Explosively Formed Projectile (EFP)
Explosive-Formed Penetrating (EFP) Warhead
Explosively Formed Penetrator (EFP) Warhead
Explosively Forged Penetrator (EFP) Warhead
A shaped charge is a concave metal hemisphere or cone (known as a liner) backed by a high explosive, all in a steel or aluminum casing. When the high explosive is detonated, the metal liner is compressed and squeezed forward, forming a jet whose tip may travel as fast as 10 kilometers per second.
Conventional shaped charges are constructed with a charge case, a hollow conical liner within the case, and a high explosive material positioned between the liner and case. A detonator is activated to initiate the explosive material to generate a detonation wave. This wave collapses the liner and a high velocity metallic jet is formed. The jet pierces the well casing and geologic formation, and a slow moving slug is simultaneously formed. The jet properties depend on the charge shape, the energy released, and the liner mass and composition. A Monroe-effect shaped-charge warhead can be expected to penetrate armor equal to 150-250% of the warhead diameter.
Shaped Charge Theory
Hydrodynamic penetration is a complex mechanism which begins to appear when the strike velocity exceeds a critical value, typically about 1,150m/s for current penetrators against rolled homogenous armor (RHA) targets. Full hydrodynamic behavior does not occur until the strike velocity reaches several kilometers per second, such as occurs with shaped charge munitions. At strike velocities less than about 1,150m/s penetration of metal armor occurs mainly through the mechanism of plastic deformation. A typical penetrator achieves a strike velocity around 1,500m/s to 1,700m/s, depending on range, and therefore target effects generally exhibit both hydrodynamic behaviour and plastic deformation.
A number of models of varying degrees of complexity have been developed to predict long rod penetrator performance. A common feature that emerges from these models is the importance of a high strike velocity to exploit more fully the hydrodynamic penetration mechanism, which, in turn, is further improved by the use of longer penetrators having higher densities relative to the target material density. This is amply supported by experimental work.
Shaped charge is indeed an extraordinary phenomenon that is beyond the scale of normal physics, which explains why its fundamental theoretical mechanism is by no means fully understood.
The shaped charge jet tip reaches 10 kms-l some 40 µs after detonation, giving a cone tip acceleration of about 25 million g. At this acceleration the tip would reach the speed of light, were this possible, in around 1.5 seconds. But of course, it reaches a terminal velocity after only 40 millionths of a second. It is difficult to think of any other terrestrial event as fast as a shaped charge jet tip. The jet tail has a velocity of 2-5 kms-l and so the jet stretches out to a length of about 8 cone diameters (CDs) before particulation occurs. The stretching occurs at a high strain rate, requiring the cone material to have excellent dynamic ductility at temperatures up to about 450°C. On reaching a target, the pressure developed between the jet tip and the forming crater can be as high as 10 Mbar (10 million atmospheres), several times the highest pressure predicted in the Earth's core.
It is universally agreed that conical liner collapse and target penetration both occur by hydrodynamic flow. However, it has been established by X-ray diffraction that the jet is solid metal and not molten. Additionally, best estimates of jet temperature by incandescence colour suggest a mean value of about 450°C, and copper melts at 1083°C at atmospheric pressure. So the following conundrum is the first confusion: The jet appears to behave like a fluid, and yet it is known to be a solid. One recent theory that would help explain this is that the jet has a molten core but with a solid outer sheath.
The hypervelocity hydrodynamic impact (unlike lower speed KE penetration) results in a mushroom head penetration, such that the hole diameter is larger than the penetrator diameter. The dynamic compressive yield stress of the target is exceeded by a factor of at least one thousand times, so that only the densities of the target and jet materials are important. Both materials flow as if they were fluids and the penetration event can be modelled quite accurately using the Bernoulli equation for incompressible flow to give the well known hydrodynamic penetration equation.
Shaped Charge History
Charles Edward Munroe was the inventor of "The Monroe Effect" in explosives in 1885. He noted that a high explosive with a cavity facing a target left an indentation. The earliest known reference to the effect appears to be 1792, and there is some indication that mining engineers may have exploited the phenomenon over 150 years ago. The Monroe Effect was rediscovered by Von Neumann in 1911, but no practical applications were developed.
