Recent Advances in Insensitive Munitions

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 next-generation 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.

MK-84 warhead prepared for loading

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, melt-castable 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.

Based on these test results, researchers replaced the binder system utilized in AFX-794 with the binder system developed for MNX-195, subsequently arriving at a new formulation— MNX-795—for use in the MK-84. They demonstrated the merits of MNX-795 in a series of small-scale, unvented FCO tests. On a scale of 1-10, with R-1 representing the mildest reaction (i.e., burn) and R-10 representing the most violent of reactions (i.e., detonation), MNX-795 demonstrated a mild deflagration reaction (i.e., R-3), whereas AFX-794 produced a severe explosion (i.e., R-8). Further, the MNX-795 formulation exhibited other desirable characteristics, such as decreased end-of-mix viscosity, shorter mix times (due to the addition of the surfactant), and a smaller change in volume upon solidification. The smaller volume change results in an end product with less surface cracking and more consistent density.

Based on these promising preliminary results, the research team next focused its efforts on the tasks required both to obtain full scale-up approval and interim hazard classification for MNX-795 and to schedule production loading trials at the designated Army ammunition plant (AAP). After receiving the appropriate approvals, AFRL researchers provided the support necessary to address the AAP's processing issues and verify its quality assurance practices. AAP personnel loaded six MK-84s (see figure) and subsequently reported equivalent or improved load quality as compared to bombs loaded with AFX-794. Furthermore, the AAP accomplished the loading procedure without altering its production processes or decreasing current throughput.

These efforts culminated in two full-scale FCO tests of MNX-795-loaded MK- 84s, successfully meeting program requirements and enabling the $27 million program to proceed to weapons qualification testing. Current engineering assessments predict that MNX- 795-loaded MK-84s will pass the four remaining IM tests, meet all program goals, and provide the much safer explosive fill required to meet future MK-84 production requirements.

Mr. John A. Cominiello and Dr. Thomas R. Krawietz, of the Air Force Research Laboratory's Munitions Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp . Reference document MN-H-05-13.