In order to find a replacement for currently used, but environmentally hazardous, delay mixtures consisting of potassium perchlorate, barium chromate (containing hexavalent chromium), and lead compounds, a collaborative research and development (R&D) project between Innovative Materials and Processes, LLC (IMP) and Ensign-Bickford Aerospace & Defense Company (EBA&D) was conducted.
The main technical objective of this R&D project was focused on the development of a multicomponent environmentally benign pyrotechnic delay system with burn-rate tunability within the required temperature range (-65 °F to 160 °F). The project focused on M201A1 and the M213/M228 fuze assemblies used by the Department of Defense (DoD). M201A1 fuzes require a 1 to 2.3 second (s) burn time from ignition of the M39A1C primer to ignition of an internal output charge. The M213/M228 fuzes have a burn time specification range of 4 to 5.5 s from initiation of the M42 primer to ignition of the C70 detonator. All reactants and additives involved in the selected delay systems were evaluated as prescribed in American Society for Testing and Materials E2552-16.
Based on the thermodynamic analyses and preliminary experimental results collected prior to the submission of the proposal, IMP proposed the use of strontium molybdate (SrMoO4) as the oxidizer in the new delay formulation. The key advantage of this replacement is that SrMoO4 is a very similar oxidant to barium chromate, while meeting all environmental standards. The proposed binary fuel of Silicon and Aluminum, as well as Silicon oxide and Diatomaceous Earth powders, also meet the environmental standards. The technical approach undertaken in this project included: 1) fundamental understanding of thermodynamic and kinetic properties of the proposed gasless reacting system; 2) mathematical modeling of combustion front propagation in the actual device geometry; 3) measurements of burn rates and determination of performance characteristics of the system understudy at different temperatures; and 4) testing of M201A1 and M213/M228 fuzes.
The comprehensive data generated in this R&D project include the effect of the pyrotechnic reactive system stoichiometry, binary fuel ratio, and fuel particle size on inverse burn rate. The developed formulations are demonstrated in two different fuze applications. The proposed gasless reacting system has shown tunable inverse burn rates between 1.33 and 11 s per inch in fuze assembles. The environmentally benign delay formulation was characterized in the M201A1 fuze assembly at -65 °F, 70 °F, and 160 °F with an average burn time of 1.76, 1.66, and 1.56 seconds with standard deviations of 0.05, 0.06, and 0.05 s, respectively. This formulation has also been successfully demonstrated in M213/M228 fuzes.
Material sensitivity characterization (electrostatic discharge, impact, and friction) and, the longterm chemical compatibility of the pyrotechnic delay; as well as, the determination of reaction kinetics and two-dimensional modeling of combustion front propagation in various cylindrical geometries using the COMSOL software is presented. All necessary kinetic parameters, like activation energy, were measured during this R&D project. Using this data, a heat flow model was generated using Solidworks Simulation Software. This model demonstrates the flow of heat through the M213/M228 fuze system and identifies locations within this specific fuze body’s geometry and material that presents propagation concerns. These analyses provided more in-depth understanding of the fuze system and its limitations. Finally, the use of resonant acoustic mixing technology has been the key aspect of making this project successful. The use of this technology has been widely documented and this contribution provides substantial evidence of its viability as a mixing technique.
IMP and EBA&D companies contributed to this project with both environmentally benign M42 primers and C70 detonators. Two primer systems were investigated as potential replacements for the legacy lead styphnate primer. Both metastable interstitial composite and DBX-1 based primer systems demonstrated excellent results. Also, IMP investigated improvement of the M67 fragmentation grenade’s entire firing train to obtain an environmentally benign system, by removing the lead styphnate and lead azide components of the C70 detonator replacing it with DBX-1. The successful results provided the first ever environmentally benign explosive train that has been demonstrated in the M67 grenade system.
Based on the performed experimental and modeling studies as well as successful testing in the fuze hardware, it can be concluded that the new environmentally acceptable delay material exhibits a wide range of tunability and meets required military performance standards. Therefore it could be considered for qualification tests in selected military applications, requiring replacement of the currently used delay mixtures.