Shaped charges were first developed after World War I to penetrate tanks and other armored equipment. A cylindrical charge that lies flat against the armour and is being initiated in one end gives a directed detonation effect so that a hole is created at the point of contact is Generation I. If that charge is equipped with a conical hole the force of the explosion will be channeled further and increases the chances for a penetration it is Generation II. The most common type of hollow charge munitions is the jet creating hollow charge, also called Hollow Charge Generation III. The other type of hollow charge munition is the projectile creating munition. It is referred to as Genetration IV. Gen I and Gen II (developed during the WW II) are predecessors to Gen III and IV but they are no longer in use in any munitions.
The "shaped charge" was introduced to warfare as an anti-tank device in World War II after its re-discovery in the late 1930s. In 1935, Henry Mohaupt, a chemical engineer [and a machine gunner in the Swiss Army] established a laboratory in Zurich to develop an effective anti-tank weapon that could be used by infantry soldiers. Henry Mohaupt was the inventor of the lined shaped charge. Other accounts mention earlier work by R.W. Wood of the John Hopkins University Physics Department as the discoverer of the metal liner principle. After the war started, Mohaupt came to the United States, and in October 1940 he took over direction of the bazooka project.
In January, 1945, Ramsey C. Armstrong founded Well Explosives Company, Inc. in Fort Worth, Texas. He decided to pursue perforating technology related to the bazooka, an anti-tank device based on the shaped charge concept. Armstrong contacted Mohaupt in Washington, DC, where he was then working for the Navy, and in October of 1946, Mohaupt and his wife made the long drive from Washington to Fort Worth.
The Beehive Charge was a six in diameter shaped charge demolition/sabotage device devised by the UK in October 1941.
The RPG-43 Ruchnaya Protivotankovaya Granata ("Hand Anti-Tank Grenade") Model 1943 was Russia's first shaped charge grenade for anti-tank purposes. It replaced the RPG-40 which was an ordinary stick grenade with an oversized high explosive head. It had an impact fuze with a 95mm diameter warhead containing 612g of TNT which could penetrate 75mm of armor.
In 1965 a Russian scientist proposed that a shaped charge originally developed for piercing thick steel armor be adapted to the task of accelerating shock waves. The resulting device, looking little like a wind tunnel, is called a Voitenko compressor. The Voitenko compressor initially separates a test gas from a shaped charge with a malleable steel plate. When the shaped charge detonates, most of its energy is focused on the steel plate, driving it forward and pushing the test gas ahead of it. Ames translated this idea into a self-destroying shock tube. A 66-pound shaped charge accelerated the gas in a 3-cm glass-walled tube 2 meters in length. The velocity of the resulting shock wave was a phenomenal 220 000 feet per second. The apparatus exposed to the detonation was, of course, completely destroyed, but not before useful data were extracted.
The US Army Ballistic Research Laboratory, an ARL predecessor organization, made several important contributions to the development of shaped-charge technology. BRL scientists delineated the penetration mechanics of the stretching, high-velocity jet of metal that is formed by the warhead, thus making it possible to design relatively light, inexpensive weapons to defend against tanks. Guided missiles, such as Shillelagh, TOW, Dragon, and Hellfire, exploited the high penetration capability of such warheads with accurate fire at long range. Further contributions included the demonstration of tandem shaped-charge warheads and the application of advanced liner material technology that increased jet velocity and ductility and provided enhanced lethality within existing weapon system envelopes.
Early work on shaped charges showed that a range of alternative constructions, including modifying the angle of the liner or varying its thickness, would result in a faster and longer metal jet. These research and development efforts to maximize penetration capabilities were based largely on trial and error. It was not until the 1970s that modeling codes could predict with any accuracy how a shaped charge would behave. While the concept of a metal surface being squeezed forward may seem relatively straightforward, the physics of shaped charges is very complex and even today is not completely understood.
In early 1997, Lawrence Livermore successfully tested a shaped charge that penetrated 3.4 meters of high-strength armor steel. The largest diameter precision shaped charge ever built produced a jet of molybdenum that traveled several meters through the air before making its way through successive blocks of steel.
