Patent Publication Number: US-7587978-B1

Title: Reactive material initiator for explosive-filled munitions

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described was made in the performance of official duties by one or more employees of the Department of the Navy, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 

   BACKGROUND 
   The invention relates generally to initiation devices for explosives. In particular, the invention relates to an initiator using reactive materials to reduce shock and thermal sensitivity for improved safety. 
   The United States Department of Defense (DoD) has sought to transition to an insensitive munitions (IM) inventory of weapons since 1999. In particular, IM compliant warheads and rocket motor are intended to diminish sensitivity to shock and/or to reduce reaction intensity in response to thermal cook-off. This enables assigning such munitions to a lower hazard classification, thereby mitigating costs for storage, transportation and handling logistics. 
   Conventional initiators for DoD weapons contain primary explosives. These chemicals include lead azide (PbN 6 ), lead styphnate (C 6 H 3 N 3 O 8 Pb) and diazodnitrophenol (DDNP, C 6 H 2 N 4 O 5 ). Such highly reactive chemicals are typically very sensitive to un-commanded shock and thermal initiation, making them potential safety hazards and are not compliant with the current Insensitive Munitions strategy. The United States Department of Transportation (DoT) lists such substances as Class 1.1A Explosives under 49 CFR §172.101 in the Hazardous Materials Table. 
   SUMMARY 
   Conventional initiators yield disadvantages addressed by various exemplary embodiments of the present invention. In particular, various exemplary embodiments provide an initiator that incorporates reactive materials, rather than explosive materials. Other various embodiments alternatively or additionally provide for concatenated initiators of such configuration. 
   Various exemplary embodiments provide an initiator for a munitions device intended to strike a target substantially along a trajectory direction. The device contains an explosive within a case. The initiator includes a sleeve, first and second anvils and an initiation pellet disposed between the anvils. The sleeve is disposed within the case and adjacent to the explosive, and defines an initiator interior aligned substantially parallel to the trajectory direction. The sleeve includes one or more orifices that extend therethrough between the explosive and the interior, the orifices being disposed adjacent to the pellet. The pellet is composed of a reactive material that chemically responds as a non-explosive to kinetic and thermal stimuli. 
   The first anvil is disposed transverse to the interior and forward of the pellet, while the second anvil is disposed within the interior behind of the pellet, wherein the second anvil translates towards the first anvil substantially along the trajectory direction in response to the device striking the target. The pellet discharges particles through the orifices, the particles being directed into the explosive in response to the second anvil translating towards the first anvil. The sleeve may be disposed within the explosive. In various exemplary embodiments, the reactive material may be a solid or powdered mixture of powdered aluminum and polytetrafluoroethylene (or other fluoropolymers), or alternatively a thermite mixture of powdered metal and metal oxide (and may include polytetrafluroethylene or other fluoropolymers). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and various other features and aspects of various exemplary embodiments will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: 
       FIG. 1  is an elevation view of an explosive projectile with an initiator in accordance with a first embodiment prior to impact; 
       FIG. 2  is an elevation view of an explosive projectile with an initiator in accordance with a first embodiment upon impact; 
       FIG. 3  is an elevation detail view of an initiator in accordance with a second embodiment prior to impact; 
       FIG. 4  is an elevation view of an explosive projectile with an initiator in accordance with a third embodiment prior to impact; and 
       FIG. 5  is a graphical view of a plot comparing time to first light versus velocity for the initiator material. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
   The embodiments described herein employ a reactive material rather than an explosive material. A reactive material represents a flammable substance that exothermally reacts by burning in response to a kinetic or thermal stimulus that exceeds the high threshold to initiate chemical response. By contrast, an explosive material combusts rapidly (i.e., by explosion or deflagration) in response to comparatively low threshold of received energy. 
   Exemplary embodiments provide an initiator that includes a disk of solid reactive material with a metal disk on either longitudinal side attached to each face of the reactive material as shown in  FIG. 1  as an axi-symmetric elevation cross-section. Artisans of ordinary skill will recognize that the cylindrical configuration as depicted is exemplary only and not limiting. The metal disks represent a hammer and an anvil respectively between which the reactive disk may be disposed. 
   This reactive material may incorporate, for example, a solid mixture of powdered aluminum and polytetrafluoroethylene (PTFE, also known as Teflon®). The reactive material may instead or additionally incorporate other fluoropolymers besides PTFE. Such materials are classified as flammable solids, rather than explosives, and thus involve less stringent handling requirements. Alternatively, other reactive materials may be used, such as a metal and metal oxide thermite mixture that may include added polytetrafluroethylene and/or other fluoropolymers. 
   For low velocity impact of the projectile against a target, a powdered mixture of aluminum and PTFE may be used for the reactive material in the initiator, because a solid mixture of aluminum and PTFE requires more kinetic energy to initiate. Other exemplary embodiments may include a series of reactive disks separated by metal disks between them. The series may be located along the centerline of the weapon or along an offset to the centerline. Alternatively, the initiator may consist of a series of reactive disks and metal anvils arranged parallel to the centerline. 
