Abstract:
An inertial delay mechanism for use in an explosive projectile is provided. The delay mechanism consists of an inertial delay fuse that is precise, doesn&#39;t require sensitive primary explosives and doesn&#39;t utilize electronic circuitry. The inertial delay fuse includes a free sliding charge element that strikes an anvil located opposite to the sliding charge element. A delay gap is provided between the sliding charge element and the anvil. Upon impact, the sliding charge element slides forward and impacts the anvil, thereby inducing a shock wave in an initiator charge that subsequently results in detonation of main charges. The design is mechanically simple and robust enough to withstand severe g-loading forces that occur during firing and penetration of a projectile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Application is a divisional of U.S. application Ser. No. 12/023,320 filed Jan. 31, 2008, the contents of which are incorporated herein by reference. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The invention was made with United States Government Support under Contract No. DTRA-99-C-0080 awarded by Defense Threat Reduction Agency and W15Qkn-04-C-1110 awarded by Army Research and Development Command. The United States Government has certain rights in the invention. 
     
    
     BACKGROUND 
       [0003]    The invention is directed to providing a delay mechanism for an explosive projectile. In particular, the invention is directed to providing an inertial delay fuse for use in explosive projectiles. 
         [0004]    In many explosive projectile applications, such as projectile based drilling or excavation, the detonation of an explosive payload carried by the projectile preferably occurs after the projectile strikes and penetrates the target. The delay in detonating the explosive payload allows the projectile to penetrate into the target a prescribed distance before detonation, thereby allowing a greater amount of material to be excavated as opposed to having the projectile detonate upon impact. Due to the velocity of the fired projectile, the delay in detonation must be short (on the order of tens or hundreds of microseconds) to allow for the delivery of the explosive payload at an appropriate depth within the target. 
         [0005]    Conventional chemical delay elements are not precise enough to be utilized for explosive projectile drilling applications. Chemical delay elements generally provide delays on the order of milliseconds with variances on the order of hundreds of microseconds as opposed to tens of microseconds. In addition, very sensitive primary explosives are required when chemical delay elements are used. The use of such sensitive primary explosives for chemical makes the handling and firing of projectiles fitted with chemical delays inherently dangerous. 
         [0006]    Electronic delays can also be utilized in projectiles. Electronic delay elements can be very precise and flexible, however, they also require complex and fragile circuitry that is relatively expensive. In addition, electronic delays require that an energy storage device be incorporated into each projectile. Available energy storage devices are relatively bulky and heavy and are not particularly well suited for use in the relatively small projectiles used for excavation. In addition, energy sources may degrade over time causing problems in the reliability of projectiles that have been stored for long periods 
         [0007]    In view of the above, it would be desirable to provide a delay mechanism that can be readily incorporated into an explosive projectile without requiring very sensitive primary explosives of conventional chemical delay devices or the circuitry of conventional electronic delay devices. Accordingly, such a delay mechanism would be less expensive to manufacture, safer to handle and more reliable. 
       SUMMARY 
       [0008]    The invention provides a delay mechanism for use in an explosive projectile. Specifically, the delay mechanism consists of an inertial delay fuse that is precise, doesn&#39;t require sensitive primary explosives and doesn&#39;t utilize electronic circuitry. The inertial delay fuse includes a free sliding charge element that strikes an anvil located opposite to the sliding charge element. A delay gap is provided between the sliding charge element and the anvil. Upon impact, the sliding charge element slides forward and impacts the anvil, thereby inducing a shock wave in an initiator charge that subsequently results in detonation of main charges. Alternatively, the anvil can be used to set off a stab detonator. The design is mechanically simple and robust enough to withstand severe g-loading forces that occur during firing and penetration of a projectile. 
         [0009]    The sliding charge element preferably includes a cup in which at least one initiator charge pellet is located. In one preferred structure, main charge pellets are also located in the cup such that the main charge pellets form part of the sliding charge element that freely slides forward upon impact of a projectile containing the fuse. In another preferred structure, the cup is retained within a delay tube and the main charge pellets are located around the delay tube such that only initiator charge pellets form part of the freely sliding charge element. 
         [0010]    In the case of use of the delay tube, the delay tube preferably includes openings adjacent to the anvil. Detonation of the main charges is accomplished through the use of a flyer-plate mechanism, in which portions of the cup pass through the openings of the delay tube to strike an explosive lead charge pellet. 
         [0011]    In an alternative embodiment, the cup includes an opening and the anvil includes a projection that fits into the opening provided in the cup. The cup moves forward upon impact causing the projection to pass through the opening and strike a conventional stab detonator such as an M55 Detonator. 
