Patent Publication Number: US-6662726-B1

Title: Kinetic energy penetrator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Benefit is claimed of U.S. Provisional Patent Application Ser. No. 60/123,380, filed Mar. 8, 1999 and entitled “Kinetic Energy Penetrator”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to kinetic energy projectiles, and more particularly to kinetic energy projectiles for defeating reactive armor. 
     2. Description of the Related Art 
     There exists an ongoing evolution of both the armor used on armored vehicles (e.g., tanks and armored personnel carriers (APC&#39;s)) and the projectiles used to defeat such armor. 
     Common anti-armor projectiles of the type fired by tank guns and artillery are typically divided into high explosive and kinetic energy subgroups. High explosive anti-tank (HEAT) projectiles typically include one or more shaped explosive charges which, upon detonation in close proximity to the armor, cause a concentrated jet to penetrate the armor. Common kinetic energy projectiles make use of a long rod penetrator to punch a hole through the armor. As implied by its name, the long rod penetrator includes an elongate, dense, heavy penetrator body or core having a relatively small cross-section. Upon impact with the armor, this small cross-section provides a concentration of impact force on the armor effective to penetrate the armor. Long rod penetrators are typically utilized in armor-piercing fin-stabilized discarding sabot (APFSDS) ammunition. 
     To defeat modern anti-tank projectiles, explosive reactive armor (ERA), also known as reactive armor (RA) and reactive explosive armor (REA), has been developed. Various ERA forms are disclosed in U.S. Pat. Nos. 4,867,077, 5,577,432, 5,413,027, 5,370,034, and 4,981,067, the disclosures of which are incorporated herein by reference in their entireties. Most ERA is modular, with individual modules formed as “boxes” which are typically rectangular prisms but may be otherwise formed. Each ERA box typically includes: an outer layer or plate (“face plate”) of steel, facing generally outward from the vehicle; a layer of explosive inboard thereof; and an additional layer or plate (“rear plate”) of steel inboard of the explosive. The ERA boxes are arrayed over the surface of the vehicle to be protected and may be directly in contact with the basal armor of the vehicle or may be held slightly spaced-apart from the basal armor. 
     When a rod penetrator impacts ERA, contact between the penetrator and the outer plate produces a shockwave which detonates the explosive layer. The explosion drives the outer plate further outward. Where the outer surface of the outer plate is not normal to the impact trajectory of the projectile, contact between the outer plate and the projectile produces a deflecting force on the projectile, deflecting both its orientation (defined by its longitudinal axis) and its subsequent trajectory (defined by the path of its center of mass) away from normal to the basal armor. Initially, the impact may bend the penetrator proximate its fore end. The penetrator will typically penetrate the outer plate producing a hole therein. Such penetration does not end the interaction between the outer plate and the projectile. A side of the penetrator will remain in contact with a side of the hole in the outer plate as the penetrator continues inward toward the vehicle and the plate continues outward. The result is a continued deflective force on the penetrator normal to its impact trajectory. 
     If applicable, a rear plate of the ERA may be driven backward (toward the basal armor) by the explosion. This further enhances deflection since the movement of the rear plate will have a component normal to the impact trajectory. Thus, upon penetration of the face plate and engagement with the rear plate, this relative movement will continuously expose new material on the rear plate to the already deflected penetrator fore end. This further deflects the projectile and provides a potentially greater dissipation of projectile kinetic energy than if the rear plate were simply affixed flat against the basal armor. When the penetrator finally reaches the basal armor, its trajectory has been deflected further off normal to the basal armor and its tip bent yet further off normal so that the projectile is more likely to deflect off the basal armor or attack such a large area of the basal armor that the penetrator will not cause penetration of the basal armor. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, in one aspect the invention is directed to an ammunition system featuring a subcaliber kinetic energy penetrator having first and second portions. The first portion preferably represents between about 9% and about 15% of the penetrator mass while the second portion is heavier, is positioned aft of the first portion, and is frangibly coupled thereto. The first portion is preferably tungsten-based while the second portion is preferably uranium-based. The former is chosen to produce a relatively wide hole in the face plate of explosive reactive armor while the latter is chosen to best perforate basal armor. The connection between the two portions is configured to rupture under pre-determined conditions, namely at a threshold torque between the first and second sections. Such rupture reduces the tendency of interaction forces between the armor and the first portion from deflecting the second portion into an ineffective, highly oblique relation to the basal armor. 
