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PBXN-109

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 would 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 had insufficient energy output to reliably initiate the new insensitive main charge explosives. Increasing the amount of booster explosive would 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.

Explosives are normally designed to provide good performance for a specific application. For example, PBXN-110, composed of 88% HMX and 12% polymeric binder, is designed to fragment a warhead's metal case and drive the fragments at high velocity, whereas PBXN-109, composed of 64% RDX, 20% aluminum, and 16% binder, is designed for internal blast applications. Other explosives have been designed to work well for underwater/underground applications such as explosive PBXN-111 composed of 20% RDX, 43% AP, 25% aluminum, and 12% binder. However, none of these explosives can optimally perform in all three missions.

Castable high explosives are usually prepared by combining an explosive ingredient, such as a nitramine, with a curable binder and optionally a reactive metal and an oxidizer. One widely used binder is hydroxy terminated polybutadiene (HTPB). HTPB is cured using conventional diisocyanate curing agents and cure catalysts.

PBXN-109, a typical HTPB explosive, contains 20 wt. % Aluminum, 64 wt. % RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), and 16 wt. % binder system containing HTPB, DOA (dioctyladipate), and IPDI (isophorone diisocyanate). The binder system may also contain a plasticizer to aid processing. Once the ingredients are mixed, the cure reaction begins which causes viscosity to increase until the polymer is fully cured. PBXN-109 has a detonation velocity of about 7600 m/s.

PBXN-103 is designed for underwater use or other low oxygen environments. It contains a large amount of ammonium perchlorate and aluminum to provide the high combustion temperature. The PBXN-103 explosive also contains high quantities of plasticizer relative to the PNC binder. PBXN-103 is a low brisance explosive, having a relatively low detonation velocity of about 6000 m/s. Because of the low detonation velocity, PBXN-103 is not well suited for use in high performance precision shaped charges and EFP's (explosively formed penetrators).

During the response of energetic materials to hazards such as thermal or mechanical stimuli, the initial low-level reaction releases sufficient energy to cause an increase in pressure and temperature, which then leads to acceleration of the reaction until a runaway condition is reached. Accurate knowledge of the reaction rates at conditions typical of those in accelerating reactions is necessary to understand and predict the violence of the ensuing explosion. Materials with reaction rates that are strongly accelerated by pressure and temperature generally give more violent responses than materials whose reaction rates are less sensitive to pressure and temperature.

The polymer bonded explosive PBXN-109 has a composition of 65% RDX, 21% aluminum, and 14% HTPB binder plasticized with DOA. For materials with a low pressure dependence of high-pressure deflagration, such as PBXN-109, the reaction rate is accelerated at a relatively low rate. This leads to relatively low violence of thermal explosions. The deflagration behavior of PBXN-109 is remarkably stable over the entire pressure range. The deflagration behavior of PBXN-109 after thermal damage is very important to the eventual violence of thermal explosion. This behavior is clearly dependent on the nature of the thermal damage. Instead of characterizing the extent and type of damage caused in PBXN-109 by high temperatures, Lawrence Livermore National Laboratory [LLNL] chose to thermally damage the PBXN-109 using time-temperature profiles close to those in actual NAWC-China Lake experiments.

Composition B gives quite violent thermal explosions. Composition B is 63% RDX, 36% TNT, 1% wax. Composition B exhibited deflagration behavior unlike that of other materials that LLNL tested. In virtually every case, the first few pellets burned relatively slowly and with apparent uniformity, but later pellets burned very rapidly and erratically. The burning of the first few pellets, shows a second-order pressure dependence, very high compared with previous results with HMX-based explosives' and AP-based propellants' and current results with PBXN-109. Second, the onset of rapid burning presumably represents a deconsolidation of the sample during the deflagration process.

With Composition B, the first few pellets burn with apparent uniformity and the following pellets burn rapidly, regardless of the initial pressure. Therefore the deconsolidation process in Composition B seems to be time dependent instead of pressure dependent as in HMX-based explosives. This may be a result of TNT in the high-temperature environment of the pressure vessel - the first few pellets may burn uniformly before the heat can penetrate and melt the TNT, leading to subsequent mechanical failure and exposure of the RDX crystal surface to flame. The deconsolidation leading to high deflagration rates seen with Composition B should drive the violence even higher.