Abstract:
A storage apparatus for a munition that contains energetic material uses a water-filled liner to absorb heat applied from the exterior of the storage apparatus. The liner is disposed between the munition and a metal storage container. The liner has segregated compartments which release water into either the storage container or the liner itself. The released water boils and forms steam. A spiral-shaped steam conduit is formed in either the wall of the storage container or in the liner itself. The steam conduit directs the steam away from the munition. The metal storage container includes a pressure relief valve to release the steam pressure. The storage apparatus delays the detonation of the energetic material in the munition.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The inventions described herein may be manufactured, used and licensed by or for the United States Government. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates in general to munitions and in particular to compliance with Insensitive Munitions (IM) standards. IM standards require that, to the extent practicable, munitions are safe when subjected to unplanned stimuli. The Fast Cook Off (FCO) test is used to simulate a situation wherein munitions are exposed to a fire. In FCO, munitions are engulfed in a flame of at least 1700° C. until the munition reacts. It is desirable for the reaction to be limited to no more than burning (Type 5 IM reaction). A detonation type of reaction (Type 1 IM reaction) is to be avoided. During a FCO test, munitions with no IM features typically demonstrate a Type 1 IM detonation in less than ten minutes. 
     In some cases, the IM features of a munition require a slow heating to function properly. In other cases, where a complete IM solution is not viable, an improvement such as delaying the onset of the Type 1 IM reaction is desirable. 
     Known munition and ammo containers have several forms, including boxes and tubes. Some of these containers have IM features for the venting of gases or for insulating the munition. The venting of gases may increase the delay time to detonation. Insulating a munition, by itself, has a minimal effect on the delay time to detonation. One known munition container is made of a composite glass-reinforced resin with meltable salts. As the salts melt, they absorb heat. However, resin or plastic containers are often not suitable for munitions because the containers do not meet leak test standards after the containers are thermally cycled. Some commercial fireproof safes and fireproof doors use a layer of felt that is impregnated with a water-based hydrogel as a means to mitigate heat damage. 
     IM venting is not feasible for some munitions. For example, the AT4 single-shot recoilless weapon is stored with the propellant in the weapon. The propellant in the AT4 is contained in its barrel, between the warhead and a counter mass. Without venting and when exposed to conditions like the FCO test, munitions such as the AT4 will detonate quickly. There is not enough time prior to detonation to evacuate or rescue nearby personnel. In some munitions, even if IM vents are present, the heating may occur distal from the IM vent and the munition may detonate before the IM vent has activated. The heating rate in situations like the FCO test is so rapid that there may not be enough time for the IM features to function prior to detonation. 
     A need exists for an apparatus and method for delaying the detonation time of munitions exposed to a FCO test. 
     SUMMARY OF INVENTION 
     One aspect of the invention is an apparatus including a closed, metal container having a central longitudinal axis. A munition is disposed in the container. The munition contains energetic material and has a central longitudinal axis that is generally parallel to the central longitudinal axis of the container. A pressure relief valve is disposed in a wall of the container for relieving steam pressure in the container. A heat-absorbing liner is disposed between an interior surface of the container and the munition. The liner has a central longitudinal axis that is generally parallel to the central longitudinal axis of the munition. The liner extends around a perimeter of the munition and extends axially along at least a portion of the munition. 
     The liner has an inner layer facing the munition and an outer layer facing the interior of the container. The inner and the outer layers define a plurality of segregated compartments. Each compartment contains water. The apparatus includes a fluid conduit having a spiral-shaped cross-section and a central longitudinal axis that is generally parallel to the central longitudinal axis of the munition. The munition is disposed internal to the fluid conduit. 
     Preferably, the central longitudinal axes of the container, the munition, the liner and the fluid conduit are all generally horizontal. 
     The plurality of segregated compartments may contain hydrogel and/or a wicking material. 
     In one embodiment, the wall of the container includes the spiral-shaped fluid conduit. 
     In another embodiment, the liner includes the spiral-shaped fluid conduit. At least one of the inner and outer layers of the liner may include a peripheral metal foil layer. The spiral-shaped conduit may be defined by adjacent wraps of the liner around the munition. 
     The invention will be better understood, and further objects, features and advantages of the invention will become more apparent from the following description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIG. 1  is a cutaway schematic side view of an embodiment of an apparatus for storing a munition. 