A shaped charge, by design, focuses all of its energy on a single line, making it very accurate and controllable. When size is added to that accuracy, the effect can be dramatic. The success of this demonstration by Lawrence Livermore at the Nevada Test Site's Big Explosives Experimental Facility would not have been possible without the combination of reliable hydrodynamic codes and diagnostic tools that verify one another.
X-radiography produces shadowgraphs that provide experienced researchers with information about the jet's velocity, density, and mass distribution. The rotating-mirror framing camera, a kind of motion picture camera, can shoot millions of frames in a second. A typical shaped-charge jet-formation experiment lasts less than 30 microseconds, and the framing camera is usually set to record an image about once every microsecond. The exposure time for the framing camera may be anywhere from 100 to 200 nanoseconds, or billionths of a second. The newest tool is the image-converter (IC) camera, which was developed at Livermore in the mid-1980s. A pulsed ruby laser is synchronized with the IC camera frames to provide illumination of the shaped charge. The electronic image tube that acts as the shutter for each image frame converts the photons of laser light reflected by the shaped charge to photoelectrons. These photoelectrons are accelerated by a high-voltage pulse onto a phosphor, where they are reconverted to photons that are then transmitted to the film. With exposure times of just 15 to 20 nanoseconds (up to ten times shorter than those of the framing camera) and a band-pass filter mounted on the camera to exclude extraneous light, the IC camera has supplied the first truly high-resolution images of the formation and early flight of a shaped-charge jet.
Shaped Charges in the Oil Industry
The most extensive use today of Shaped Charges is in the oil and gas industry, where they open up the rock around drilled wells. Shaped charges are used in the oil and gas industry and in other fields to pierce metal, concrete, and other solid materials. In an oil or gas well, a metallic casing is cemented to the borehole walls to maintain the borehole integrity. Shaped charges are incorporated in a hollow carrier gun or a strip positioned in the casing. The shaped charges are activated to pierce the well casing and the geologic formation at the hydrocarbon producing zone. The hydrocarbons enter the casing through such perforations and are transmitted to the well surface.
Unlike the ripping affect achieved by bulk cutters, shaped charges are intended to sever targets by jetcutting. Shaped charges utilize special housings that are designed to create a cavity or void between the explosive material and target wall. Employing a phenomenon known as the Monroe Effect, the shock wave produced at detonation accelerates and deforms the shaped housing into a high-velocity (24,000-27,000 fps) plasma jet within the void space. The formed jet is able to cut through steel targets of various thicknesses based upon the void shape and the "stand-off" distance to the target wall. Because the "cutting" efficiency of shaped charges is several times greater than that of bulk charges, they can often greatly reduce the net explosive weight needed to sever similar-sized targets.
Linear-shaped charges (LSC) have a void shaped into a chevron or inverted "V" along its entire length, and they are designed to cut linearly through its target. Subcontractors use LSC's on a wide range of decommissioing targets in many different configurations depending on cutting requriements.
Prior to perforation technology, wells were "open hole" or "shot hole" (barefoot) completions, sometimes employing liners. But the perforated casing completion was an important and necessary development as wells got deeper, and reservoir conditions became more and more complex. Gun perforators have been successfully used as a well completion method since at least 1927; the first patent was 1926, but it did not work. Early gun perforators were "bullet" devices, utilizing actual projectiles (usually steel bullets) to penetrate the well casing. The lined shaped charge perforator a/k/a the jet perforator or jet charge has displaced the old bullet perforators
Conical-shaped charges (CSC) have the cavity created in the shape of a cone designed to cut round holes and to penetrate deep into targets. Industry's primary use of CSC's is in the development of perforating guns; multiple CSC assemblies placed down boreholes and detonated to penetrate through the drill casing and into the surrounding geologic strata for the extraction of hydrocarbons. The use of steel charge cases instead of zinc cases eliminates the decrease in formation productivity and damage to completion components associated with the detonation by-products from zinc-based charges and reduces the cost for special completion fluids. The shaped charges are designed and arranged to assist in optimum orientation and deployed with a tubing-conveyed perforating system to provide an innovative, effective solution for perforating and increasing productivity in long horizontal intervals.