     An exemplary second section length is between 400% and 700% of the first section length. 
     The penetrators of the invention may be utilized with a variety of known sabot structures including push and pull type sabots. In a push-type sabot, propellant gases are substantially trapped behind a sealing flange or other protuberance located relatively aft along the projectile and typically aft of an additional flange. In a pull-type sabot, the sealing flange is relatively forward along the projectile and may be ahead of an additional flange or feature which helps maintain the projectile centered within the tube. Exemplary push-type sabots are disclosed in U.S. Pat. Nos. 5,155,295 and 5,359,938 while an exemplary pull-type sabot is disclosed in U.S. Pat. No. 5,063,855. The disclosures of these patents are incorporated herein by reference in their entireties. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a projectile according to principles of the invention chambered in a weapon. 
     FIG. 2 is a diagrammatic view of a flight path of the projectile of FIG.  1 . 
     FIG. 3 is a cross-sectional view of a penetrator of the projectile of FIG.  1 . 
     FIGS. 4-8 are semi-schematic views showing stages of interaction between the penetrator of FIG.  3  and vehicle armor. 
     FIG. 9 is a cross-sectional view of a first alternate penetrator. 
     FIG. 10 is a cross-sectional view of a second alternate penetrator. 
     FIG. 11 is a cross-sectional view of a third alternate penetrator. 
     FIG. 12 is a cross-sectional view of a fourth alternate penetrator. 
     FIG. 13 is a cross-sectional view of a fifth alternate penetrator. 
     FIGS. 14 and 15 are semi-schematic views showing stages of interaction between the penetrator of FIG.  11  and vehicle armor. 
     FIG. 16 is a cross-sectional view of a sixth alternate penetrator. 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     FIG. 1 shows a weapon  10  having a tube  12  extending from a chamber  14  at the aft end of the tube to a muzzle  16  formed by a fore end of the tube. The tube extends along a central longitudinal axis  200  and may have a smooth bore or inner surface  18  with a diameter D of an exemplary 120 mm which is characteristic for the main gun of a modern NATO main battle tank (e.g., the U.S. M1-A2). 
     An ammunition round  20  is provided having a case  22  accommodated within the chamber  14 . The case extends from a base to a mouth and is substantially filled with a propellant  24 . A saboted projectile  26  is accommodated within the mouth of the case  22 , an aft portion extending into the case  22  and a fore portion extending into the tube  12 . The projectile, shown as a long rod penetrator  28 , includes a body  30 . The projectile and penetrator have a common central longitudinal axis  201  which, while the projectile is in the weapon, is coincident with the axis  200 . 
     The body  30  extends from a fore end  32  to an aft end  34  and bears a plurality of (for example, six) fins  36  extending generally radially outward proximate the aft end  34 . Centrally along the body  30 , the penetrator bears interlocking features  38  engageable with mating interlocking features  40  of the sabot  42  (shown as an exemplary push-type sabot). The features  38  and  40  may be formed as screw-like threads or as annular thread-like grooves/protrusions engaged with each other so as to be effective to prevent relative longitudinal movement of the penetrator and sabot body. 
     The sabot  42  has a body formed in an exemplary three segments or petals. The three petals are identical to each other which facilitates a balanced sabot and smooth discard of the sabot. The petals are separated from each other along three planar interfaces at 120 degree angles about the penetrator axis  201 . The assembled sabot body fully encircles a major portion of the penetrator body. The sabot body includes fore and aft protuberances  50  and  52  dimensioned to cooperate with the bore  18  so as to maintain the projectile substantially centered along the tube axis  201 . In the exemplary embodiment, the petals, and thus the sabot body, are primarily formed of a composite material. Suitable composite materials include carbon and/or aramid fiber in an epoxy or other resinous matrix. 
