Tests to study the detonability of solid rocket propellants

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In late 1957, the Navy Bureau of Naval Weapons9 initiated a program to study the detonability of solid rocket propellants. Sensitivity data were required to assess the safety characteristics of large missiles that were to be launched from submarines. At first, the effort was focused on determining whether confined, small diameter charges of high energy composite propellants made of ammonium perchlorate, aluminum, and cast plastic binders could be detonated. The Naval Ordnance Laboratory (NOL), White Oak10 developed the Large Scale Gap Test (LSGT) to measure the shock sensitivity of solid propellants.

The NOL Large Scale Gap Test was essentially a modification of the test developed by Eyster, Smith, and Walton.11 In the NOL LSGT test, polymethyl methacrylate disks are placed between a standard booster explosive, the donor, and the test material, the acceptor to attenuate the shock. The acceptor charge is confined in a steel tube.

The thickness of plastic barrier is varied until a detonation of the test material is achieved in 50% of the trials. The thicker the plastic barrier required to prevent detonation of the acceptor charge, the lower the shock sensitivity of the material under test. A steel witness plate is used to determine if the test material detonated.

The propellant formulations tested were sensitive to impact as measured by drop-weight impact machines but were much less sensitive to shock as measured by the LSGT than the standard high explosives. The ammonium perchlorate, aluminum, and polyurethane binder composite formulations did not detonate even when no plastic barrier was used between the donor and the propellant. Double base propellants made with nitroglycerine and nitrocellulose did detonated however, and some of these caused more damage to the laboratory test chambers than the high explosive compositions then used in service munitions.

In 1958, Navy scientists questioned the validity of extrapolating the data from the NOL LSGT to support the shock sensitivity assessments being made for large composite rocket propellants. The explosive sensitivity experts were concerned about the possibility that a large Polaris rocket motor could detonate when the missile was launched from a submarine. Although the Polaris propellant did not detonate in the small diameter shock sensitivity tests there was concern that the propellant grain in the rocket motor might be larger than the critical diameter12 of the propellant formulation used. If the size of the propellant charge exceeded the critical diameter, the motor might detonate when the missile was launched from a submarine.

The Bureau of Naval Weapons tasked the Naval Surface Warfare Center, White Oak Laboratory to perform large-scale shock sensitivity tests to determine whether the Polaris rocket motor could be detonated. These tests, called the BEAUREGARD13 tests, were configured essentially like the NOL Large Scale Gap test
discussed above but much larger. It consisted of 730 pounds of Composition B (RDX, TNT, and wax) high explosive detonated in contact with about 2000 pounds of propellant. No attenuator was used since we did not believe that the propellant would detonate. The diameter of the propellant acceptor charge tested was the same as the largest cross sectional area of one web of the propellant grain used in the Polaris rocket motor.

Beauregard test set-up

The tests were performed at the Naval Air Warfare Center, China Lake, CA in 1958 and 1959. The shots were fired at ambient temperature and then at -65°F and +150°F. The Polaris propellant did not detonate in the BEAUREGARD test but it did contribute some to the blast overpressures measured. The question regarding the conditions or size which could result in a detonation of the Polaris propellant was not answered.14

During the BEAUREGARD test series, Mr. Harry Taylor of NSWC, China Lake told us of some work with nitrosol propellants that led him to believe that these materials might make very powerful explosives. The nitrosol propellant consisted of pelletized nitrocellulose, ammonium perchlorate, aluminum, and some nitrate ester plasticizers. He suggested that we perform some gap tests at the White Oak laboratory to evaluate the sensitivity and the potential for using this propellant material in warheads.

When the nitrosol propellant test charges were fired at White Oak to determine their shock sensitivity, the blast from these low detonation velocity – high positive impulse explosives literally blew nails out of the bombproof roof. Dr. Donna Price, Dr. Jim Ablard, and Dr. Sig Jacobs, and other senior explosives scientists at the White Oak Laboratory, were impressed with the results and believed that these would make powerful underwater explosives.. As a result Mr. Max Stosz was assigned the task of reformulating the propellant to optimize the formulation for potential use in underwater weapons.

His work resulted, eventually, in the development of PBXN-103, a very powerful underwater explosive comprised of ammonium perchlorate, nitrocellulose, metriol trinitrate and triethylene glycol dinitrate plasticizers, aluminum powder, and stabilizers. Though this explosive was ready for application in the mid 1960s, it had one disadvantage that delayed its use in Navy weapons.