Demolition Shaped Charge
The Charge, Demolition, Shaped, 150mm is designed to make holes of considerable depth and breadth in a variety of materials. It consists of a 150mm diameter conical steel liner with three removable legs which provide a standoff of 145mm. The Charge, Demolition, Shaped, 150mm contains 3.1 kg of HE and its total mass is 4.9 kg.
Target Material Depth of Hole (mm) Armour Plate 178 Mild Steel 250 Hard Rock (Granite) 380 Reinforced concrete 760 Soft rock (Sandstone) 910
Shaped Charges and Explosived Disposal
Shaped Charges are used as a remote clearing device.
COPPER CONE - Using the jet-forming copper cone, the charge produces a jet which may be used to pierce holes, typically through 75mm (3 in) of mild steel or greater thicknesses of concrete or brickwork. It may be used for causing the detonation or deflagration of steel-cased ammunition without any risk of inadvertent disturbance of the target before firing. The usual explosive load is between 20 and 50g.
COPPER EXPLOSIVELY-FORMED PROJECTILE - A wide angled copper cone, essentially a slightly domed disc, generates an explosively-formed projectile (EFP) which may be used to penetrate robust targets at much greater ranges than the jet-forming cone. This enables the VULCAN to be used as a de-armer and disruptor device. It punctures 10mm thick steel at a range of at least 1,500mm.
ALUMINIUM PROJECTILE The aluminium projectile is able to deliver a powerful blow to shell fuses and bomb pistols thus removing them or jamming their mechanisms. It thus provides a low-priced, disposable, alternative to de-armers using heavy steel barrels.
MAGNESIUM INCENDIARY CONE - The jet formed by this cone is less penetrating than that formed by the copper cone but it is a less powerful initiator of detonation. It is used to penetrate even thick-walled shells or bombs and ignite the explosive or pyrotechnic filling. In this application it is much less likely to cause inadvertent high order detonation than other, more conventional, charges. It thus provides a reliable means of bringing about a "low order" deflagration event. The usual explosive load for this purpose is between 30 and 40g.
WATER PROJECTILE A conical cavity is formed in the explosive, water is poured into the cavity, and a plastic cone is inserted to retain the water. The charge thus becomes a shaped charge, able to penetrate steel-cased munitions with thicknesses of up to 10mm, and to disperse their contents with minimal risk of detonation. Charges are quickly assembled and robust.
High Explosive Anti-Tank (HEAT) Armor Piercing Shaped Charge
Armor piercing shells comprise a special type of anti-tank ammunition which is provided with a hollow charge warhead. In principle, a hollow charge comprises an outer casing, a metal cone and an explosive. When the explosive detonates, the metal cone is squeezed together and a metal jet is formed which, with great force, penetrates even very thick and hard armor. Due to its good effect in armored targets, the hollow charges have long constituted a serious threat to armored vehicles.
The High Explosive Anti-Tank (HEAT) rounds take a cone-shaped shaped charge warhead to targets. This shaped charge warhead, with its inherent blast and fragmentation capability, also provides additional weapon defeat capability. A copper shaped charge liner and wave shaper are contained within the warhead.
A sophisticated heavy two-stage shaped-charge warhead is capable of piercing armor of equivalent to 900mm thickness. A triple-shaped charge warhead offers 50mm more penetration. The RPG-7 grenade, with a shaped-charge warhead, has very good armor penetration (330 mm), capable of defeating most types of armored vehicles. Even a small 440 gram shaped-charge explosive is extremely destructive, and can penetrates more than 14 inches (35.6 cm) of armor. The M77 submunition's antimateriel capability is provided through a shaped charge with a built-in standoff, which can penetrate up to four inches of armor. The smaller artillery-delivered M46 submissions have a shaped charge warhead that penetrates 2.75 inches of homogeneous armor.