     The fore protuberance  50  is formed as an annular scoop. The aft protuberance  52  is longitudinally broader than the fore protuberance or scoop  50 , forming a bulkhead which largely retains propellant gases behind it and provides the principal positioning of the saboted projectile along the tube axis  200 . From the scoop  50  to the bulkhead  52 , the sabot body increases in diameter which then decreases or tapers from the bulkhead to the aft end of the sabot. Ignition of the propellant increases the pressure within the case  22 , propelling the saboted projectile forward along the axis  200  through the tube. An exemplary muzzle velocity is from about 1,375 to about 1,650 meters per second (m/s). Upon exit from the muzzle, the sabot petals peel off and discard from the penetrator  28 . The penetrator  28  then proceeds along its flight path  202  to the target vehicle  60  (FIG.  2 ). Exemplary impact velocities are from about 1,000 m/s to nearly the muzzle velocity. 
     FIG. 3 shows details of the long rod penetrator  28  and the ERA box  62 . The ERA box is mounted atop the vehicle basal armor  64 . Shown in simplified form, the ERA box includes a sandwich of an outboard steel face plate  66 , an explosive layer  68  and an inboard rear plate  70 . The inboard surface of the rear plate  70  is held spaced apart from the outboard surface of the basal armor by a separation S. In the illustrated situation, the exemplary flight path  202  is somewhat oblique to the basal armor in the vicinity of impact. If such a flight path is anticipated by the designers of the ERA box, the orientation of the reactive armor sandwich parallel to the basal armor is common. Where the anticipated flight path is more normal to the basal armor, the sandwich may, however, be set at an angle to the basal armor to better deflect the incoming penetrator. Thus, in the illustrated example, the basal armor  64  and base plate  66  share a common outward directed normal  203 . At the point of impact, an angle θ separates the flight path  202  from the normal  203  and its complement α separates the flight path  202  from the plane of the face plate  66 . The flight path  202  and normal  203  define opposite upslope and downslope directions  210  and  212  along the face plate  66 . The upslope and downslope directions are coplanar with the flight path  202  and normal  203 . The upslope direction has a component parallel to the at-impact trajectory and the downslope direction has a component antiparallel to such trajectory. 
     The body  30  of the penetrator  28  has a length L from the fore end  32  to the aft end  34 . Extending aft from the fore end  32  there may be an aerodynamic ballistic tip or nose  72 . The nose  72  may have a conical, ogival, or other drag-reducing surface profile. The nose  72  may be formed of composite material, aluminum, or other lightweight material which has a relatively small effect on projectile penetration. Aft of the nose  72  is a first penetrator section  74  formed of an ultradense material such as uranium (depleted), tungsten, tungsten or uranium-based alloys, and tungsten or uranium-based composites. Aft of the first penetrator section  74  is a second penetrator section  76 , also formed of an ultradense material. In the exemplary embodiment, the first and second penetrator sections are frangibly coupled to each other via a combination of interfitting features and a threaded stud  78 . The interfitting features include a reduced diameter portion  80  of the second penetrator  76  adjacent the fore end thereof and a matching cylindrical bore or pocket  82  formed in the aft end of the first penetrator section  74  and receiving the reduced diameter portion  80 . The threaded stud  78  extends centrally and longitudinally from within the reduced diameter portion  80  through the base of the cylindrical bore  82  and into the first penetrator section  74 . In the exemplary embodiment, the first penetrator section  74  has a characteristic length L 1  and the second penetrator section  76  has a characteristic length L 2 , such lengths being measured relative to an average separation plane  204  bisecting the interfitting features securing the two sections together. The first and second penetrator sections  74  and  76  further have respective characteristic diameters D 1  and D 2 . In the exemplary embodiment, the first and second penetrator sections are largely cylindrical so that D 1  and D 2  are the median and modal diameters of the respective sections and are substantially equal to the average diameters of the respective sections (ignoring the relatively small local variations due to the interlocking features  38 ). 
