Abstract:
A modified 40 mm grenade round designed to breach doors without throwing a significant amount of shrapnel into a building&#39;s interior or back toward the shooter. The modified round includes a forward extension on the ogive. The extension is rigidly connected to a thrust column which transmits an impact load directly from the ogive&#39;s nose cap to the striker on the fuse assembly. This configuration detonates the explosive charge within the projectile while the explosive is still well outside the door. This early detonation throws a pressure wave again the door&#39;s exterior, forcing the door inward.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 12/657,405. The parent application was filed on Jan. 19, 2010. It lists the same inventor and remains pending as of the date of filing of the present application. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of projectile delivery systems. More specifically, the invention comprises an explosive projectile designed to breach a door while producing very little shrapnel. 
     2. Description of the Related Art 
     Although the components of the present invention can be applied to many different types of projectiles, they were primarily developed as a component of existing 40 mm grenade weapons (such as the U.S. Army&#39;s M-433). While those skilled in the art will be familiar with such weapons, a brief description may nevertheless be helpful. 
       FIG. 1  depicts prior art 40 mm grenade round  10 . Its two main components are case  12  (which houses the propulsion system) and projectile  14 . The grenade round is designed to be fired from a variety of weapons. One example is the U.S. Army&#39;s M-203 grenade launcher which is typically slung beneath the barrel of a rifle such as the M-16A2. 
     The launching of a 40 mm grenade involves the same principles as a conventional rifle cartridge. The main difference, however, is the size and mass of the projectile. A typical shoulder-fired military weapon launches a projectile weighing less than 30 grams at a relatively high velocity (700-1,000 meters per second). In contrast, a 40 mm grenade weapon launches a projectile weighing over 200 grams at a relatively low velocity (70-80 meters per second). Thus, while the operating principles between the two types of weapons are the same, they can be said to operate in different regimes. 
     The unified 40 mm grenade round  10  is placed in the launching weapon and then fired. Case  12  remains within the weapon. Projectile  14  is propelled down the weapon&#39;s bore. Rifling ring  26  engages internal rifling on the firing weapon&#39;s bore and spins the projectile in order to stabilize it in flight. 
     The leading end of the projectile assumes the form of ogive  28 . Those skilled in the art will know that the term “ogive” sometime refers to a specific profile used for missile nose cones. However, the term is also more broadly used to mean the nose portion of any flying projectile. In this disclosure, “ogive” is given the latter meaning. Thus, it may assume a wide variety of shapes. The ogive generally contains the arming and detonating mechanisms. The volume between the ogive and the rifling ring typically contains the explosive. 
       FIG. 2  shows the same 40 mm grenade round of  FIG. 1  cut in half to reveal its internal details. Projectile  14  includes a hollow volume defined by the combination of ogive  28 , casing  36 , and aft closure  38 . These three components are joined together by suitable means, such as threaded engagements. 
     Explosive  34  is contained within casing  36 . Fuse assembly  30  is contained within ogive. The fuse assembly activates spitback detonator  32  when the projectile strikes a target object (assuming it has been appropriately armed). The spitback detonator then initiates explosive  34 . Casing  36  is typically scored to form a series of rectangles which will break into relatively small pieces when the explosive detonates. 
     The propulsion system contained within case  12  is often referred to as a “high-low” system. While a detailed discussion of this system is beyond the scope of this disclosure, a brief description may aid the reader&#39;s understanding of the environment in which the present invention operates. The “high” part of the system refers to high pressure chamber  18 . This chamber is often created by the insertion of a metallic case filled with propellant into base  16 . The open end of the metallic case is closed by burst diaphragm  22 . A primer is contained in the opposite end. 
     A mechanical striker is used to detonate this primer which then causes the propellant within the high pressure chamber to initiate. This action ruptures the burst diaphragm. The expanding propellant gases are then metered through nozzle  24  into low pressure chamber  20 . These relatively low pressure gases act against the aft end of aft closure  38 , thereby propelling the projectile down the firing weapon&#39;s bore. For a more detailed discussion of the propulsion system of the M-433, the reader may wish to review U.S. Pat. No. 7,004,074 to Van Stratum (2006), which is hereby expressly incorporated by reference. 
     A detailed description of the fuse assembly is likewise beyond the scope of this disclosure. However, a fuse assembly typically contains a number of safety features designed to prevent accidental detonation. For example, in some embodiments, the fuse can only be armed when the projectile first experiences a violent forward acceleration followed by a rotation at a minimum rotational velocity. The presence of these two cues indicates that the projectile has been intentionally and successfully fired from a weapon. The fuse assembly will then arm itself during flight. Once armed, any sudden deceleration (such as the projectile impacting a surface) will initiate spitback detonator  32  and explode the grenade. 
