Abstract:
A new mechanism substantially reduces the vulnerability of explosive load munitions to thermal stimuli, such as fire or heat during transport and storage, thus enhancing personnel safety and the survivability of adjacent munitions. The mechanism includes a threaded fuze adapter made of plastic and having a melting temperature that is lower than the auto-ignition temperature of the explosive. The adapter secures a fuze or metal closing plug to an explosive loaded projectile and is designed to permit venting of combustion gases through the nose of the projectile upon auto-ignition of the explosive, thereby preventing detonation of the explosive and fragmentation of the projectile body. A plastic or metal ring is utilized to support the body of an explosive loaded projectile within a fiberboard packing tube, thus allowing the fuze to readily separate from the projectile body upon the melting of the plastic threaded fuze adapter and subsequent combustion of the explosive during an unplanned thermal stimulus event. An intumescent coating is deposited on the metal ammunition container that is used to package explosive loaded cartridges, to reduce the rate of thermal stimuli to the munitions, thereby ensuring that the plastic fuze adapter of the present invention reaches its melting temperature prior to the explosive attaining its auto-ignition temperature.

Description:
REFERENCE TO PREVIOUS APPLICATIONS 
   This application is a continuation of application Ser. No. 10/122,109 filed on Apr. 11, 2002 now abandoned by Roger Wong, et al. for Mechanism For Reducing The Vulnerability Of High Explosive Loaded Munitions To Unplanned Thermal Stimuli, which application itself claims benefit under 35 USC 119(e) of provisional application 60/282,884 filed Apr. 10, 2001, the entire file wrapper contents of which applications are hereby incorporated by reference herein as though fully set forth at length. 

   U.S. GOVERNMENT INTEREST 
   The invention described herein may be made, used, or licensed by or for the U.S. Government for U.S. Government purposes. 

   FIELD OF THE INVENTION 
   The present invention relates in general to the field of Insensitive Munitions (IM) used by the U.S. Armed Forces, and it particularly relates to a new mechanism for reducing the vulnerability of explosive loaded munitions to unplanned thermal stimuli. 
   BACKGROUND OF THE INVENTION 
   High explosive munitions are an essential part of the arsenals of armed forces. Logistic operations of the armed forces frequently involve the transportation of high explosive munitions from manufacturing plants to ammunition storage depots, Ammunition Supply Points (ASP) and magazines, throughout the world. For military sites located within a national boundary, ground transportation is preferred and commonly conducted by rail or trucking freight. For military sites located overseas, the transportation of munitions includes ships and airplanes. 
   Explosive loaded munitions are transported and stored in manners intended to minimize risks of accidental detonation. However, accidents such as an overturned tractor trailer, a train derailment, or a cargo plane crash can occur during transport of the munitions. In some instances, the ensuing fire and heat resulting from the accident could provide sufficient thermal stimuli to cause the munitions to detonate. In such an event, a chain explosion could result from sometimes a single munition explosion. To minimize such a risk of accidental explosions, the United States Department of Defense requires that all munitions and weapons withstand unplanned stimuli such as heat from fire, shock from blast, and impact from fragments and bullets. This requirement is termed Insensitive Munitions (IM). 
   To meet the Insensitive Munition requirement, munitions must pass Fast Cook-Off (FCO) and Slow Cook-Off (SCO) test requirements, as established by MIL-STD-2105B, “Military Standard for Hazard Assessment Tests for Non-Nuclear Munitions”. In a typical Fast Cook-Off test, the munition is engulfed in the flames of a jet fuel (or gasolene) fire exhibiting a minimum average temperature of 1,600° F., to assess its response to rapid heating. In the Slow Cook-Off test, the munition is heated in a closed chamber at a linear rate of 6° F. (or 50° F.) per hour until a reaction occurs, to assess its response to gradual heating. The FCO and SCO tests are considered to be passed if the munition exhibits a Type V response where the test items only burn or scatter parts less 50 feet away from the burn pan or test oven. 
   SUMMARY OF THE INVENTION 
   It is a feature of the present invention to provide a new mechanism for high explosive munitions that substantially reduces the vulnerability of explosive load munitions to thermal stimuli such as fire or heat during transportation and storage, thus enhancing personnel safety and the survivability of adjacent munitions. Further, the munitions design of the present invention is capable of meeting the Insensitive Munitions requirements according to the MIL-STD-2105B specifications. 