         FIG. 2  is a sectional view along the line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a schematic front view of a flattened liner for a munitions container. 
         FIG. 4  is a sectional view along the line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a schematic transverse sectional view of a munitions container having a spiral-shaped fluid conduit formed in its wall. 
         FIG. 6  is a schematic transverse sectional view of a liner having a spiral-shaped fluid conduit. 
         FIG. 7  is a partially cutaway perspective view of an embodiment of an apparatus for storing a munition. 
         FIG. 8  is a graph of temperature as a function of time from a test of one embodiment of an apparatus for storing a munition. 
     
    
    
     DETAILED DESCRIPTION 
     A novel apparatus for storing a munition uses water or water compositions, such as hydrogel, to absorb heat. The heat source is external to the stored munition. Water has a heat of vaporization of 2257 joules per gram. The heat of vaporization of water is 540 times greater than the heat needed to raise the temperature of one gram of water 1 degree C. Water maintains its boiling temperature until it is evaporated. 
       FIG. 1  is a cutaway schematic side view of one embodiment of a munition container  10  and  FIG. 2  is a sectional view of  FIG. 1 . Container  10  is a closed container having a central longitudinal axis A. Container  10  is made of metal, for example, steel. A munition  12  contains energetic material  16  and is stored in container  10 . Munition  12  has a central longitudinal axis B. Axis B of munition  12  is generally parallel to axis A of container  10 . A pressure relief valve  14  is disposed in a wall  18  of container  10  for relieving steam pressure in container  10 . 
     A heat-absorbing liner  20 , shown in dashed lines in  FIGS. 1 and 2 , is disposed between an interior surface  22  of container  10  and munition  12 . Liner  20  has a central longitudinal axis C that is generally parallel to axis B of munition  12 . Liner  20  extends around the entire perimeter  24  of munition  12  and extends axially along at least a portion of munition  12 . Liner  20  may extend around the perimeter  24  of munition  12  multiple times. Liner  20  has an inner layer  26  that faces munition  12  and an outer layer  28  that faces interior surface  22  of container  10 . In  FIG. 2 , container  10 , munition  12  and liner  20  are shown as cylindrical shapes for clarity. However, container  10 , munition  12  and liner  20  may have other shapes and each need not be the same shape as the other. It is preferred that axes A, B and C are all generally horizontal. 
       FIG. 3  is a schematic front view of liner  20  in an unrolled or flattened position.  FIG. 4  is a sectional view along the line  4 - 4  of  FIG. 3 . In the flattened position, liner  20  is generally rectangular with a dimension D along its axis C and a dimension E that is orthogonal to dimension D. Dimensions D and E may be selected as needed for a particular munition. Inner layer  26  and outer layer  28  of liner  20  form a plurality of segregated compartments  30  with seams  32 , such as the array of rows and columns of compartments  30  shown in  FIG. 3 . The number of rows and columns of compartments  30  may be varied. The size of the compartments  30  may be varied. Each compartment  30  contains at least water  36 . Preferably, each compartment  30  contains a water-based gel such as a hydrogel  38 . Each compartment  30  may also contain wicking material  34 . Liner  20  may be made of, for example, two sheets of plastic that are heat-sealed to form seams  32  and compartments  30 . 
     As liner  20  absorbs heat generated external to container  10 , one or more compartments  30  may burst or fail. The water  36  or hydrogel  38  from the burst compartments will collect in the bottom of liner  20  and/or in the bottom of container  10 . As the water  36  or hydrogel  38  in the bottom of liner  20  and/or container  10  boils, steam is produced. Thermal protection for munition  12  occurs by directing the steam that is produced through a fluid conduit having a spiral-shaped cross-section. The munition  12  is disposed internal to the spiral-shaped fluid conduit. 