     A typical fuse assembly is the M-550 fuse used by the U.S. Army. A discussion of the details of the fuse assembly is beyond the scope of this disclosure. However, the reader wishing to know these details is referred to U.S. Pat. No. 5,081,929 to Mertens (1992). 
     The assembly shown in  FIGS. 1 and 2  functions very well.  FIG. 3  shows projectile  14  flying toward a target.  FIG. 4  shows the projectile striking a target and detonating. Target surface  42  is in this example a reinforced piece of concrete (a hard target). The explosion throws shrapnel  40  in all directions away from the point of impact.  FIG. 5  shows the result, with void  44  being blown into target surface  42 . The prior art projectile is primarily intended as an anti-personnel weapon, and the wide dispersal of shrapnel is obviously effective in this regard. 
       FIG. 6  shows an idealized depiction of the detonation of explosive  34 . Explosive pressure is generally emitted in a direction normal to the surface of the volume of explosive. As the explosive volume depicted is cylindrical, it will emit lateral pressure wave  50  (roughly in the shape of an expanding cylinder), forward pressure wave  46 , and rearward pressure wave  48 . The shape of these pressure waves determine in large part how shrapnel created by the explosion will fly. 
     It has long been known to use a 40 mm grenade as a door breaching round. However, it is not optimal in this role. In anti-insurgency operations, soldiers must often penetrate occupied buildings. In many instances, it is not known whether the occupants are hostile. However—hostile or not—the occupants will not voluntarily open the door. Thus, the door must be breached. 
       FIGS. 7 and 8  shows the use of a prior art 40 mm grenade round in this role. In  FIG. 7 , projectile  14  impacts door  52  at a significant velocity (typically about 70 meters per second). Ogive  28  knocks breach  54  into the face of the door. The sudden deceleration initiates the fuse assembly, so spitback detonator  32  initiates the explosive.  FIG. 8  shows the result. The expanding pressure waves from the exploding projectile destroy the door and explosion  58  sends flying debris  56  into the occupied structure. Persons within the structure may be injured or killed. 
     In addition, debris from the door and the casing of the projectile itself may be thrown back toward the shooter. This fact forces the shooter to stand back a considerable distance (such as 30 meters). It is more desirable to station the soldier or soldiers preparing to enter a structure much closer to the door, so that there will be little delay between the detonation of the grenade and their entry. 
     Thus, while the prior art 40 mm grenade, round is effective in breaching doors, it may produce unwanted collateral damage and may unduly delay the entry of a security team into a structure. A system which can breach the door without throwing significant shrapnel would therefore be preferable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a modified 40 mm grenade round designed to breach doors without throwing a significant amount of shrapnel into a building&#39;s interior or back toward the shooter. The modified round includes a forward extension on the ogive. The extension is rigidly connected to a thrust column which transmits an impact load directly from the ogive&#39;s nose cap to the striker on the fuse assembly. This configuration detonates the explosive charge within the projectile while the explosive is still well outside the door. This early detonation throws a pressure wave again the door&#39;s exterior, forcing the door inward. 
     The projectile includes primarily plastic components which fracture into light and small debris when the explosive detonates. The projectile preferably also includes bore-riding cylindrical surfaces in the body and the ogive. These surfaces minimize balloting and resulting off-axis wobble as the projectile exits the muzzle of a weapon. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view, showing a prior art 40 mm grenade round. 
         FIG. 2  is a perspective view with a cutaway, showing internal details of the prior art grenade round. 
         FIG. 3  is a perspective view, showing a prior art projectile in flight. 
         FIG. 4  is a perspective view, showing the detonation of the projectile upon striking the target. 
         FIG. 5  is a perspective view, showing the resulting damage to the target. 
         FIG. 6  is a perspective view, showing the expanding pressure waves caused by the detonation of a cylindrical volume of explosive material. 
         FIG. 7  is an elevation view, showing a prior art projectile striking a door. 
         FIG. 8  is an elevation view, showing a prior art projectile destroying a door. 
         FIG. 9  is an elevation view, showing a projectile made according to the present invention. 
         FIG. 10  is a perspective view with a cutaway, showing internal details of the propulsion assembly. 
         FIG. 11  is a perspective view with a cutaway, showing the body with its contained explosive. 
         FIG. 12  is a perspective view with a cutaway, showing the ogive with an attached fuse assembly. 
         FIG. 13  is a section view, showing one embodiment of a snap fit between the propulsion assembly and the body. 
         FIG. 14  is an elevation view, showing the projectile flying toward a door. 
         FIG. 15  is an elevation view, showing the detonation of the projectile. 
         FIG. 16  is a perspective view, showing a door being blown open by the present invention. 
         FIG. 17  is a perspective view, showing a steel door being blown open by the present invention. 
       
