Consideration of the energy transfer mechanisms involved in SPETC ignition systems

Solid propellant electrothermal-chemical (SPETC) gun concepts have evolved over the past decade. A common difficulty experienced by many of the "large-scale" applications of SPETC technology has been the excessive electrical energy and power requirements needed to produce significant performance benefits. However, one aspect of SPETC gun technology that has attracted a lot of interest is in the area of plasma ignition due to its lower power requirements. It has been shown by various groups that plasma ignition offers benefits over a conventional gunpowder ignition system at large scale. High-energy electrical discharges previously used for ignition can potentially lead to adverse internal ballistics. Effort has recently been directed at reducing the ignition energy stimulus with the result that electrical discharge energies of a few tens of kilojoules are now routinely used to ignite large caliber gun system propellant charges. This not only gives improved internal ballistics but also reduces the cost, size, and complexity of SPETC gun components. An understanding of the energy transfer mechanisms involved during the ignition phase, which allows the electrical energy to be transferred to the surface of propellants, is central to reducing the electrical energy requirements, and these mechanisms are the subject of this paper. There is some evidence for radiative energy transfer being the dominant transfer mechanism for some higher power plasma igniters. However, with low power density and low energy ignition, the radiant flux is negligible in the timescale under consideration, suggesting that radiation is not the dominant process. Supporting experimental evidence from measurements of the radiant flux incident upon the propellant grain surface is offered. These measurements have been performed with the use of embedded optical fibers in translucent propellant formulations. The light collected by these fibers is input to an absolutely calibrated time-resolved spectrograph. A new hypothesis is put forward, which proposes that the energy transfer mechanism for low-power SPETC ignition is dominated by a process of metallic vapor deposition. Modeling and experimental results are presented, which support this hypothesis.