The origin of the Insensitive Munition concept

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As is often the case, the evolution of the Navy’s view on Insensitive Munitions was not completely devoid of human experience. Rather, it followed several disastrous incidents involving military hardware and emerged from experience gained serendipitously by Navy scientists while trying to solve other problems.

The “beginning” that led to the Insensitive Munitions concept was in the mid 1950’s and was twofold. First, there was a need then for solid rocket propellants with high specific impulse to meet the operational requirements of the new tactical and strategic weapons. This requirement resulted in the development of some high-energy solid propellants that had some, but not all, of the sensitivity characteristics of high explosives.

Second, aerodynamic heating problems were encountered with munitions carried externally on high-speed aircraft and with warheads on some tactical missiles. There was at least one case at the White Sands, New Mexico test range where a TNT-based explosive warhead on a surface to air missile had exploded violently before reaching the target.

The cause of the missile warhead explosion was attributed to aerodynamic heating. TNT has a melting point of about 176°F. Upon melting, it expands by about 10 to 12 percent by volume. Air friction resulting from a missile or an aircraft traveling at high velocities can raise the temperature of a warhead and cause the melting of some of the TNT. Because of the volume expansion, molten explosive may be forced into critical areas where pinching, friction or heat can cause it to react explosively.

After the premature event with the surface to air missile at the White Sands test range, the Navy laboratories developed Plastic Bonded Explosives (PBXs) for use in Navy warheads. These first PBX’s were pressed materials and were based on technology developed at the Los Alamos National Laboratory, Los Alamos, NM and Livermore National Laboratory, Livermore, CA. Thus, the first Navy PBX compositions were similar to some of the materials used in nuclear weapons. These explosives were comprised of crystals of HMX8 held together with a small quantity of a thermoplastic binder.

Pressed PBX’s solved the aerodynamic heating problem for some but not all warheads. These were difficult to load into warheads. Large hydraulic presses were used to compress the explosive in a die and then the resulting explosive charge was machined to size to fit into the warhead case. Pressed PBXs could not be used in large munitions like bombs or in warheads that contained internal plumbing to accommodate electrical fuze connections. The cost of machining the explosive charge to fit through warhead loading ports and, once inserted, made to conform around the internal plumbing were prohibitive.

To solve this problem, the Naval Air Warfare Center, China Lake, CA developed a castable composition, PBXN-101. PBXN-101 was comprised of HMX in a polyester thermosetting plastic binder. It was developed for use as the main charge in the SHRIKE anti-radiation missile warhead. This was the first warhead to use a cast PBX composition. The SHRIKE air to surface, anti-radar missile was introduced into the Fleet in 1956. PBXN-101 was a brittle explosive optimized for performance and, by modern standards, was quite sensitive to shock, heat and bullet and fragments impact. PBXN-101 is no longer approved for use in Navy weapons.

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8HMX is 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane.