Understanding the pyroelectric combustion behaviour of metallized electrically controlled solid propellants

Abstract

Solid rocket propellants that demonstrate variable burning rates are highly desirable in various propulsion applications. Electrically controlled solid propellants (ECSPs) are one such kind that ignite and combust only when external electric power is applied, which enables to adjust/control their combustion rates. However, the knowledge on their combustion mechanisms is scant in the limited literature. In this study, pyroelectric combustion behaviour of metallized ECSPs, based on lithium perchlorate (LP) and poly vinyl alcohol (PVA), containing 5%, 10%, and 15% tungsten (W) are investigated, in comparison to the non-metallized baseline propellant. Theoretical performance parameters, heat release and mass loss from decomposition process, and combustion rates are obtained. Pyroelectric combustion of ECSPs involves electrolytic decomposition of LP producing oxidizer species, which react with fuel species from PVA and/or W in thermochemical reactions, to generate heat and products including LiCl, HCl, LiOH, WO3, etc. Results indicate that addition of W reduces the theoretical adiabatic combustion temperature and overall heat release from exothermic decomposition by 19% and 36% respectively, in ECSP-M15 containing 15% W, compared to the baseline case. However, relatively low voltage response time for metallized ECSPs implies that their ionic conductivity is higher than the baseline ECSP. More importantly, combustion rates of all ECSPs increase exponentially with increase in applied initial voltage in the 100–500 V range. Inclusion of W enhances the combustion rates significantly by 4.9 times at 100 V and by 1.7 times at 500 V for ECSP-M15 relative to the baseline propellant. This signifies that electrochemical mechanisms become the rate controlling step at high electric power, in governing the pyroelectric combustion behaviour of ECSPs. Further, these propellants were successfully tested for multiple burn/no-burn conditions under the application/removal of the external electric power, which is highly desirable for various propulsion systems.