         
           
                 
               
                 
                 
                 
                 
               
             
                 
                     
                 
                 
                   REFERENCE NUMERALS IN THE DRAWINGS 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                   10 
                   40 mm grenade round 
                   12 
                   case 
                 
                 
                   13 
                   propulsion assembly 
                   14 
                   projectile 
                 
                 
                   16 
                   base 
                   17 
                   side wall 
                 
                 
                   18 
                   high pressure chamber 
                   20 
                   low pressure chamber 
                 
                 
                   22 
                   burst diaphragm 
                   24 
                   nozzle 
                 
                 
                   26 
                   rifling ring 
                   28 
                   ogive 
                 
                 
                   30 
                   fuse assembly 
                   32 
                   spitback detonator 
                 
                 
                   34 
                   explosive 
                   36 
                   casing 
                 
                 
                   38 
                   aft closure 
                   40 
                   shrapnel 
                 
                 
                   42 
                   target surface 
                   44 
                   void 
                 
                 
                   46 
                   forward pressure wave 
                   48 
                   rearward pressure wave 
                 
                 
                   50 
                   lateral pressure wave 
                   52 
                   door 
                 
                 
                   54 
                   breach 
                   56 
                   flying debris 
                 
                 
                   58 
                   explosion 
                   68 
                   door frame 
                 
                 
                   70 
                   steel door 
                   72 
                   steel bar door 
                 
                 
                   74 
                   detonation assembly 
                   76 
                   body 
                 
                 
                   78 
                   upper extreme 
                   80 
                   lip 
                 
                 
                   82 
                   cylindrical side wall 
                   84 
                   aft closure 
                 
                 
                   86 
                   aft pocket 
                   88 
                   lip receiver 
                 
                 
                   90 
                   female thread 
                   92 
                   bulkhead 
                 
                 
                   94 
                   detonator pocket 
                   96 
                   male thread 
                 
                 
                   98 
                   hammer weight 
                   100 
                   striker 
                 
                 
                   102 
                   thrust column 
                   104 
                   nose cap 
                 
                 
                   106 
                   sloping side wall 
                   108 
                   cylindrical side wall 
                 
                 
                   112 
                   threaded engagement 
                   114 
                   extension 
                 
                 
                   116 
                   groove engaging protrusion 
                   118 
                   standoff ogive 
                 
                 
                     
                 
               
            
           
         
       