Chemically, the energetic nitrate ester binder and the plasticizers in PBXN-103 were similar to that used in single and double-base rocket and gun propellants. Because nitrate ester propellants decompose by an autocatalytic chemical reaction, a surveillance program is required to monitor the depletion of the stabilizer molecules used in the compositions. Until that time, there were no Navy high explosive compositions in service weapons that required a surveillance program.

What finally forced a Navy decision to use PBXN-103 was the discovery that H-615, the explosive used in the MK-46 torpedo warhead, would probably not defeat the intended submarine targets. Thus, PBXN-103 became the first solid rocket propellant-type high explosive used in warheads.

The NAVORD document16 that approved PBXN-103 for operational use in the MK-46 Torpedo warhead included the following provision:

“… because this explosive is chemically similar to a missile propellant, it has similar storage requirements, and proper surveillance must be maintained to assure that, with the passage of time, chemical stability remains at an acceptable level. The Naval Ordnance Station/Indian Head (NOD/IH) will establish and monitor a program for proper chemical surveillance of the explosive…”

As follow-on to the BEAUREGARD test series mentioned above, the Navy funded the SOPHIE program. This was to further explore the conditions under which composite ammonium perchlorate, aluminum, and polyurethane binder rocket propellants could be made to detonate. Varying amounts of the high explosive RDX were added to the propellant formulation to reduce the critical diameter of the charges.

These sensitivity tests and other tests that followed confirmed that these relatively insensitive composite materials could be made to detonate when RDX was added and, when detonated, were indeed very powerful explosives. The results showed that modifying composite propellants to make them detonable might be one way to avoid the surveillance problem encountered with PBXN-103. The SOPHIE program follow-on efforts resulted, eventually, in the development of PBXN-105, an ammonium perchlorate, aluminum, RDX, energetic plasticizer, and polyurethane binder explosive. The Aerojet Company, Sacramento, CA. working under a Navy contract developed PBXN-105. This explosive was essentially a modified POLARIS rocket motor propellant. Eventually PBXN-105 replaced PBXN-103 as the main explosive charge in MK-48 torpedo warheads.

As one can surmise from the above discussion, the Navy scientists who performed the sensitivity tests on the castable propellant formulations were also involved in the development and testing of high explosives for warheads. It was only natural that they recognized the performance and safety improvements made possible with these less sensitive materials and programs were proposed to apply the propellant technology to warheads. Thus, it was no accident then that the insensitive PBX compositions that became available for use in Navy warheads in the 1960’s had physical, chemical, and sensitivity characteristics that were similar to that of the tough rubbery solid propellants used in rocket motors.

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9The portions of BUWEPS that was involved in this later became the Naval Ordnance Systems Command and still later, the Naval Sea Systems Command. Other portions became the Naval Air Systems Command.
10The Naval Ordnance Laboratory, White Oak has undergone a series of name changes over the years. As of this writing, it is a part of the Naval Surface Warfare Center, Indian Head Division. For the remainder of this document, the new name will be used.
11Eyster, A. E., Smith, L.C. and Walton, S.R., “The Sensitivity of High Explosives to Pure Shocks”, NOLM 10,336, July 1949.
12Critical diameter is the smallest cross sectional area of a bare test material that can sustain a stable (steady-state) detonation reaction. Critical diameter is sometimes called the “failure diameter” of an explosive material.
13Banas, L. S., “BEAUREGARD” Critical Diameter Tests of Second Stage Minuteman Wing VI Motor Propellant, AFRPL-TR-65-5, Air Force Rocket Propulsion Laboratory, Edwards AFB, CA, January 1965.
14Jaffe, I, Beauregard, R, Amster, A, Detonability of Solid Propellants III. Shock Sensitivity of Large Diameter Charges of Polaris Propellant, NAVORD Report 6289, March 1959.
15H-6 is a high-explosive composition comprised of RDX, TNT, Aluminum, and D-2 desensitizer wax.
16NAVORDNOTE 8010, ORD-0332B:CFD of 17 March 1970; Subj: Use of PBXN-103 in the Torpedo (MK-46) Warhead MK-103 MOD 1.