       FIG. 5  is a schematic transverse sectional view of one embodiment of a spiral-shaped fluid conduit  40  formed by the double-wall of container  10   a . Conduit  40  has a central longitudinal axis F that is generally parallel to axis B of munition  12 . Munition  12  and liner  20  are disposed internal to spiral-shaped fluid conduit  40 . The entrance  42  to conduit  40  is preferably located vertically at least as high as axis F of conduit  40 . Water  36  or hydrogel  38  from burst compartments in liner  20  collects in the bottom of container  10   a , boils, and enters entrance  42  of conduit  40 . The steam flowing through spiral-shaped conduit  40  absorbs heat being applied to container  10   a  from the external environment outside container  10   a  and carries the heat away from munition  12 . When the steam pressure in conduit  40  is high enough, the steam will exit container  10   a  via relief valve  14 . 
       FIG. 6  is a schematic transverse sectional view of another embodiment of a spiral-shaped fluid conduit  44 . Conduit  44  is formed by multiple wraps of liner  20  around munition  12 . Conduit  44  has a central longitudinal axis G that is generally parallel to axis B of munition  12 . When liner  20  is used as conduit  44 , one or both of the inner layer  26  ( FIG. 4 ) and outer layer  28  of liner  20  include a peripheral foil layer or foil coating  46  and/or  48 , respectively. The foil layer(s)  46 ,  48  contain and guide the hot steam. Foil layer(s)  46 ,  48  on adjacent wraps of liner  20  from spiral-shaped conduit  44 . Foil layers  46 ,  48  may be made of a metal, for example, aluminum. 
     The entrance  50  to conduit  44  is preferably located vertically at least as high as the axis G of conduit  44 . Water  36  or hydrogel  38  from burst compartments in liner  20  collects in the bottom of liner  20 , boils, and enters entrance  50  of conduit  44 . The steam flowing in spiral-shaped conduit  44  absorbs heat being applied to container  10  from the external environment outside container  10 . The steam leaves conduit  44  at conduit exit  52  and enters container  10 . When the steam pressure in container  10  is high enough, the steam will exit container  10  via relief valve  14 . 
       FIG. 7  is a partially cutaway schematic perspective view of one embodiment of an apparatus for storing munition  12 . A layer of heat insulating material  54  may be wrapped around the exterior of metal container  10 . The heat insulating material  54  may be, for example, fiberglass. A layer of wicking material  56  may be placed adjacent to the interior surface of container  10 . Wicking material  56  may be, for example, felt. Multiple wraps of liner  20  with foil layer(s)  46 ,  48  ( FIG. 4 ) are disposed around munition  12  and adjacent to wicking material  56 . Protective dunnage  58  may be used between liner  20  and munition  12 . Dunnage  58  may be a packing material, for example, cardboard. 
     TEST RESULTS 
     A heating test was conducted on an AT4 single-shot recoiless weapon loaded with its propellant. To establish a baseline measurement, an AT4 weapon was placed in a wooden container without a liner  20  and heated. The AT4 weapon detonated in about 7 minutes. Another baseline measurement was made using computer simulation to calculate the time required to detonate the AT4 when placed in a steel container. In the simulation of the steel container, the time to detonation was also about 7 minutes. The reaction time of 7 minutes is, from the perspective of munitions in general, exceptionally long because the propellant in the AT4 is insulated by a barrel. Most munitions have a thin-walled cartridge case and, during a FCO test, will react in a matter of seconds. Such a short time period does not enable another type of IM feature to activate. 
     A liner  20  was constructed using two sheets of plastic that were heat-sealed to form compartments  30  and water was placed in the compartments  30 . A vapor barrier sheet having a foil layer was placed on the exterior of one of the plastic sheets of the liner  20 . The liner  20  was wrapped around the AT4 weapon three times and secured to the AT4 weapon with duct tape. The AT4 weapon and liner  20  were placed in a PA116 steel container  10  with a vent  14 . The test time was limited to about 22 minutes. The flame temperature fluctuated and averaged about 1700 degrees F. 
     The AT4 weapon with the liner  20  did not react after a burn time of 22 minutes. The apparatus was examined after the burn test and water was still retained in the spiral-shaped conduit  44 . The surface of the AT4 weapon under the liner  20 , including plastic parts of the AT4, decals on the AT4, and the duct tape, were all intact.  FIG. 8  is a graph of the temperature on the top surface of the AT4 (inside the conduit  44 ) as a function of time. Because water from the burst compartments  30  pools at the bottom of liner  20 , the cooling at the top of the AT4, as shown in  FIG. 8 , is from the steam in conduit  44 . 
     While the invention has been described with reference to certain embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.