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 9  shows a perspective view of a grenade round made according the present invention, including a cutaway to reveal internal details. The round includes three major components. These are: detonation assembly  74 , body  76 , and propulsion assembly  13 . 
     The round is made to be fired from a rifled bore. Propulsion assembly  13  remains in the breech end of the bore when the round is fired. Detonation assembly  74  and body  76  together form a projectile which flies downrange as a unit. Acceleration of the projectile is accomplished using the same “high-low” pressure system as for the prior art. The propellant within high pressure chamber  18  is initiated. Burst diaphragm  22  then ruptures and meters the expanding propellant gas into low pressure chamber  20  (which is the void between the aft end of body  76  and base  16 ). Body  76  contains explosive  34 . The explosive is initiated by fuse assembly  30 , which will be explained in more detail subsequently. 
       FIG. 10  shows a more detailed view of propulsion assembly  13  (a cutaway is again included to reveal internal features). The reader will observe how expanding gases from high pressure chamber  18  are metered through nozzle  24  into the low pressure chamber. Case  12  includes base  16  joined to side wall  17 . Side wall  17  includes a protruding lip  80  proximate upper extreme  78 . This lip is configured to engage a corresponding feature on the body so that the propulsion assembly and the body can be attached. 
       FIG. 11  is a cutaway view of body  76 . Aft closure  84  is preferably formed integrally with cylindrical side wall  82 . Explosive  34  is contained within this interior volume. The explosive is preferably covered by a thin bulkhead  92 . This bulkhead preferably includes detonator pocket  94 . The bulkhead covers the forward end of the explosive mass. It provides a moisture seal. It also tends to prevent spalling upon the initiated of the round. 
     Female thread  90  is provided on the forward portion of cylindrical side wall  82 . This feature is used to join the body to detonation assembly  74 . Rifling ring  26  is provided on the aft end of body  76 . The body may be made from a relatively soft material such as reinforced plastic. The rifling ring is intended to engage the grooves and lands in the firing bore so that the projectile may be rotationally accelerated as it travels down the bore. It is therefore preferable to make the rifling ring out of tougher material—with aluminum being a good choice. 
     Those skilled in the art will know that a rifled bore is often defined by two distinct diameters. First is the “bore diameter” or “land diameter.” Second is the “groove diameter,” which is the diameter of a circle passing through the deepest part of the rifling grooves. The bore is defined by a set of “lands” (the protruding areas between the grooves). While the lands are often thought of as having considerably ore surface area than the grooves, this is not always the case. In fact, many modern rifled bores have more groove area than land area. 
     Rifling ring  26  includes one or more groove engaging protrusions which have a diameter greater than the land diameter of the rifled bore. The rifling ring also includes lip receiver  88 , which is configured to receive lip  80  on case  12  in a snap-fit configuration. 
     Aft closure  84  preferably includes aft pocket  86 , which serves several functions. Turning briefly back to  FIG. 9 , the reader will observe that including the aft pocket increases the volume of low pressure chamber  20 . Increasing this volume tends to spread the recoil impulse over a longer time interval—which decreases the weapon “kick” perceived by the shooter. The presence of the aft pocket also provides space to include a “base bleed” (the reader may wish to review U.S. Pat. No. 7,802,520 to Van Stratum (2010), which is hereby expressly incorporated by reference) gas generator (a device which injects gas behind the projectile in flight in order to reduce base drag) or a tracer (a source of bright light in the back of the projectile which allows the shooter to more accurately observe the trajectory). 
       FIG. 12  shows detonation assembly  74  in detail. A cutaway is again provided. The detonation assembly has two main components—fuse assembly  30  and standoff ogive  118 . These two components are joined together by any conventional means—with threaded engagement  112  being one example. 
     Spitback detonator  32  is attached to the aft end of fuse assembly  30 . The fuse assembly is in reality a complex mechanism which is only represented in a conceptual form in  FIG. 12 , striker  100  is an element which activates the detonation sequence. It is akin to the firing pin in a rifle. The striker may include additional features such as a hammer weight or weights  98 . While the striker may assume many forms, it is configured to activate the fuse assembly when it receives an impact load directed toward the rear of the projectile. 
     The fuse assembly typically contains a set-back safety device and a rotation safety device. Both these safety devices must be in the “fire” position in order for the striker to produce the detonation of spitback detonator  32 . The set-back safety device must receive a sharp acceleration in the forward direction in order to switch from a “safe” configuration to a “fire” configuration. This occurs when the projectile is fired from the rifled bore. 
     The rotation safety device is switched from a “safe” position to a “fire” position when the projectile rapidly rotates through a defined number of rotations. This occurs when the rifling in the bore of the launching weapon rotationally accelerates the projectile to the spin rate it will experience in flight. The rotation safety device generally requires multiple rotations so that the weapon will have an “arming distance”—meaning that the fuse assembly cannot be fired until it has traveled a specific distance. 
     For prior art rounds, the arming distance is generally 14 m to 28 m. This requirement means that the safety devices are set so that no specific round out of a large sample will be armed before it has traveled 14 m and every specific round out of a large sample will be armed after it has traveled 28 m. Once the safety mechanisms are armed (in the “fire” position) a blow to striker  100  will actuate the fuse assembly. Spitback detonator  32  will then be fired and the explosive within the projectile will be initiated. 
     Standoff ogive  118  includes nose cap  104 . The forward tip of the nose cap extends a significant distance beyond what would be the tip of a conventional ogive. This distance is denoted as extension  114 . 
     The outer wall of standoff ogive  118  has three distinct regions in the particular embodiment shown. Cylindrical side wall  108  exists in the aft region. The thicker wall of nose cap  104  exists in the forward region. Sloping side wall  106  joins these two regions together into a unified whole. Of course, the separate regions may be formed by multiple independent parts linked together. In the embodiment shown, however, the three regions are formed as one integral piece. 
     A significant design feature of the present invention is the rapid transmission of impact forces experienced by nose cap  104  to striker  100 . Thrust column  102  is provided for this purpose. While the thrust column may assume many geometric forms, it is important that it be relatively stiff. 
     In the embodiment shown, the thrust column is a hollow cylinder that is integrally molded with the balance of the standoff ogive. Graphite reinforced NYLON may be used for the standoff ogive and this provides sufficient stiffness. In other embodiments, a relatively soft material could be used for the nose cap and side walls, with a stiff metal cylinder being used for thrust column  102 . Whatever configuration is used, the forward portion of the thrust column is connected to the nose cap while the aft portion rests against striker  100 . Since the striker in the embodiment of  FIG. 12  includes one or more hammer weights  98 , the aft portion of thrust column  102  bears against these hammer weights. 
     In the embodiments shown, body  76  is joined to detonation assembly  74  by engaging male thread  96  (on the aft end of fuse assembly  30 ) and female thread  90  on the forward end of body  76 . Uniting these two subassemblies creates a projectile. The projectile must be joined to the propulsion assembly so that they remain an integral unit up until the time when the grenade round is fired. 
     It is preferable to mold standoff ogive  118 , body  76 , and case  12  as plastic components. The use of plastic allows novel joining techniques. The reader will recall from  FIG. 10  that side wall  17  of case  12  includes lip  80 . From  FIG. 11 , the reader will also recall that rifling ring  26  includes lip receiver  88 . These features allow the projectile assembly to “snap” into the open mouth of case  12 —thereby uniting the propulsion assembly with the projectile. 
       FIG. 13  shows a sectioned elevation view of this snap engagement. As body  76  is forced downward (with respect to the orientation shown in the view) Lip  80  on side wall  17  will snap into lip receiver  88 . This snap engagement will hold the components together until the weapon is fired. Other joining features could certainly be substituted, but a snap feature is inexpensive to produce and easy to use in the assembly process. 
       FIGS. 14-17  illustrate the operation of the present invention.  FIG. 14  shows projectile  14  flying toward door  52 . The nose cap of the standoff ogive extends well forward of the body. The nose cap strikes the target first and transmits the striking force back through the thrust column directly to the striker of the fuse assembly. The fuse assembly is thus struck while the body of the projectile (and the explosive it contains) is still well outside the door. 
       FIG. 15  shows the detonation of the projectile. Explosion  58  shatters the body and other components of the projectile—creating flying debris  56 . A strong pressure wave is projected forward toward door  52 —which causes it to bow inward as shown. 
