaptable to a wide-range of battle field scenarios to include any type of land, sea, air or spaced-based launch platforms and longer-range, higher-speed engagements. The warhead may be configured for use as an offensive weapon or for countermeasures.
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.
A typical scenario for the use of a dual-mass fragmentation warhead from a launch platform to intercept and destroy a threat is illustrated in
As shown in
A forward-firing fragmentation assembly 50 is positioned in the opening around the dome-shaped end of the explosive. The assembly suitably includes a dome-shaped layer 52 of metal fragments 54 that are expelled in the forward-firing pattern with a mass efficiency of at least 70% upon detonation of the explosive. Pre-formed fragments are generally preferred because they have a known size and shape upon detonation and retain a mass efficiency near 100%. The fragments may be shaped (rectangular, square or other unique shapes) for a particular threat. For ease of assembly the fragments are typically formed in a mold held by an epoxy that is pulverized on detonation.
As will be described in more detail with reference to
A containment ring 56 may be placed around the periphery and aft of the dome-shaped layer. This ring provides a degree of confinement of the pressure wave to direct fragments axially instead of radially. The ring contains the explosive blast momentarily (e.g. a few milliseconds) but long enough to direct the pressure wave in a forward direction before the ring is itself pulverized. The ring contributes to reducing or eliminating any tails of the pattern beyond the prescribed half-angle. The ring may be extended forward to provide additional confinement to narrow the half-angle as desired. The ring could be extended to span the entire length of the case. A variable-thickness pattern shaper may be inserted between the explosive and fragment layer to slow portions of the wave front to further shape the forward-firing pattern.
A side-firing fragmentation assembly 60 is positioned in the tapered void space 36 around the aft section 42 of explosive 38. The assembly suitably includes a volume of metal fragments 64 that are expelled in the side-firing pattern with a mass efficiency of at least 70% upon detonation of the explosive. Pre-formed fragments are generally preferred because they have a known size and shape upon detonation and retain a mass efficiency near 100%. The fragments may be shaped (rectangular, square or other unique shapes) for a particular threat. For ease of assembly the fragments are typically formed in a mold held by an epoxy that is pulverized on detonation. The relative size, shape and number of fragments in the forward and side-firing assemblies may be configured for a particular threat. In a typical embodiment, the pre-formed fragments in the side-firing assembly are suitably smaller in size and greater in number than the pre-formed fragments in the forward-firing assembly in order to maximize fragment packaging density and increase the number of fragments in the lethality cone “pattern density”.
In general, the side-firing pattern can be more difficult to control than the forward-firing pattern and thus typically will have a larger half-angle. As will be shown in the simulations, the pressure wave in the aft section of the explosive tends to move in a generally sideways or lateral direction expelling the metal fragments in the side-firing pattern. The taper of the containment structure and the mass of the safe and arm device and interceptor behind the side-firing fragmentation assembly provide a measure of confinement to control the half-angle. A base plate 66 may be placed between the assembly and the safe and arm device to provide additional confinement to prevent fragments from being expelled backwards. Additional confinement can be achieved by placing one or more containment rings fore or aft of the side-firing fragmentation assembly.
One might assume that in this configuration the forward and side-firing patterns would be initiated simultaneously or that the side-firing pattern, given its proximity to the aft detonation, would actually occur slightly prior to the forward-firing pattern. In a typical engagement scenario like that shown in
One might further assume that the removal of a portion of explosive 38 to create the tapered void space would reduce the total energy imparted to the forward-firing fragmentation assembly and degrade the lethality of the weapon. However, as the simulations will again demonstrate, for an L/D (length/diameter) optimized forward-firing aft-initiated warhead a tapered aft portion of the explosive represents “dead” volumetric space. In other words, explosive in that space does not contribute to the total energy in the forward propagating wave. Essentially the single-point detonation expands as the pressure wave moves forward until it fills the diameter of the casing. Suitably, the taper of the containment structure and explosive are optimized for a given warhead to maximize the tapered void space without reducing the total energy in the forward propagating pressure wave. In a particular warhead for a particular threat, the void space could be enlarged to increase the available volume of metal fragments for side firing at the cost of energy, hence velocity of the fragments expelled in the forward pattern. Alternately, the void space could be decreased to accommodate a reduced mass of fragments for a side-firing pattern.
In warhead analysis, the detonation pressure wave is simulated using CTH analysis models.
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The CTH analysis model clearly demonstrates (a) that the proper tapering of the explosive and containment structure to create the void space for the side-firing fragmentation assembly does not degrade the forward energy of the pressure wave, (b) that conforming the shape of the forward-firing fragmentation layer to the shape of the pressure wave front increases fragment velocity and pattern uniformity, (c) that the pressure wave will expel fragments 76 in a side-firing pattern and (d) that the side-firing pattern can be delayed with respect to the forward-firing pattern. Other warhead configurations and configurations of the forward and side-firing fragmentation assemblies may be employed within the scope of the dual-mass warhead architecture.
The threat detection, guidance, navigation and control systems either on the launch platform or the interceptor delivering the warhead 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 forward and side-firing patterns is typically 1,000 times or 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 half-angle of the volume and preferably no further (a certain percentage of fragments will stray outside the volume). 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.
Different embodiments of the forward-firing fragmentation assembly are depicted in
A shown in
c illustrates a forward-firing fragmentation assembly 120 that utilizes a flat fragmentation layer 122. Fragments 124 are cast in an epoxy or held in a cup that is pulverized upon detonation. A layer 126 such as RTV holds the assembly in place. A nose cone 128 is positioned on the front of the warhead for aerodynamics. A pattern shaper 130 is placed between the fragment layer 126 and a conformally shaped surface of the explosive 132. 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 120 to shape the pattern density of expelled metal fragments. In the embodiment shown, the conformal aft surface of the pattern shaper has a concave conical shape with radius R1 and slope S2 and a concave annular shape around the periphery starting at radius R2 with slope S2. This non-planar interface progressively slows the propagation velocity of the pressure wave with increasing radius from the long body axis up to a radius R1 and progressively increases the propagation velocity of the pressure wave with increasing radius from a radius R2>R1 so that the number of expelled fragments per unit area is approximately uniform over a prescribed solid angle upon detonation of the explosive. Retaining ring 134 placed around the periphery and at least coextensive with fragmentation layer 120 provides confinement albeit for a few microseconds that emphasizes the expelled fragments axial velocity over their radial velocity. The design of the retaining ring and the concave annular shape of the pattern shaper are jointly optimized to bring the tails of the distribution of the expelled fragments in to the prescribed solid angle.
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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.
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