In an effort to supply an environmentally friendly propellant for medium caliber ammunition applications that also improves insensitive munitions (IM) characteristics, the U.S. Army Research, Development and Engineering Command (RDECOM) Armament Research, Development and Engineering Center (ARDEC) teamed with ATK to design and produce small quantities of a solventless propellant for the 25mm M793 round. The M793 TP-T is a low-cost target practice cartridge ballistically matched to the M792 HEI-T (High Explosive Incendiary with Tracer) and the MK210 HEI-T rounds.
A propellant manufactured using the solventless process was considered essential to achieve the desired goals of meeting existing performance levels for the M793 round, eliminating environmental problems, and improving the IM characteristics. The solventless propellant manufacturing process relies on energetic plasticizing agents mixed with nitrocellulose to create the appropriate material properties that allow for extrusion of the propellant grains through a heated press. The plasticizers in the propellant formulation give the final product more ductile material properties relative to propellants that are manufactured using a solvent process. This ductility was expected to give the material improved response to impact related IM threat scenarios such as bullet impact, fragment impact, and shaped charge jet impact.
The final formulation that was settled upon was given the identifier, PAP8386. The formulation and grain geometry were determined using computer modeling to estimate the available chemical energy and ballistic efficiency. Once the formulation and geometry were set, propellant was manufactured using the described solventless process. Following manufacture, the base grain was characterized both chemically and physically. This provided verification that the process was capable of matching the target levels for the formulation and physical dimensions within a reasonable tolerance.
The PAP8386 propellant formulation for medium caliber ammunition applications removes several toxins and potential carcinogens and is manufactured using a solventless process that eliminates volatile organic compounds. Tests have shown improved hazard sensitivity and mechanical properties relative to existing fielded propellants in the Army inventory. Ballistic requirements were met in the platform chosen for development, and the propellant was incorporated as part of a system-level approach to improving the IM response of medium caliber ammunition. With the IM testing complete, the next phase involves scaling up the developmental work to full scale production. The results of that effort would be used to complete all qualification testing.
The closed bomb test, a standard device used to measure gasification rates for energetic materials, demonstrated that the PAP8386 propellants burned slightly more slowly than the legacy 25mm propellant and provided roughly equal pressure levels. It provided a strong indication that PAP8386 would be appropriate for the M793 Round.
The final proof of performance for any propellant system is gun testing in the final configuration. Initial firings were carried out in the M793 cartridge using uncoated base grain PAP8386 at ambient conditions (21°C). Charge weights were varied to determine optimal load conditions. These tests indicated that uncoated PAP8386 burned too rapidly for an efficient application of the available energy.
To further improve the interior ballistic efficiency, deterrent coatings are applied to the exposed propellant surfaces as part of the finishing process. Deterrent coating at Radford Army Ammunition Plant was accomplished on a small scale using a proprietary system. A combination of penetrant and inhibitor deterrents was applied to the grains as part of a coating optimization investigation. Multiple deterrents were implemented based on previous work involving highly plasticized propellants. A closed bomb test provided a strong indication that the proper blend of coated and uncoated material would be able to achieve the performance requirements of the M793 cartridge.
After the deterrent coating process was completed, additional ballistic tests were conducted. A blend of uncoated and coated propellants was shot in an effort to meet the ballistic targets. Once again, all testing was carried out at ambient temperatures (21°C) in the M793 cartridge. From the intermediate tests, a final blend configuration was chosen for a final set of ballistic tests. The blend was able to meet the ambient performance requirement of the M793 cartridge. Additional ballistic testing was then conducted across the operating temperature. While hot propellant performance was well within specification, the candidate propellant had increased ballistic pressure at cold temperatures. This was attributed to thin walled propellants and grain fracture under extreme cold conditions coupled with base-pad ignition.
An ignition system utilizing a flashtube and three booster pellets was selected to continue the propellant development work. The rounds with this ignition system showed a slight increase at cold temperatures, but this increase was well within acceptable limits. This configuration was able to reach the -46°C requirement from the performance specification for the M793 round without exceeding the allowable pressure. Velocity and action time for these rounds also remained within acceptable limits.
To develop the potential IM improvements fully, a system-level approach was selected. In addition to the new propellant, which should provide some gains in IM response, a new cartridge case and container were used in the IM testing. The cartridge case and container were both designed and manufactured by ARDEC and are intended to prevent dangerous pressure build-up in the event of a fire.
The IM testing was conducted in March 2010 at National Technical Systems in Camden, Arkansas. The IM testing was guided by MIL-STD-2105C and the recommendations of the Joint Services Insensitive Munitions Technical Panel. Tests conducted include Fast Cook-Off, Slow Cook-Off, Bullet Impact, Fragment Impact, Shaped Charge Jet Impact, and Sympathetic Detonation. IM testing of the system combining the new propellant and the vented case demonstrated clear improvements over current systems. In none of the tests was a reaction worse than a Type IV experienced.