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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.

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.

To reduce the chance of accidental explosions and fires, the Navy, Air Force, and Army are replacing existing main charge explosives with new, more insensitive explosives such as PBXN-103 and PBXN-109. For safety, the Navy, Air Force, and Army are replacing present main charge explosives with insensitive main charge explosives having critical diameters greater than 1 inch. The critical diameter for an explosive is the minimum diameter mass of that explosive that can be detonated without being heavily confined. Future underwater and bombfill explosives will have critical diameters greater than one inch.

Two examples of these insensitive main charge explosives are PBXW-124 (27% NTO, 20% RDX, 20% aluminum, 20% ammonium perchlorate, and 13% binder by weight) which has a critical diameter of between 3 and 4 inches, and PBXW-122 (47% NTO, 5% RDX, 15% aluminum, 20% ammonium perchlorate, and 13% binder by weight) which has a critical diameter of 7 inches.

Existing booster explosives and fuses have insufficient energy output to reliably initiate the new insensitive main charge explosives. Increasing the amount of booster explosive will increase the weapon's sensitivity and the chance of an accidental detonation. Moreover, the existing Department of Defense (DOD) inventory of fuses and booster explosives is very large and cannot be replaced without considerable cost. What is needed is an inexpensive method of reliably initiating the new, more insensitive main charge explosive while at the same time reducing the chance of the accidental initiation of a fuse booster system.

There is a desire to move away from trinitrotoluene (TNT) based fills since there is no longer a CONUS producer of TNT in existence. Therefore, it is desirable to develop General Purpose (GP) bombs (500 and 2000 pound class), which contain a non-TNT based high explosive fill that also meets all of the IM criteria as specified in MIL-STD-2105B.

Reaching a 1.2.3J hazard classification allows for significantly higher storage densities than the previous highest classification, which means that it saves in facility storage, handling, and transportation expenditures.

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