     In the exemplary embodiment, the initial impact of the penetrator with the face plate  66  will substantially crush a relatively non-robust nose  72 . For simplicity of illustration, the nose  72  has therefore been removed from the following figures. FIGS. 4-8 show a simplified sequence of stages of the interaction between the penetrator and the vehicle armor. Contact/engagement between the fore end of the first penetrator section  74  and the face plate  66  produces an inwardly directed shockwave within the reactive armor sandwich, initiating explosion of the explosive layer  68 . Such explosion drives the exemplary face plate  66  outward ( 206 ) and drives the rear plate  70  inward ( 208 ) toward the basal armor  64 . The outward direction is coincident with the local outward-directed normal  203  to the basal armor. The angle θ separates the at-impact flight path or trajectory of the projectile from the outward direction  206 . Interaction between the face plate  66  and the first penetrator section  74  produces a force on the first penetrator section having a component F P  antiparallel to the impact trajectory and a component F N  normal to the impact trajectory. The presence of the component F N  tends to deflect the trajectory of the penetrator away from the at-impact trajectory and to an even more oblique relation with the basal armor. Furthermore, the force F N  produces a torque on the penetrator which will tend to rotate the penetrator axis  201  out of alignment with its trajectory or flight path  202  and to yet a more oblique relation to the basal armor. With a conventional rod penetrator, both of these effects will decrease the likelihood of achieving perforation of the basal armor. With the two-section penetrator however, the interaction between the first penetrator section  74  and the face plate  66  can initiate rupture of the frangible coupling between the first and second sections, isolating deflection to the first section. This will reduce the amount by which the second penetrator section  76  is deflected as compared with a conventional monolithic penetrator. Furthermore, deformation is advantageously confined to the first penetrator section  74  and, if such section is made relatively highly deformable, such as if made from a tungsten alloy, a further reduction in the deflection of the second penetrator section may occur as tungsten produces a relatively large hole. 
     The combination of the kinetic energy of the long rod penetrator and the energy imparted to the face plate by the explosion of the explosive layer produces a hole  90  (FIG. 5) in the face plate having a perimeter  92 . The hole  90  will typically be larger than the cross-sectional area of the penetrator body, even when such hole is projected along the longitudinal axis of the penetrator. Advantageously, there is subsequently an interval during which the penetrator is not in contact with the face plate  66 . During this interval (the “clearance interval”), the face plate proceeds outward while the penetrator proceeds along its flight path toward the vehicle basal armor. At some point during this interval, the first penetrator section  74  or, more particularly, the portion thereof remaining after ablation caused by perforation of the face plate, will come into contact with the rear plate  70  as the latter moves inward. 
     During an interval wherein the long rod penetrator is engaged to the inward moving rear plate (“rear plate engagement interval”), as there is a component of the rear plate&#39;s velocity which is parallel (rather than antiparallel) to the trajectory of the penetrator, there will be a tendency for the first penetrator section  74  (or portions thereof) to slide along the rear plate thus exposing a large amount of the rear plate&#39;s outboard surface to the penetrator. During this rear plate engagement interval, the component of the engagement force between the rear plate and the penetrator normal to the penetrator trajectory is not passed between the first penetrator section  74  and second penetrator section  76  due to the rupturing of the frangible coupling between the two sections. Thus, during this rear plate engagement interval, the force will continue to deflect the first section  74  relative to the second section  76  (FIG.  6 ). The rear plate engagement interval may last beyond the end of the clearance interval. Thus, during this rear plate engagement interval, the second penetrator section  76  is further spared deflection forces which would otherwise deflect a monolithic penetrator. 