         [0012]    An inner surface of the cup is preferably shaped to focus a shock wave into the initiator charge. For example, a concave portion is formed on the inner surface of the cup that faces the initiator charge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be described with reference to certain preferred embodiments thereof and the accompanying drawings, wherein: 
           [0014]      FIG. 1  is a cross-sectional view of a projectile incorporating an inertial fuse in accordance with a first embodiment of the invention; 
           [0015]      FIG. 2  is a cross-sectional view of a projectile incorporating an inertial use in accordance with a second embodiment of the invention; 
           [0016]      FIG. 3  is a cross-sectional view of a preferred cup structure used in the embodiment of  FIG. 2 ; and 
           [0017]      FIG. 4  is a cross-sectional view of a projectile incorporating an inertial fuse in accordance with a third embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    An explosive projectile  10  incorporating an inertial delay fuse in accordance with a first embodiment of the invention is shown in  FIG. 1 . The projectile  10  includes a penetrating nose cone  12 , a casing  14 , a sabot  16  and a pusher plate  18  that allows for acceleration in a gun bore. A nose charge  20  and a nose charge initiator  22  are provided within the nose cone  12 . A sliding main charge element  24  is provided within the casing  14 . The sliding main charge element  24  includes an initiator charge pellet  26  (PSTN), several main charge pellets  28  (Pax-11) and a tamper  30  that are located within a sliding cup  32  (preferably 7075 aluminum). The sliding main charge element  24  is placed at the rear of the projectile  10  such that a machined tab  34  of the sliding cup  32  is retained by an edge of the casing  14 . The tab  34  holds the sliding cup  32  in a fixed position until the projectile  10  impacts a target. At that point, the tab  34  breaks and allows the sliding cup  32  to slide forward as will be described in greater detail below. An anvil  36  made of a dense material (for example HD 18.5 Tungsten Alloy) is placed at the front of the projectile  10  adjacent to the nose cone  12 , such that, a delay gap is provided between a front face of the sliding cup  32  and a face of the anvil  36 . The anvil  36  is screwed into a coupler  38 , which is also threaded to accept and hold the nose cone  12  to the casing  14 . In the above-described configuration, the projectile  10  essentially consists of two primary masses, namely, the sliding main charge element  24  and the penetrating nose cone  12 , which are accelerated together when fired from the bore of a gun. 
         [0019]    In operation, the nose cone  12  is slowed down by forces transferred to the nose cone  12  when the projectile  10  strikes a target. The sliding main charge element  24 , however, essentially retains its velocity, as the tab  34  of the sliding cup  32  breaks free from the casing  14  due to the large applied forces, thereby allowing the sliding main charge element  24  to slide freely toward the anvil  36  through the delay gap. The sliding main charge element  24  builds forward velocity relative to the decelerating nose block  12  as it passes through the delay gap. After a predetermined period defined, in part, by the length of the delay gap, the sliding cap  32  strikes the anvil  36  and a high pressure shock wave is created that propagates back through the sliding cap  32  and into the initiator charge pellet  26 , where the shock wave runs up to a detonation wave. The detonation wave transfers into the main charge pellets  28  located adjacent to the initiator charge pellet  26  causing full detonation of the sliding main charge element  24 . The tamper  30  (preferably made of Copper) is provided to add mass and increase the time at pressure as the sliding main charge element  24  detonates. The high pressure resulting from the detonation of the sliding main charge element  24  in turn launches a shock wave in the forward direction that propagates back through the anvil  36 , the coupler  38  and into the nose charge initiator  22 . The shock wave runs up to a detonation wave in the initiator charge  22  causing the nose charge  20  to detonate and thereby fracture the nose cone  12 . 
         [0020]    As will be readily appreciated by those skilled in the art, the delay in detonation can be precisely set by changing factors including, but not limited to, the length of the delay gap, the total projectile mass, the mass of the sliding main charge  24 , the shape of the nose cone  12 , and the strike velocity. Accordingly, the delay time between impact and detonation can be precisely controlled on the order of microseconds to compensate for weak or strong targets, desired depth of penetration, etc. using a very simple and robust mechanical structure. Accordingly, the deficiencies of conventional chemical and electrical fuses can be avoided. 
         [0021]    A second embodiment of the invention will now be described with reference to  FIG. 2 . The second embodiment primarily differs from the first embodiment in that only a sliding initiator charge element is used instead of a sliding main charge element. As shown in  FIG. 2 , an explosive projectile  40  is shown that includes a casing  42 , an anvil  44  located in the front of the casing  42 , a delay tube  46  fitted along a central axis of the casing  42 , several main charge pellets  48  (for example PAX-11) that surround the delay tube  46 , a first stage nose pellet  50  and second stage nose pellet  52  (for example PBX-9407), a base plate  56 , a sliding initiator charge element  58 , an end cap  60  that screws into the casing  42 , a sealing O-ring  62 , a sabot  64  and a sabot retainer  66 . 
         [0022]    As shown in  FIG. 3 , the sliding initiator charge element  58  includes a sliding cup  68 , preferably manufactured from AZ31B Magnesium, which retains a first stage initiator charge pellet  70  (PETN), several second stage initiator charge pellets  72  (PETN) and a hammer element  74  (preferably Tungsten). The sliding cup  68 , as in the first embodiment, also includes a tab  76  that is used to hold the sliding initiator charge element  58  in place until the projectile  40  impacts a target. In the illustrated embodiment, the tab  76  is a machined circular lip that extends around the entire circumference of the end of the sliding cup  68 . The tab  76 , however, may be formed of one or more tab elements instead of a single circular lip. An inner surface of the sliding cup  68  also preferably includes a concave portion  76  that focuses a shock wave into the first stage initiator charge pellet  70  as will be described in greater detail below. 
         [0023]    As in the case of the first embodiment, the second embodiment uses the built up velocity difference between the penetrating nose of the casing  42  and the sliding initiator charge element  58 , caused by the impact of the projectile  40  on a target, to both delay and initiate the explosive train. Unlike the first embodiment, however, the main charge pellets  48  are separated from the sliding cup  68  such that the main charge pellets  48  do not move. Instead, only the first and second stage initiator charge pellets  70 ,  72  contained within the sliding cup  68  move down the delay tube  46  and pass through the delay gap. After a predetermined time period determined, in part, by the length of the delay gap between the initial location of the sliding cup  68  and the anvil  44 , the sliding cup  68  strikes the anvil  44  causing a shock wave to travel rearward into the first initiator charge pellet  70 . The shock wave subsequently runs up to a detonation wave and is transferred to the second initiator charge pellet  72 . The detonation wave is preferably transferred to the first and second stage nose charge pellets  50 ,  52  through a flyer-plate initiation mechanism. Specifically, portions of the sliding cup  68  are blown outward in the radial direction into transfer holes  80  provided in the delay tube  46 . The fragmented portions of the sliding cup  68  act as mini flyer-plates that impact the first stage nose charge pellet  50  causing it to run up to detonation. Detonation then propagates through the second stage nose charge pellet  52  and into the main charge pellets  48 . Delay time can be adjusted in the same manner as in the first embodiment. As shown in the illustrated embodiments, the end of the delay tube  46  is preferably expanded in diameter to provide a volume to mitigate the gas pressure buildup. 
         [0024]    In this embodiment, the hammer  74  performs a function similar to the tamper  30  of the first embodiment, by increasing the time at pressure when the sliding initiator charge element  58  detonates. The length of the sliding initiator charge element  58  is preferably adjusted such that the hammer  74  ends up in a location adjacent to the transfer holes  80 , such that the mass of the hammer  74  assists in directing the detonation shock wave to push the fragments of the sliding cup  68  through the transfer holes  80 . It is preferable that the mass of the hammer  74  be greater than the combined mass of the other elements of the sliding initiator charge element  58 . The increased mass of the hammer  74  provides a benefit in that the tab  78  of the sliding cup  68  can be made of a thickness (for example four thousands of an inch) that is easily machined. Without the heavy hammer  74 , the tab  78  would have to be much thinner (for example two thousands of an inch) to insure breakage upon impact of the projectile  40  on a target. 
         [0025]    The provision of the delay gap in “parallel” with the main charge in the second embodiment of  FIG. 2  rather than in “series” as provided in the first embodiment of  FIG. 1 , allows both for a shorter projectile and a longer delay gap while minimizing fuse volume. A shorter projectile translates into a lighter projectile and a shorter cartridge, while a longer delay gap translates into a higher slapping velocity, and consequently a more reliable functioning of the initiator. The need for a nose charge is also eliminated in the embodiment of  FIG. 2 , as the first and second stage nose charge pellets  50 ,  52  also serve to break up the nose of the projectile  40 . Another benefit of the “parallel” delay gap configuration is a lower strike velocity to deliver the main charge to a given depth in a target. In contrast, the “series” delay gap of the first embodiment serves to reduce the deceleration pressure in the main charge during penetration because the main charge is free to slide. Thus, a more shock sensitive explosive can be utilized in the main charge of the first embodiment. 
         [0026]      FIG. 4  illustrates a modification of the projectile  40  illustrated in  FIG. 2 . Like components are indicated with the same reference numerals. in the third embodiment illustrated in  FIG. 4 , a modified cup  82  is provided with an opening  84 . In this case, a modified anvil  86  is provided with a needle like projection  88  that passes through the opening  84  in the modified cup  82  and strikes a conventional military grade stab detonator  88  (preferably an M55 detonator). Accordingly, detonation is initiated through the use of a stab detonator instead of inducing a shock wave into an initiator charge as in the embodiments illustrated in  FIGS. 1 and 2 . 
         [0027]    The invention has been described with reference to certain preferred embodiments thereof. It will be understood, however, that modifications and variations are possible within the scope of the appended claims. For example, while the embodiment of  FIG. 1  preferably includes the use of a nose cone charge to fragment the nose cone. While the fragmentation of the nose cone is desirable in excavation applications, it may not be necessary in other projectile applications. Accordingly, the nose cone charge can be eliminated if not required for a particular application. Further, the number of main and initiator charge pellets may be varied depending on the required application. In addition, while the use of the tamper  30  and hammer  74  are preferable, these elements may also be eliminated depending on the particular application. Still further, the structural configuration of the illustrated components may also be varied as long as the concept of using mechanical inertia to cause detonation is employed.