   To this end, the new munitions design of the present invention incorporates a number of novel design features, including the following:
         1. A threaded fuze adapter made of plastic having a melting point lower than the auto-ignition temperature of the explosive, secures a fuze or metal closing plug to an explosive loaded projectile and is designed to permit venting of combustion gases from the burning explosive through the nose of the explosive loaded projectile, thereby preventing an accidental detonation of the explosive loaded projectile.   2. A plastic or metal ring is incorporated into a fiberboard packaging tube of the present invention to support the projectile body of an explosive loaded cartridge, thus allowing a fuze or metal closing plug to readily separate from the projectile body upon the melting of the plastic threaded fuze adapter of the present invention and subsequent combustion of the explosive during an unplanned thermal stimulus event.   3. An intumescent coating is deposited on a metal ammunition container that package the explosive loaded cartridges in accordance with the present invention to reduce rate of thermal stimuli to the munitions, thereby ensuring that the plastic fuze adapter of the present invention reaches its melting point prior to the explosive attaining its auto-ignition temperature.       

   The foregoing and other features and advantages of the present invention are realized by a mechanism that incorporates the following design features such as a threaded plastic fuze adapter, an improved packaging tube with a support ring for the projectile body, and an intumescent coating deposition on a metal container holding the improved packaging tubes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention and the manner of attaining them, will become apparent, and the invention itself will be understood by reference to the following description and the accompanying drawings, wherein: 
       FIG. 1  is a side view of an explosive loaded cartridge made according to the present invention; 
       FIG. 2  is a cutaway view of the explosive loaded cartridge of  FIG. 1 , shown with a threaded plastic fuze adapter of the present invention for securing the fuze to the nose of the explosive loaded projectile; 
       FIG. 3  is comprised of  FIGS. 3A and 3B  that respectively illustrate a cross-sectional view and a side view of the plastic threaded fuze adapter of  FIG. 2  made according to the present invention; 
       FIG. 4  is side view of a conventional fiberboard packaging tube shown supporting the fuze of an explosive loaded cartridge of  FIG. 1 ; 
       FIG. 5  is a side view of a fiberboard packaging tube for securing the explosive loaded cartridge of  FIG. 1  incorporating the projectile body support ring of the present invention; 
       FIG. 6  is comprised of  FIGS. 6A ,  6 B,  6 C, wherein  FIG. 6A  is a side view of the support ring of  FIG. 5 ,  FIG. 6B  is a front (or rear) view of the support ring, and  FIG. 6C  is a cross-sectional view of the support ring of  FIG. 6B ; and 
       FIG. 7  is comprised of  FIGS. 7A and 7B , that respectively illustrate a side view and a front view of a metal ammunition container made according to the present invention for storing the explosive loaded cartridges of  FIG. 1 . 
   

   Similar numerals in the drawings refer to similar elements. It should be understood that the sizes of the different components in the figures might not be in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  illustrate an explosive loaded cartridge  10  incorporating a threaded fuze adapter  12  ( FIG. 2 ) made according to the present invention. An exemplary explosive loaded cartridge  10  is the 60 mm M720A1 cartridge, to be manufactured and used by the U.S. Armed Forces. The explosive loaded cartridge  10  is comprised of a number of major components, namely: a fuze  16 , a threaded fuze adapter  12 , a projectile body  14 , a main charge explosive  28 , a tail fin  22 , a propelling charge  26  and an ignition cartridge  24 . Each of these components will now be described in more detail. 
   The fuze  16  is generally a threaded body  50 , with a tapered midsection  44  and nose  46 . The fuze  16  of the explosive loaded cartridge  10  is secured to the threaded opening  32  of the projectile body  14  via the plastic fuze adapter  12 . An exemplary fuze  16  used in conjunction with the exemplary 60 mm M720A1 cartridge is the Multi-option M734A1 fuze. 
   The aft section of the fuze body  50  is typically comprised of a threaded metal base  42 , with a Safing and Arming (S&amp;A) device (not shown), an explosive lead charge (not shown) and an explosive booster  48  within its interior volume. The S&amp;A device, lead charge and booster  48  are designed to initiate and detonate the main charge explosive  28  in the projectile body  14 . The S&amp;A device generally holds a stab or electrically initiated detonator, which is “out-of-line” with the lead charge and booster  48  until the explosive loaded cartridge  10  has been fired from the weapon and the fuze  16  has armed. Following a firing of the explosive loaded cartridge  10 , the armed fuze  16  detonates the main charge explosive  28 , after it electronically senses or impacts the target. The ensuing explosion causes the projectile body  14  to break up into lethal fragments. The lead charge and booster  48  are typically made of a pressed explosive, such as Composition A5 (also known as COMP A5) and PBXN 5. PBXN-5 explosive is used for the booster  48  in the exemplary Multi-option M734A1 fuze because it reacts less violently than COMP A5 explosive when subjected to thermal stimuli. The threaded metal base  42  is engaged to the interior surface  205  ( FIGS. 3A ,  3 B) of the plastic fuze adapter  12  of the present invention, which, in turn, is engaged to the threaded opening  32  of the projectile body  14 , by means of its exterior threaded surface  210  ( FIGS. 3A ,  3 B). When the fuze  16  is engaged to the projectile body  14 , the booster  48  is situated immediately adjacent to the main charge explosive  28  inside the projectile body  14 . 
   A plurality of slots  54  are typically formed on the exterior surface of the fuze body  50 , at the base of the tapered midsection  44 . The slots  54  are designed to accept a wrench or fork-shaped tool for assembling the fuze  16  to the projectile body  14 . A metal, U-shaped clip  62  ( FIG. 4 ) is commonly inserted into the slots  54 , to support an explosive loaded cartridge  10  within a fiberboard packaging tube  60  ( FIG. 4 ). The metal packing clip  62  ( FIG. 4 ) is not utilized when the explosive loaded cartridge  10  is packaged in an improved fiberboard packaging tube  70  ( FIG. 5 ) of the present invention, which will be a subject for a further description in subsequent details. 
   The tapered midsection  44  and nose  46  of the fuze body  50  generally house the mechanical and/or electronic components that initiate the detonator in the S&amp;A device after the fuze  16  electronically senses or impacts the target. An electronic, radio frequency (RF) transceiver/firing circuit board and a turbine alternator are housed under the truncated, conical shaped, metallic windshield and plastic nose assembly of the exemplary Multi-option M734A1 fuze. 
   Referring now to  FIGS. 3A and 3B , the threaded fuze adapter  12  of the present invention is generally made of a cylindrical ring  200  having a threaded interior surface  205  as well as a threaded exterior surface  210 . The fuze adapter  12  attaches the fuze  16  to the projectile body  200  by means of its interior and exterior threaded surfaces  205 ,  210 . As used in conjunction with the exemplary 60 mm M720E1 cartridge, the threaded fuze adapter  12  has a nominal inside thread diameter (I.D.) of 1½ inches, nominal outside thread diameter (O.D.) of 1 11/16 inches and a length (L) of approximately 0.64 inch. 
   The threaded fuze adapter  12  is made of a material, e.g. an ionomer plastic, having a melting temperature sufficiently below the auto-ignition temperature of the main charge explosive  28  in the projectile body  14 . A typical melting point of the fuze adapter material would be about 200° F. For the exemplary 60 mm M720A1 cartridge, a FORMION R  Fl-120 plastic is utilized. During an unplanned thermal stimulus such as an exposure to external heat or fire source, the threaded fuze adapter  12  is melted upon reaching its melting temperature prior to the main charge explosive  28  reaching its auto-ignition temperature. Upon melting of the threaded fuze adapter  12 , the fuze  16  is no longer physically secured to the projectile body  14 , thereby enabling the fuze  16  to freely separate from the projectile body  14 . As the thermal stimulus continues to heat the explosive loaded cartridge  10 , the main charge explosive  28  begins to burn upon reaching its auto-ignition temperature. The burning explosive  28  produces combustion gas, which generates pressure within the internal volume of the projectile body  14 . As the gas pressure begins to build, the fuze  16  is expelled from the projectile body  14 , thereby enabling the combustion gas to pass through and out the threaded opening  32 . Venting of the combustion gas and relief of the internal pressure prevents the burning reaction of the main charge explosive  28  from developing into an uncontrolled detonation and fragmenting the projectile body  14 . Thus, the threaded fuze adapter  12  enhances personnel safety and the survivability of adjacent munitions in a fire, by preventing accidental explosion and fragmentation of an explosive loaded cartridge  10 . 
   The projectile body  14  is generally made of a forged, high fragmenting, steel alloy shell. The thickness of the projectile body  14  is roughly 3/16 inch to ¼ inch. The ogival shape of the projectile body  14  is designed to reduce the aerodynamic drag on the explosive loaded cartridge  10  during flight. Approximately mid position of the projectile body  14 , an obturating ring  20  is circumferentially fitted within an external groove. During launch, the obturating ring  20  expands to seal the high pressure, propellant combustion gas behind the projectile body  14  as it travels up the mortar barrel. The sealing action allows the explosive loaded cartridge  10  to be propelled the maximum distance downrange. The projectile body  14  has a threaded opening  32 , which accepts the fuze  16  via the threaded fuze adapter  12  and allows the main charge explosive  28  to be loaded therein. The projectile body  14  connects to the tail fin  22  via a threaded boss  34  located at the aft end. 