     The clearance interval ends when the penetrator comes into contact with the perimeter  92  of the hole  90 . This occurs at a location X C  on the penetrator which, advantageously, falls along the second penetrator section  76  at a distance L 3  aft of the separation plane  204  (FIG.  7 ). After this point, the side of the penetrator (or more particularly, the side of the second penetrator section  76 ) will be in sliding contact with the perimeter  92  at the downslope side thereof. The sliding engagement continues to slow the penetrator, wearing away the face plate and enlarging the hole and also wearing material off the side of the penetrator. The engagement further continues to deflect the penetrator trajectory into a more oblique relationship to the basal armor. This “hole engagement interval” extends until either the projectile has passed through the face plate or the hole  90  has been enlarged so that the hole reaches the boundary of the lateral periphery of the face plate (FIG.  8 ). If/when the location of engagement between the plate and second penetrator section is ahead of the center of gravity C G2  of the second section, the engagement will further deflect the axis of the second section into a more oblique relationship with the basal armor. However, once the point of engagement is aft of the center of gravity, the engagement will deflect the axis (but not the trajectory) back toward normal to the basal armor. 
     Eventually, the second section  76  impacts the surviving portion of the rear plate  70  and the basal armor  64  (FIG.  7 ). As the deformation and severance of the first section  74  has spared the second section  76  from much of the deflection which would otherwise be produced by the ERA, the second section advantageously has sufficient remaining kinetic energy and remains at a trajectory effective to perforate the basal armor  64 . Especially on a tank, the basal armor is still quite formidable. Thus the second section  76  should represent the majority of the total projectile mass and should retain a majority of the total projectile kinetic energy. With a monolithic projectile, it is estimated that the major bending and fracture produced by the action of the ERA extends along a region of about the forwardmost 10% to 14% of the penetrator (absent the ballistic tip or nose). Thus, selection of the length L 1  to represent about 14% of the combined length should leave the second section  76  sufficiently undamaged. With a substantially constant penetrator diameter and with the first and second sections formed of similarly dense materials, the first section would represent a corresponding 14% of the combined penetrator mass while the second section represents substantially the remaining 86% (the nose and fins representing a very small portion). Such an amount should be effective to defeat the basal armor. More broadly, the first section may represent about 9% to about 15% of the combined penetrator mass with the second section representing substantially the remainder. A broadly preferred range for combined penetrator mass is from about 4.0 to about 4.6 kg. For a 120 mm NATO APFSDS round, an exemplary combined penetrator mass is about 4.5 kg. 
     To the extent that the size of the initial hole in the face plate is maximized, the deflection of the trajectory of the second piece  76  is minimized. This results from the extension of the duration of clearance interval provided by the larger hole. There may be a number of ways to achieve this. One way is to select the adiabatic shear properties of the respective first and second sections  74  and  76  to respectively provide a maximum size hole in the face plate and a maximum penetration depth in the basal armor. By way of example, tungsten and depleted uranium are two materials of nearly the same density (19.35 g/cm 3  and 19.05 g/cm 3 , respectively) Typical tungsten and uranium-based materials used in penetrators have densities lower than the pure materials, typically, however, in excess of 17.0 g/cm 3 . The adiabatic shear properties of tungsten relative to uranium are such that with identical mass and cross-sectional area projectiles, a tungsten projectile will produce a larger diameter hole in a given armor plate through which both projectiles are capable of passing, while a uranium projectile will penetrate plates at thicknesses which the tungsten projectile can not penetrate. When an exemplary tungsten penetrator impacts with rolled homogenous armor (RHA) its leading end experiences a mushrooming deformation producing a relatively large yet shallow hole. In distinction thereto a depleted uranium penetrator interacts with the armor more like a chisel, without the same mushrooming deformation, producing a deeper yet narrower hole. Accordingly, tungsten-based materials may be preferred for the first penetrator section  74  while uranium-based materials are preferred for the second penetrator section  76 . 
