A new explosive formulation improves insensitive munitions characteristics without affecting production processes.

AFRL scientists from the High Explosives Research and Development facility successfully developed, demonstrated, and transitioned a nextgeneration melt-castable explosive formulation. The new formulation, MNX-795, exhibits significantly improved insensitive munitions (IM) characteristics—a requirement for the formulation’s intended use in the MK- 84 bomb.

On eight different occasions, the original melt-castable formulation developed for the MK-84, AFX-794, failed to pass the IM criteria in simulated fuel fires known as fast cook-off (FCO) tests. The MK-84s used in these tests employed a variety of pressure venting technologies, but in all instances, their engineered vents failed to function quickly enough to reduce the deflagration/ burn reaction to a level sufficient for meeting IM criteria. When the scientists tested an AFX-794 configuration with venting enabled prior to burn initiation, the AFX-794 did meet IM criteria, indicating that current venting techniques function too slowly to adequately vent the formulation’s decomposition gases. Scientists confirmed their hypothesis after conducting a subsequent MK-84 FCO test in which the explosive began to vent from the center of the bomb via the arming well, causing another failure. To provide the additional time necessary for these venting technologies to function, scientists proposed the addition of a thermal coat to the warhead’s exterior skin in order to reduce heat transfer to the explosive. However, because the thermal coat would increase the difficulty of fitting warheads to the current guidance kits, the recommendation was not adopted.

Scientists instead chose to leverage technology recently developed for an explosive formulation called MNX-195. The Army is evaluating MNX-195 as a potential IM-compliant, Composition B explosive replacement. As part of the evaluation effort, researchers modified MNX-195’s original wax-based, meltcastable binder system—the same binder employed in AFX-794. This new technology, described in Air Force Patent Disclosure AF-746, replaces a wax constituent used in first-generation, plasticized, wax-based binders with a liquid phosphate-based surfactant. During MNX-195 development, researchers observed that the incorporation of this surfactant reduced reaction violence during FCO testing. Specifically, the high-energy MNX-195 exhibits a reaction more benign than that of a comparable explosive formulation, despite the fact that MNX-195 contains more crystalline energetic material. The scientists expected the surfactant additive to modify the explosive’s burn rate by quenching radicals generated during decomposition (i.e., by acting as a fire retardant during prolonged thermal events). Upon further investigation, they found that the new binder reduced gas generation in these events by a factor of 6. The buildup of decomposition gases is the primary mechanism for propagating a reaction in FCO events, since the decomposition gases are autocatalytic.