   In particular,  FIG. 1  shows a target  100  being approached by a warhead projectile or weapon  110 , such as a bomb, bullet, or missile launched thereagainst. To enhance penetration, the projectile  110  includes a hardened nose  120 , composed from a structurally rigid material, such as steel. Behind (i.e., aft of) the nose  120 , the projectile  110  includes a case  130  containing an initiator  140  and an explosive fill region  150  disposed annularly around the projectile&#39;s longitudinal axis or centerline  160 . The initiator  140  and explosive fill  150  may be separated by an axi-symmetric sleeve or housing  170  that includes angularly disposed vent holes or orifices  180 . The centerline  160  may be substantially parallel to and preferably co-linear with a trajectory direction of the projectile  110  towards the target  100 . The orientation represented for the projectile  110  identifies the lower end (adjacent the target  100 ) as forward and the upper (or distal) end as aft. 
   The explosive fill region  150  may be disposed in an annular torus or annulus disposed around the sleeve  170 . The initiator  140  includes a metal disk or anvil  190  and a reactive material disk or initiation pellet  200  along the centerline  160  disposed within the interior of the sleeve  170 . The metal disk  190  may be composed of a substantially rigid metal, such as steel. The reactive disk  200  may be confined longitudinally between the nose  120  (representing an upstream anvil) and the metal disk  190  (representing a downstream hammer), and also restrained radially by the sleeve  170 . 
   The sleeve  170  may include a void behind of the metal disk  190 . Alternatively, such a void in the sleeve  170  may be forward of the metal disk  190 , provided that the latter&#39;s translation through the former remains unimpeded. The metal disk  190  may be restrained to avoid translation along the centerline  160  during launch or gun-ejection. 
   Those of ordinary skill will recognize that although an axi-symmetric cylindrical disk is depicted, alternative solid shapes may be used to be represented by the term “disk” as provided in the instant disclosure. Alternatively, the nose  120  may include an additional metal disk (not shown) between the nose  120  and the reactive disk  200 , thereby enabling the nose  120  to have a less rigid composition. The initiator  140  may be activated to ignite the explosive in the fill region  150  when the projectile  110  violently contacts (i.e., striking) the target  100 . 
   Thicknesses of the metal and reactive disks  190 ,  200  may be tailored to determine the optimum time for initiation, namely to tune the initiation time for the selected explosive material for the fill region  150  and its geometry. Typically, the reactive disk  200  may possess a shorter axial length (i.e., in the longitudinal direction) than the metal disk  190 , and optionally may also have a slightly smaller radius. Alternatively or in addition, the particle size of the aluminum powder may additionally or alternatively be used to tune the initiation sensitivity, as laboratory experiments demonstrate significant dependency of the initiation time of the reactive material on the aluminum particle size. 
   The reactive material initiator  140  may be disposed in the sleeve  170  radially inward of the explosive fill region  150  (annularly extending from the sleeve) of the projectile  110 . As shown in  FIG. 2 , upon impact with the target  100  (having traveled in the trajectory direction), the momentum and mechanical energy of the projectile  110  in flight may be converted to compress the reactive disk  200  to deform as flattened disk  210 . Alternatively, multiple disks, as shown in tandem between corresponding anvils in  FIG. 4  and discussed subsequently, may be employed. 
   As the compressed disk  210  squeezes, reactive material discharges through the vent holes  180  into the explosive fill  150  as hot glowing reactive particles  220 . The kinetic to compressive energy conversion may be attributed to the momentum difference of the nose  120  being interrupted in translation by the target  100  and the metal disk  190  whose inertial motion is impeded substantially less by the reactive disk  200 , which has only a small fraction of the target&#39;s mass. The relative motion difference between the nose  120  and the metal disk  190  is represented by the directional arrow  230 . The metal disk  190  travels towards the nose  120  along a translation direction corresponding to the trajectory and substantially parallel to the centerline  160 . 
   The purpose of the metal anvils  120 ,  190  is to compress the reactive disk  200  therebetween. The resulting energy transfer to the reactive disk  200  and the consequential radial extrusion of reactive material, forced through the vent holes  180 , applies frictional, shear and/or compression heating to the reactive particles  220 . These heated reactive particles  220  transfer thermal energy to the explosive material in the fill region  150  beyond its reaction threshold, thereby initiating the surrounding explosive material therein to explode in the target&#39;s proximity. 