     Another way to extend the clearance interval is to provide the second section  76  with a smaller characteristic diameter than the first section  74 . FIG. 9 shows one such penetrator  28 ′ having first and second sections  74 ′ and  76 ′ respectively. The first and second sections may each have varying diameters along their respective lengths. For example, as shown in FIG. 10, a second alternate penetrator  28 ″ includes first and second sections  74 ″ and  76 ″. The presence of a relatively large diameter portion of the second section  76 ″ proximate its fore end and tapering toward its aft end is advantageous to extend the clearance interval. This is because such a profile allows the side of the second section to be in very close proximity but not in contact with the hole perimeter  92  as the second section passes through the hole  90 . However, if the second section  76 ″ is relatively wide at its fore end, this will tend to disperse contact forces between the second section and the basal armor and thereby reduce penetration. In this regard, the presence of the reduced diameter portion  80  may bypass this effect by concentrating the impact forces over a correspondingly reduced area. 
     Another alternate penetrator  100  is shown in FIG.  11 . Such a penetrator may be formed as a “drop-in” replacement for the penetrator  28  of the saboted projectile  26 . The fins and ballistic tip may be similar to those of the penetrator  28 . The penetrator  100  may have first, second, and third penetrator sections  101 ,  102  and  103 . Each of the three sections  101 - 103  is formed of an ultradense material. The first section  101 , at its fore end, carries the ballistic tip or nose. At its aft end, the first section is frangibly coupled to the fore end of the second section  102  by interfitting features which may be similar to those coupling the first and second sections  74  and  76  of the penetrator  28 . At its aft end, the second section  102  is frangibly coupled to the fore end of the third section  103  by similar interfitting features. Dimensionally, the first section  101  may be similar to or the same as the first section  74  of the penetrator  28 . In such a case, the second and third sections  102  and  103 , combined, may have similar dimensions to the second section  76  of the penetrator  28 . By way of example, the combined length L 101  and L 102  of the first and second sections  101  and  102  may be approximately 25-30% of the overall length L 101 +L 102 +L 103  of the three assembled sections  101 - 103 . The length L 101  may still be the exemplary approximately 14% of such total length. While the penetrator  100  represents a three-piece modification of the basic two-piece configuration of the penetrator  28 , FIGS. 12 and 13 show alternate penetrators  100 ′ and  100 ″ which represent similar modifications of the penetrators  28 ′ and  28 ″. In such a three-sectioned penetrator  100 , both the second and third sections  102  and  103  are preferably performed of depleted uranium or such other material as may be chosen to produce a relatively deep yet narrow perforation. In such a configuration, the first section preferably still achieves its function of producing a relatively wide hole in the face plate. Deformation of the second section and its separation from the third section preferably further allow the third section to maintain a more normal relation to the basal armor. FIGS. 14 and 15 illustrate two sequential stages in this process. 
     FIG. 16 shows an alternate penetrator  120  which may be otherwise similar to the penetrator  28  of FIG. 3 except as to the form of the frangible coupling between first and second penetrator sections  122  and  124 . A tubular internally-threaded collar or sleeve  126  secures abutting externally-threaded reduced-diameter rear and fore portions  128  and  130  of the first and second penetrator sections, respectively. Other coupling configurations may alternatively be used. An important factor is that the coupling have sufficient strength to maintain engagement between the sections during expulsion from the weapon and flight to the target but not be so robust that it transfers sufficient force to induce bending of the aft portion(s) upon target impact of the fore portion(s). Various aspects of engagement forces and torques may be borne in disproportionate amounts by different portions of the coupling. By way of example, in the threaded stud embodiment of FIG. 3, the threaded stud provides substantially all the tensile strength to transmit aerodynamic drag forces from the fins on the second section to the first section to prevent longitudinal disengagement. The aft end of the first penetrator section  74  at the pocket  82  has sufficient strength to resist torsional forces such as associated with projectile yaw and pitch upon launch. However, it is weak (frangible) enough to quickly break upon impact without transmitting substantial force to the second section. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the multi-piece penetrator may be formed as a drop-in replacement for any of a number of conventional substantially monolithic penetrators used in a variety of weapons. Accordingly, other embodiments are within the scope of the following claims.