Explosively Formed Projectile (EFP)
Wide angle cones and other liner shapes such as plates or dishes do not jet, but give instead an explosively formed projectile or EFP. The projectile forms by dynamic plastic flow and has a velocity of 1-3 kms-l . Target penetration is much less than that of a jet, but the hole diameter is larger with more armour backspall.
The concept of using explosive energy to deform a metal plate into a coherent penetrator while simultaneously accelerating it to extremely high velocities offers a unique method of employing a kinetic energy penetrator without the use of a large gun. A typical explosively formed projectile (EFP) is comprised of a metallic liner, a case, an explosive section, and an initiation train. Very often there is also a retaining ring to position and hold the liner-explosive subassembly in place. EFP warheads are normally designed to produce a single massive, high velocity penetrator. After detonation, the explosive products create enormous pressures that accelerate the liner while simultaneously reshaping it into a rod or some other desired shape. The EFP then hits the target at a high speed, delivering a significantly high mechanical power.
An EFP warhead configuration may be comprised of a steel case, a high-explosive charge, and a metallic liner. Explosively formed penetrator (EFP) warheads have been designed to project a single massive high velocity penetrator to attack the top of armored vehicles. Such armor perforation capability needs further improvement to counter new generations of harder armored vehicles, without resorting to a larger caliber weapon system. In developing a warhead configuration that meets system constraints and also meets performance requirements, several parameters in the warhead configuration must be redesigned to achieve an optimum configuration. Several warhead configurations have been developed to accommodate varying system constraints.
Explosively Formed Penetrator warheads can defeat the target at very long standoffs. EFP warheads consist of an explosive billet and a metal liner. When the explosive is detonated, the detonation wave forms the liner into a high-speed long rod penetrator and propels the penetrator towards the target at speeds greater that Mach 6.
An EFP must be aerodynamically stable so as to strike the target within a small miss distance and a small angle of obliquity. In the U.S., extensive work has focused on forming EFPs with canted fins, to induce spin-up. By forming canted fins on an EFP, improvements in aerodynamic stability can be realized.
Current anti-armor ordnance employ explosively formed elongated penetrators for piercing armored vehicles and equipment. Such penetrators are generally one of two types: rearward folding or forward folding. In forward folding types a warhead containing an explosive charge drives the periphery of a metal plate, referred to as a liner, forward with an axial velocity greater than the axial velocity of the central portion causing the periphery to fold over and converge forward of the central portion and form an elongated penetrator. In rearward folding the explosive charge drives the periphery of the liner forward with an axial velocity less than the axial velocity of the central portion causing the periphery to fold over and converge rearward of the central portion to form the elongated penetrator.
In these approaches, then, the axial velocity component is critical in determining the final desired shape of the penetrator and this is a well accepted technique. However, in certain applications, for example, where the explosively formed penetrator is delivered from the warhead assembly of a missile or projectile, the explosively formed penetrator encounters the skin of the missile or projectile during the critical earlier stages when the liner is being formed into the penetrator shape by the folding action of the periphery over the center. The engagement of the liner with the skin radically alters the axial velocities of the periphery thereby disrupting the folding. This disruption of the forming process causes the penetrator to fragment or otherwise lose its effectiveness as a penetrator. To avoid this, provision is made to remove the impeding portion of the skin using clearing charges or skin just prior to the liner folding action cutting devices which significantly increase the cost and complexity of the systems.
In consequence of the development that has taken place on the protection side through the introduction of composite armor, active armor, etc., the importance of improving the penetrability of the warhead has, however, increased. Developments have therefore led to increasingly longer and heavier hollow charges. In certain cases this can be accepted, typically for all-target shells etc., but for severely weight-optimized designs, with limited space etc., this method is inappropriate. With state-of-the-art technique, therefore, limit has been reached in practice as regards the length and weight of the charges.
