As mentioned above, the detonation of munitions on the flight deck of the aircraft carriers and the in-bore explosion of projectiles in large caliber guns motivated the Navy to change the direction its fiscal year 1970 explosives research and development program. Rather than continuing the search for higher energy explosives, we directed the scientists at the Navy Laboratories to concentrate their research on explosives that were insensitive to heat and gun set-back pressures. This, we believed, could provide solutions to the explosive materials sensitivity problems mentioned above. The knowledge gained at White Oak and China Lake some years earlier while studying the detonability of solid rocket propellants would serve as a starting point for the development work.
The Naval Surface Warfare Center, White Oak, MD issued a number of contracts to private industry. Under one of these contracts, Aerojet General Corporation scientists modified the propellant used in Polaris missile to make it detonable. Aerojet had been involved in the BEAUREGARD and SOPHIE propellant sensitivity studies mentioned above and had manufactured Polaris rocket motors for the Navy.
Laboratory and field test data had established that modified composite rocket propellants were relatively insensitive to shock and were difficult ignite. Further, once ignited it was difficult to maintain combustion at ambient pressures. The propellants under study often extinguished when the pressure in the combustion chamber was dropped to ambient atmospheric pressure.
For these reasons we believed that, once modified to make them detonable on demand, these propellant type compositions could solve the Navy projectile in-bore explosion problem. There were no data available then that indicated that compositions comprised of RDX or HMX in inert rubbery binders would improve the impact, friction, and thermal sensitivity characteristics of the explosives.
Laboratory test data soon showed that PBXN-106, the formulation developed by Aerojet was considerably less sensitive to shock than Composition A-3 or Composition B explosives. The energy content was about the same for all three explosives and thus there would be no degradation in warhead performance. Eventually, PBXN-106 was recommended for use in the Navy 5-inch projectiles.
Laboratory investigations with PBXN-106 provided the first indication that formulations made with RDX or HMX in tough rubbery binders reacted very mildly when exposed to intense fires and were insensitive to the bullet impact. Mr. Harry Heller of the White Oak laboratory first reported this improvement in sensitivity to heat and bullets to me in late 1969.
In the early 1970’s, The Naval Air Warfare Center, China Lake, CA was also conducting tests with PBXs made with rubbery binders. China Lake scientists loaded one of their explosives, PBXN-107, into a ZUNI warhead and conducted a JP-5 fuel-fire cook-off test. The ZUNI warhead provided heavy confinement for the explosive. Another ZUNI warhead was loaded with composition B and subjected to the same test. ZUNI warheads loaded with Composition B, the usual explosive load for that weapon, detonated in the fire. In marked contrast, PBXN-107 merely split open the steel case and left the warhead metal parts hanging from the stand. In this case, the explosive burned with no violent reaction observed.