   With more specific reference to  FIG. 2 , the main charge explosive  28  is typically a melt-castable explosive such as Composition B (also known as COMP B) and PAX-21. PAX-21 explosive is used in the exemplary 60 mm M720A1 cartridge because of its reduced shock sensitivity and predictable behavior in unconfined burns. COMP B explosive and PAX-21 explosive have auto-ignition temperatures of about 380° F. and 314° F., respectively. As used herein, auto-ignition temperature means a temperature at which the explosive  28  starts to combust upon subject to thermal stimuli. 
   The tail section  30  of the explosive loaded cartridge  10  is comprised of a tail fin  22 , a propelling charge  26  and an ignition cartridge  24 . The tail fin  22  generally consists of a plurality of fin blades attached to a cylindrical boom which assembles to the projectile body  14 . The fin blades are circumferentially attached to the boom at equal angular spacing and are generally trapezoidal shaped with rounded corners. The tail fin  22  provides the necessary stability control to maintain a proper flight path of the explosive loaded cartridge  10  to the target. An exemplary tail fin  22  used in conjunction with the exemplary 60 mm M720A1 cartridge is the six bladed, aluminum alloy M27 fin. The propelling charge  26  is fitted onto the boom of the tail fin  22 , between the fin blades and projectile body  14 . The ignition cartridge  24  is housed within the boom of the tail fin  22 , opposite the end that assembles to projectile body  14 . A plurality of vent holes  40  extending through the boom of the tail fin  22  enables the ignition cartridge  24  to ignite the propelling charge  26 . 
   The propelling charge  26  is generally comprised of horseshoe shaped, containers filled with a propellant charge  36 . Typically, the containers are made of a combustible, felted fiber material and the propellant charge  36  is a single or double based propellant. An exemplary propelling charge  26  used in conjunction with the exemplary 60 mm M720A1 cartridge is the four-increment, M235 propelling charge. 
   The ignition cartridge  24  is designed to function and ignite the propelling charge  26  when the explosive loaded cartridge is fired from the weapon. It typically has a percussion primer  38 , black powder pellet and a propellant charge therein. An exemplary ignition cartridge  24  used in conjunction with the exemplary 60 mm M720A1 cartridge is the M702 ignition cartridge. The explosive loaded cartridge  10  is fired from the weapon by loading it, tail section  30  first, into the muzzle of the mortar barrel. Upon release, it slides down the barrel and impacts a firing pin at the bottom. The firing pin strikes and initiates the percussion primer  38  of the ignition cartridge  24 . The percussion primer  38  initiates the black powder pellet, which in turn ignites the propellant charge contained within the ignition cartridge  24 . The hot combustion gas and flame from the ignition cartridge flashes through the vent holes  40  in the fin boom and ignites the propelling charge  26 . The combustion gas pressure generated by the ignition cartridge  24  and propelling charge  26  propels the explosive loaded cartridge  10  up the barrel and out to the target. In order for the threaded fuze adapter  12  to function as described earlier upon an occurrence of a thermal stimulus, an improved packaging method for the explosive loaded cartridge  10  is provided by the present invention. To understand the need for a new packaging method of the present invention, it might be beneficial to describe a conventional munition packaging method according to a prior art. 
   With reference to  FIG. 4 , a conventional packaging method for the explosive loaded cartridge  10  includes a fiberboard tube  60  and a U shaped, metal support clip  62 . The fiberboard tube  60  is generally made of a cylindrical casing with a stationary end cap  66  and a removable end cap  68 . The explosive loaded cartridge  10  is encased within the fiber tube  60  and is positively restrained by the metal support clip  62  attached to the fuze  16  via the wrench slots  54 . A small gap exists between the fuze  16  of the explosive loaded cartridge  10  and the stationary end cap  66 . In the event of an unplanned thermal stimulus, the metal support ring  62  and stationary end cap  66  would restrain the fuze  16  in place and prevent the release of the fuze  16  upon a melting of the threaded fuze adapter  12 . The pressure generated by the combustion gas upon the ignition of the main charge explosive  28  would build up and might not be sufficiently relieved from the broken joint resulting from the threaded fuze adapter  12 , thus causing a potential detonation of the explosive loaded cartridge  10  with an ensuing fragmentation of the projectile body  14 . 