In order to increase the penetrability, hollow charges differing from conventional hollow charges have also been developed in recent times. These charges can, for instance, comprise an auxiliary body disposed in front of or integrated with the metal cone of the charge so that upon initiation of the charge it generates a slug which follows behind the actual penetration jet and penetrates and enlarges the hole made by the penetration jet. Alternatively, the hollow charge may have a warhead with two complete hollow charges, so-called tandem hollow charges, which after the projectile is fired accompany each other as an integral unit during the greater part of the travel towards the target, only to separate at a predetermined distance from this and to continue towards the target at mutually slightly different velocities along largely the same trajectory and thereafter to hit the target with a sufficient interval of time to enable the charge which reaches the target first to detonate the explosive in any active armour before the second charge reaches the target, so that this latter charge penetration jet is able to work without disturbance and also is assisted by the penetration work already performed by the first charge which has already detonated within the same confined area of the charge.
In order to function in the intended manner each of the two hollow charges in such a tandem hollow charge must have its own ignition system with associated safety device. To separate the two hollow charges, it is also necessary to have a smaller parting charge, e.g. a powder charge, between the two charges in order to impart to each of these its own velocity change.
It is realized that the penetrating ability against active armor can be increased significantly through two such interacting charges. It is also realized, however, that the warhead of the projectile will be significantly more expensive with two complete hollow charges, each including its own ignition system and a parting charge.
A tandem-projectile can be adapted in particular for compartmentalized targets (multi-layer armor). Two armor-penetrating devices are incorporated in the shell body of a tandem shell. These armor-penetrating devices distinguish each other with respect to their moment of impact; the rear one of the two devices encompasses a shaped hollow explosive charge arrangement. Such a device has two coaxial shaped hollow explosive charges arranged one behind the other. When the device impacts on a target the rear shaped hollow explosive charge is the one that first becomes operative. From a lining forming part of the rear charge a pointed spike is formed; this spike is adapted to be ejected through the forward hollow charge by passing through an opening disposed in the apex of the forward charge and produces a channel in the armor plate of the target. The forward shaped hollow explosive charge is thus ignited with delay relative to ignition of the rear shaped hollow explosive charge. The pointed spike formed by the forward shaped hollow explosive charge thus follows the said channel produced by the rear shaped hollow explosive charge and becomes operative at a preselected position.
It has been observed that difficulties occur when the compartmentalized reinforced target is impacted obliquely by the afore-described known projectile. The difficulties can be attributed to the strongly reduced cross section of the penetrating channel produced by the spike of the first shaped hollow explosive charge as compared to the cross section of such a channel when a perpendicular impacting of the shell and the target occurs. Consequently, only a relatively small surface area is damaged which is a drawback. A further drawback resides in the behavior of such a known shell relative to an active armor. What is meant here by an active armor is an arrangement of explosive charges in the region of the outer armor, by the activation of which the spike formed by the shaped hollow explosive charge is disturbed and is made ineffective with respect to the main armor.
Advanced armor techniques employ a small armor explosive charge that can deform the explosive cone of a shaped charge or deflect the armor piercing subcaliber round normally used in destroying armor such as on a tank. Therefore, there is a need for a multi-warhead that has the capability of defeating armor that is protected with an outer explosive charge arrangement that is designed to defeat a round that has a single blow effect. Therefore, it is an object of this invention to provide a multi-warhead that has a multiplicity of subcaliber warheads that are designed to strike the target and destroy the small protective explosive charges around the armor and then deliver the main warhead to the armor proper for piercing the armor in an effective manner.
Two major applications have evolved for explosively formed projectiles or warheads, namely, long-standoff sensor-fuzed submunitions and medium standoff, close-overflight missiles. The former application, which is the more traditional one, requires the formation of a single-piece EFP capable of flying in a stable fashion to the target. This refinement has led to the flared EFP rod and, more recently, to the finned EFP rod designs.
For the medium or short-standoff applications, a new type of EFP was developed. The need for an aerodynamic shape is not necessary for these applications because of the short distance the EFP must travel, hence, the length of the rod was increased and the flared tail was eliminated from the design. In fact, some of these rods are purposely stretched beyond their breaking point and fracture into several pieces resulting in greater total length.
Prior art devices have tried to solve this problem of selectable effects through the use of different or multiple initiation points for the shape charge munition. The complex shape of the detonation wave produced was intended to interact with the liner causing it to break up into a number of individual fragments. The problem with this approach is that it requires a relatively complex initiation scheme.
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