Advancements in the Operation and Fabrication of Electrically Controlled Solid Propellants

Abstract

An experimental study was conducted to investigate and compare the ignition and combustion characteristics of nitrate and perchlorate-based electrically controlled solid propellants (ECSPs). Sodium and lithium-based nitrates and perchlorates were formulated with polyethylene oxide as the binder and carbon black as a conductive additive via a solution casting process. The resulting propellants formed a polymer electrolyte via the formation of complex between the salt and polymer which was either confirmed by Fourier-transform infrared spectroscopy analysis or suggested by differential scanning calorimetry. Each nitrate-based oxidizer provided faster ignition than its perchlorate counterpart across a range of applied voltages, with the LiNO3-based propellant yielding the fastest ignition among all the propellants. IR imaging of the ignition process revealed that the nitrate-based propellants initially heated at the cathode whereas perchlorate-based formulations preferred the anode, highlighting some of the fundamental differences in the ignition mechanisms. The nitrate-based propellants exhibited higher pressure deflagration limits than the perchlorate-based propellants, with the NaNO3-based propellant establishing the highest pressure deflagration limit (> 13.8 MPa). Strand burning experiments were conducted for the nitrate-based propellants and a modified St. Robert's Burning Rate Law was established accounting for varying both pressure and voltage. The nitrate-based propellants exhibited a greater sensitivity to voltage than previously reported sensitivity of perchlorates. The results of this study indicate that while perchlorate based ECSPs hold an advantage in terms of performance, nitrate-based propellants are advantageous for faster responsiveness and a greater operating envelope. A second study explored the direct ink writing of ECSPs using an ultraviolet and thermally curable binder based on polyethylene glycol diacrylate. This work presents the first known fabrication of an ECSP using additive manufacturing techniques, as well as the first example of an electrically ignited solid propellant with ammonium perchlorate as the sole oxidizer. Ammonium perchlorate and lithium perchlorate propellants with carbon black additive concentrations from 0 to 5 wt% were formulated and investigated to determine the influence of the additive on curing properties. Thermal decomposition, ignition delays, and burning rates were investigated for propellants containing 2.5 wt% carbon black. Cure depth results revealed diminishing cure depths with increasing carbon black concentrations due to UV absorption by carbon black. The cure depths of propellants ranged from 0.18 to 4.68 mm. Ignition delay experiments exhibited an inverse relationship between ignition delay and applied voltage for the lithium perchlorate propellant and no voltage dependence of the ignition delay for the ammonium perchlorate propellant. The ammonium perchlorate propellant showed considerably lower ignition delays compared to those of the lithium perchlorate-based propellant at all voltages. The lithium perchlorate propellant demonstrated significant ignition delay sensitivity with print orientation, while the ammonium perchlorate propellant did not. Pressurized combustion experiments demonstrated the capability to throttle the burning rate of lithium perchlorate propellant by changing the voltage magnitude and illustrated higher burning rate sensitivity to voltage rather than pressure.