   With reference to  FIG. 5 , it illustrates an improved packaging enclosure  70  according to the present invention; the explosive loaded cartridge  10  is encased within a fiberboard tube  70 . An exemplary fiberboard tube  70  for use with the exemplary 60 mm M720A1 cartridge is the PA  164  fiber tube. The fiberboard tube  70  is generally made of a cylindrical casing with a stationary end cap  72  and a removable end cap  74 . The tail fin  22  of the explosive loaded cartridge  10  is positioned against the removable end cap  74 , thus enabling the explosive loaded cartridge  10  to be loaded or removed in a rearward manner. 
   The fiberboard tube  70  has an overall length sufficiently greater than the length of the explosive loaded cartridge  10  such that a sufficient space  76  exists between the tip of the nose  46  of the fuze  16  and the stationary end cap  72 . A support ring  78  provides a positive restraint of the explosive loaded cartridge  10 . 
   With further reference to  FIGS. 6A ,  6 B,  6 C, the support ring  78  is attached to the fiberboard tube  70  and engaged with the explosive loaded cartridge  10  in the ogive region of the projectile body  14 . The support ring  78  is generally formed of a plastic cylindrical shell  82  with a circular flange  80  that is peripherally located along the mid section of the cylindrical shell  82  of the support ring  78 . 
   When used for supporting the exemplary 60 mm M720A1 cartridge, the cylindrical shell  82  typically has a nominal inside diameter (I.D.) of approximately 2.15 inch, and a length of approximately 0.875 inch. The outer surface of the cylindrical shell  82  is generally 2.40 inches in diameter. The inner surface of the cylindrical shell  82  is comprised of a straight surface  84  and two tapered surfaces  86 . 
   The straight inner surface  84  spans approximately one third the length of the cylindrical shell  82 , while the tapered inner surfaces  86  occupies the remaining length of the cylindrical shell  82 . The tapered inner surface  86  has a taper that is generally conforms to the curvature of the projectile body  14  at the point of contact there between to provide a positive restraint of the explosive loaded cartridge  10  within the fiberboard tube  70 . When used for supporting the exemplary 60 mm M720A1 cartridge, the tapered inner surface  86  has a taper angle (T) of approximately 70. 
   The circular flange  80  is generally formed on the outer surface of the cylindrical shell  82  at the mid length. When used for supporting the exemplary 60 mm M720A1 cartridge, the flange  80  has a thickness of about 0.125 inch and an outer diameter of 2.69 inches. The flange  80  is designed to secure the support ring  78  to the fiberboard tube  70 . 
   In a manner of the storage of the explosive loaded cartridge  10  in the fiberboard tube  70  as illustrated in  FIG. 5 , in the event of an unplanned thermal stimulus, upon the melting of the threaded fuze adapter  12 , the space  76  enables the fuze  16  to freely and completely separate from the projectile body  14 , thereby enabling the threaded fuze adapter  12  to achieve its fullest function as intended. 
   With reference to  FIG. 7 , a plurality of fiberboard tubes  70  each containing an explosive loaded cartridge  10  are packaged inside a metal ammunition container  90 . An exemplary metal ammunition container  90  for storing the exemplary 60 mm M720A1 cartridge is the PA  124  metal container, which holds eight of the explosive loaded cartridges  10  packaged inside fiber tubes  70 . 
   A further novelty of the present invention is an intumescent coating  92  deposited onto the exterior or interior surface of the metal ammunition container  90 . The intumescent coating  92  is typically used in the construction industry to protect structural members such as steel beams in fires. 
   In the event of an unplanned thermal stimulus such as an external heat or fire source, the intumescent coating  92  on the metal ammunition container  90  insulates the fiberboard tubes  70  and the explosive loaded cartridges  10  packaged therein from the fire, and further abates the rate of heating of the explosive loaded cartridges  10 . The gradual heating inside the metal ammunition container  90  ensures that the threaded fuze adapter  12  reaches its melting temperature prior to the main charge explosive  28  reaching its auto-ignition temperature, thus preventing an accidental detonation of the explosive loaded cartridge  10 . 
   It should be understood that the geometry, compositions, and dimensions of the elements described herein can be modified within the scope of the invention and are not intended to be the exclusive; rather, they can be modified within the scope of the invention. Other modifications can be made when implementing the invention for a particular environment.