Patent Grant
9329010
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.
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.
In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals.
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.
A heat-absorbing liner 20, shown in dashed lines in
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.
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.
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.
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.
The inventions described herein may be manufactured, used and licensed by or for the United States Government.
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