The introduction of castable plastic bonded explosives, PBX’s, in Navy munitions

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In the late 1960’s and early 1970’s, the Navy introduced the use of Plastic Bonded Explosives as the main charge in underwater weapons. As mentioned above, the MK-46 torpedo warheads were loaded with PBXN-103, a double base propellant type formulation. This was soon followed by loading PBXN-103 in the MK-48 torpedo warhead.

The reason these explosives were used was not because of their desirable sensitivity characteristics. Rather, it was because they increased the lethal area of underwater munitions by at least 50 percent compared to the TNT, RDX, and aluminum powder formulations that they replaced. It was an added benefit however, that these explosives were considerably less sensitive to shock and impact but weapon designers did not appreciate this fact at that time.

The guiding criteria for Navy explosive research and development programs until about 1967 were to strive for maximum performance within acceptable handling and transportation safety limits. Generally, this meant trying to pack as much energy as possible into a given volume; the higher the energy/density, the better. The sensitive PBXN-101 formulation mentioned previously was an example of this focus on energy. There was no great desire to fund programs to utilize propellant technology to develop less sensitive explosives because the sensitivity of munitions to extremes of heat, shock and impact was known but not recognized as a problem.

Navy munitions were tested to confirm that they were safe to manufacture, transport, store, and use. All of the explosives used by the Services met the Department of Transportation safety requirements for transportation and storage. In addition, the warhead loading production-base in the United States was almost entirely designed for melt-cast TNT-based explosives. Manufacturing solid propellant type compositions required the use of high-shear mixers. The government explosives loading plants did not have many of these mixers and there was no great desire to give this work to the commercial propellant manufacturers.

Program managers argued that the cost of loading cast-cure solid propellant type formulations in high-shear mixers would add an unnecessary cost to weapons. Vulnerability of weapon stores and ships was of no concern. Safety engineers accepted, as a fact of life, that munitions were dangerous and could be detonated by enemy action. Also, in some cases, the total store could detonate. Accidents involving fires and munition explosions and gun-barrel explosions were not expected to happen very often. These were believed to be only remote possibilities.

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