       FIG. 16  depicts the effect on a door in a perspective view. The pressure wave breaks the portion of door frame  68  containing the striker assembly, which allows the door to swing inward as shown. 
       FIG. 17  depicts the effect on a different type of door. Steel door  70  opens inward in this assembly while steel bar door  72  opens outward. The operation of the standoff ogive causes the explosive to detonate between the two doors, blowing steel door  70  inward and steel bar door  72  outward. A steel door frame typically will not break. However, the pressure wave impacting the steed door will deform it sufficiently to disengage the bolt with the striker and blow it open. 
     Material selection is significant to the advantages provided by the present invention. Returning now to  FIGS. 9-12 , some of these features will be explained. Case  12  of propulsion assembly  13  is preferably molded from glass reinforced NYLON. Cylindrical side wall  82  and aft closure  84  of body  76  are preferably molded of graphite reinforced NYLON. Standoff ogive  118  is also preferably molded of graphite reinforced NYLON (a very tough material). 
     Looking specifically at  FIG. 9 , the reader will observe that explosive  34  is surrounded by body  76  and the fuse assembly of detonation assembly  74 . It is important to prevent electrical potential from building between components in contact with the explosive, as an electrical discharge could produce an accidental detonation. 
     In the prior art, the components surrounding the explosive tend to be conductive metal. In the present invention, however, the components tend to be non-conductive plastics. One way to resolve this problem is to coat the graphite reinforcing fibers in the plastic with conductive nickel. Another approach is to apply a thin conductive layer (such as deposited nickel) to the interior surface of the plastic components. Either approach provides sufficient conductivity to eliminate the problem of electrostatic discharges. 
     The use of plastic components throughout the projectile greatly reduces the production of harmful shrapnel upon detonation. The metal components of the fuse assembly tend to fly forward toward the door, where they are broken into even smaller particles. The metal of the rifling ring breaks into aluminum fragments having high surface area and low mass. These decelerate rapidly. The remaining plastic fragments are very small and produce little damage. As a result, a soldier firing the breaching projectile at a door may stand as close as 10 m. The arming range of the fuse assembly should be adjusted to 9-14 m (as opposed to 14-28 m for the prior art). 
     The use of plastic components in combination with appropriate geometry also serves to reduce “balloting” as the projectile accelerates down the bore and out the muzzle. “Balloting” refers to a precessing yaw of a projectile&#39;s centerline as it travels down the bore and exits the muzzle. 
     Ideally, the projectile&#39;s centerline remains perfectly concentric with the centerline of the rifled bore. In prior art grenade rounds, only a portion of the projectile&#39;s external surface engages the bore of the firing weapon. A forward “bore riding” ring is usually provided along with an aft rifling engaging ring. Balloting could be largely eliminated by providing the projectile with a smooth cylindrical surface sized to closely slide within the land diameter of the rifled bore (the “bore diameter”). However, many grenade launching weapons use soft metal barrels (such as thin walled steel or aluminum tubing). A close sliding fit with a metal projectile will quickly wear out the bore in such weapons. 
     The present invention uses much softer plastic materials, however. Even a soft bore material can endure many firings of a soft plastic projectile without significant degradation. Geometry is preferably included to minimize balloting.  FIG. 11  shows cylindrical side wall  82 . This is sized to be a close sliding fit within the land diameter (while the groove engaging protrusions of rifling ring  26  are sized to fit within the groove diameter and be larger than the land diameter). 
     Turning now to  FIG. 12 , the reader will recall that standoff ogive  118  includes cylindrical side wall  108 . This is also sized to be a close sliding fit within the land diameter. Thus, the projectile has cylindrical surfaces which closely ride the land diameter along most of its length. This prevents projectile balloting as the projectile travels down the bore and as it exits the muzzle. 
     The preceding description contains significant detail, but it should not be construed as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. As an example, the shape of the standoff&#39;s ogive&#39;s side wall could be modified while still providing the basis function of the present invention. Many other alterations will occur to those skilled in the art. Thus, the scope of the invention should be fixed by the following claims, rather than by the examples given.