Insensitive Munitions (IM)
Under their normal conditions of use, modern munitions are both effective-they provide an essential military capability- and relatively safe-they are very unlikely to explode or burn spontaneously-despite the fact that they are composed primarily of very hazardous material. Under very severe conditions, however, their dangerous nature comes to light. The energetic materials-high explosives, gun propellants, rocket propellants-that are found in munitions of all types are sensitive to heat and to mechanical shock, so they may be triggered by fire or by impact with bullets or fragments. Such secondary effects are significant: in the Gulf War, for example, most of the disabling damage to fighting vehicles was found to be caused by their own munition payloads, inadvertently triggered by unwanted stimuli.
If a round is hit by a shape charge jet, it is initiated. As a result, the fragments that are generated by the blast then strike the other rounds that are adjacent to it. The latter rounds then initiate, contributing to the overall reaction and damage sustained by the vehicle, crew, and other munitions. The mechanisms of reaction for the initiation of the surrounding rounds are due to the blast and fragments impinging on the aforesaid adjacent round.
The probability of sympathetic detonation can be reduced in several ways. This can be done by reconfiguring the ammunition compartments within the vehicle. It can also be accomplished by packaging the ammunition with anti-fraticide materials. However, each of the aforesaid solutions will reduce the amount of space available for the storage of ammunition. The most acceptable solution to the problem is to reduce the sensitivity of the energetic material to sympathetic detonation Incorporating less sensitive energetic material will reduce the vulnerability of initiation from the cited threats without reducing the number of rounds stored in the vehicle. It has been found that by reducing the vulnerability to sympathetic detonation of the energetic materials used in these munitions, the probability of catastrophic reaction can be minimized.
There is a desire to move away from trinitrotoluene (TNT) based fills since there is no longer a CONUS producer of TNT in existence. A range of energetic materials can be used in low-risk munitions: explosives and propellants less vulnerable than their predecessors to both slow and rapid heating ("cook off") and to impact by bullets or fragments of exploding shells. For warheads, efforts concentrate on the replacement of explosives such as TNT, which is very sensitive to heat and shock, by more stable plastic-bonded explosives, which are better able to withstand adverse condi-tions. For gun propellants, the single, double and triple-base formulations now in service can be replaced by others based on components that are more energetic, but less sensitive. These new explosives and gun propellants are made primarily with energetic crystals such as RDX and HMX, contained in new energetic binders and plasticizers. Some of these formulations not only match the performance of the munitions they replace, they improve on it.
An insensitive munition is one that will not detonate under any conditions other than its intended mission to destroy a target. If it is struck by fragments from an explosion or hit by a bullet, it will not detonate. It also will not detonate if it is in close proximity to a target that is hit. In extreme temperatures, the missile will only burn (no detonation or explosion). This increased safety allows greater numbers of missiles to be packaged, handled, stored, and transported in smaller containers. Passing these requirements addresses higher levels of safety performance and means the system is safer to operate in any environment while maintaining its highly lethal performance. It also allows for cost saving opportunities for the government.
The development of explosive compositions for military applications is also motivated by the need for insensitive explosives with high energy output. This problem has always plagued the military, but in recent years it has become more critical. Increased performance requirements on munitions are making it necessary to utilize higher energy explosives. Consequently, explosives tend to become more sensitive and vulnerable to sympathetic detonation as the energy content of the formulation increases.
Explosive compositions have traditionally been developed along three basic avenues. The first of which takes an energetic filler such as cyclotrimethylene trinitramine cyclotetramethylene tetranitramine, pentaerythritol tetranitrate, etc, . . . , and combines it with an energetic binder such as trinitrotoluene or nitrocellulose. These compositions exhibit high energy output with lower concentrations of energetic filler but they tend to be too sensitive for new military applications. The second approach is to combine a high percentage of explosive filler in an inert binder usually an organic wax or polymer. By varying the percentage of explosive filler, the sensitivity and energy output of the material can be changed. Typically, one can improve the vulnerability of the composition by lowering the concentration of filler but this will also lower the energy output. The objective then becomes finding the concentration of binder that lowers the sensitivity to an acceptable level while maintaining as high an energy output as possible.
The third approach is to synthesize new energetic molecules. The explosive formulations developed to date using the techniques described above have not yielded high energy output explosives that demonstrate a low enough susceptibility to sympathetic detonation to be considered for use in insensitive munitions. Previous efforts have failed in this respect in that they did not discover the proper combination filler or binder (i.e. in either chemical type or concentration level) to yield these properties.
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