   Alternatively, the metal disk  190  as a steel anvil may be secured at the aft end of the sleeve  170  by a set of discrete shear pins disposed to project radially inward in a plane perpendicular to the centerline  160  distributed with angular separation of e.g., 120°=⅔π or 90°=½π. In lieu of the pins, a frangible obstacle, such as a ceramic or metal lip ring could be employed on the interior cylindrical wall surface of the sleeve  170 . As an example, the pins or ring can be composed of a shear-failure-prone metal, e.g., Ti-6 Al-4 V alloy. See “Reverse-Ballistic Impact Study of Shear Plug Formation and Displacement in Ti6Al4V Alloy”, W. H. Holt et al.,  Journal of Applied Physics , v. 73 no. 8, pp. 3753-59, Apr. 15, 1993. The pins or ring (not shown) inhibit longitudinal translation of the metal disk  190  within the sleeve  170  during launch or gun-barrel ejection. Upon impact with the target  100 , inertial forces of the metal disk  190  shear the pins or shatter the ring, for a suitably determined acceleration load for impact. This releases the metal disk  190  to travel longitudinally through the sleeve  170  at high relative speed towards the reactive disk  200  for initiation of the explosive  150 . Such a configuration may enhance handling safety by avoiding contact between the metal disk  190  and the reactive material disk  200  prior to impact with the target  100 . 
     FIG. 3  shows a detail of the initiator unit  140  as a second embodiment. An anchor, represented by an inner cylindrical tube  230 , or alternatively a set of retainer pins, is disposed behind the anvil components inhibit aft translation along the sleeve  170 . Such devices avoid shifting the projectile&#39;s center of gravity during launch or gun-ejection due to inertial forces. An anvil  240  with a hard metal dowel  250  may represent (or substitute for) the metal disk  190 . The anvil  240  may include a recess in which the dowel  250  may be disposed (as shown), or be integrally connected thereto or else be separately disposed in tandem within the sleeve  170 . The dowel  250  may preferably be composed of hard steel, whereas the anvil  240  may be a softer metal. 
   A shear plate  260  may be disposed forward of the dowel  250 , and an annular shim (or spacer)  270  may be disposed between the shear plate  260  and the reactive material disk  200 . The shear plate  260  may preferably be composed of titanium, due to its behavior to produce a shear slug (heated by adiabatic shearing) upon being struck by the dowel  250  and driven between the shim  270  towards the reactive material disk  200 . The hot shear slug forms a layer of molten metal on its periphery. In this configuration, the reactive material disk  200  is protected from accidental initiation during handling, due to the high acceleration conditions required to produce the molten metal layer on the shear slug and hence conditions suitable for the reactive material to be ignited. Alternatively, an appropriately thick (i.e., rigid) shear plate  260  may be placed in contact with the reactive material disk  200 . The hot shear slug can thereby be driven into the reactive material. 
   As mentioned previously, an alternative third embodiment for the initiator may include a stack of initiator units  140  disposed in tandem series along a longitudinal array for multipoint initiation, as illustrated in  FIG. 4 . In the multipoint initiation scheme, the sleeve  170  may include several layers of reactive material disks  200  may be sandwiched between metal disks  190 . Groups of vent holes  180  disposed longitudinally along the sleeve  170  may be disposed radially adjacent to the reactive disks  200 . All the initiators may activate when the target  100  is struck to ignite the explosive in the fill region  150  at multiple initiation sites corresponding to the vent holes  180  for more homogeneous initiation than with a single initiator  140 . 
   Alternatively, a plurality of sleeves  170  may be disposed within the fill region  150 . Each of these sleeves  170  may include one or more initiator units  140  (disposed in tandem series). A single sleeve  170  may preferably be aligned coincident with the centerline  160 , but may alternatively be offset therefrom. A plurality of sleeves  170  may be disposed in a preferably symmetrical pattern parallel to and around the centerline  160 . Another alternative includes rectangular, cruciform, oval or other cross-sectional geometries for the sleeve  170 . Yet other alternative geometries may have dimensional or cross-sectional non-uniformity along the length of projectile  110  for the sleeve  170  in which to contain one or more initiator units  140 . 
   Advantages from incorporating a warhead initiator using reactive rather than explosive materials include: (1) classification as a flammable solid, thereby being IM compliant, and (2) reduction in logistics costs rendering safer to handle, store and transport than conventional initiators. 
     FIG. 5  shows the results from experiments performed at Naval Surface Warfare Center-Dahlgren Division to compare confined and unconfined reactive material samples as a plot  300 . A solid mixture of powdered aluminum and PTFE was pressed into sample disks. Sample disks (confined by steel anvils on either end and struck by a launched steel disk) reacted (to produce light) at a much lower impact velocity than unconfined sample disks of the same reactive material similarly struck. 
   The plot  300  graphically compares launch velocity (in ft/sec) of the steel disk as the abscissa  310  and the time (in μsec) to initiation (as determined by first light observed) as the ordinate  320 . The initiation velocity of 531 ft/sec represents a threshold line  330  determined by Los Alamos National Laboratory above which direct impact fails to initiate. A first curve  340 , based on solid squares, shows direct impact results asymptotically approaching the threshold. A second curve  350 , based on solid diamonds, crosses the threshold  330  reaches into the initiation region on the left. 
   The initiator  140  may be integrated into the projectile  110  as assembled to exploit advantages derived from the reduced hazard from un-commanded initiation as compared to a conventional initiator equipped with explosives. Alternatively, the initiator  140  may be stored separately for field assembly with the projectile  110  into the shell  170 , as desired. 
   While certain features of the embodiments of the invention have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments.