The Effects of Fuel-To-Oxidizer Ratio on HAN Based Electric Solid Propellants

Electrically activated solid propellants (ESPs) are a new type of solid propellant that is quickly proving to have a wide range of benefits for spaceflight and other applications. They will only ignite when an electrical current is applied, and their combustion rate depends on the power applied to the propellant. When the electrical current is stopped, the combustion of the propellant also stops. Because of these properties, they have a lot of potential in the field of space propulsion, where the benefits of a solid rocket engine are not attainable due to limits with traditional solid propellants. Currently, most ESPs use a stoichiometric fuel-to-oxidizer ratio. This provides the most efficient combustion of the fuel and oxidizer; however, fuel-rich ratios may offer tactical or system-level performance benefits, depending on the specific application, including reduced flame temperature and alternative products (e.g., CO, H2). This project aims to investigate the effects on the propellant when the fuel-to-oxidizer ratio of the propellant is increased from stoichiometric, i.e., toward fuel-rich ratios. Initial calculations were performed using CEA software to predict the combustion characteristics and propulsive performance of ESPs. From this, the predicted trends of increasing the fuel-to-oxidizer ratio were determined, with several significant mixtures being determined. Once this had been done, two of these mixtures had exhaust products measured, with oxidizer: fuel ratios of 80:20 and 60:40. Attributes measured from the experimental testing would then allow for the CEA theoretical calculations to be compared with the experimental results to determine the accuracy of the calculations. From this data, it was found that there was an error of 7.78% and 4.97% between the propellants tested, indicating that the CEA and experimental data differ, though not by a large amount, and thus the CEA calculations can likely be trusted for calculating performance at different fuel-to-oxidizer ratios. Electrical parameters of the propellants through varying fuel-to-oxidizer ratios were also measured. The oxidizer: fuel ratio of the propellants was 80:20, 60:40, 55:45, 45:55, and 35:65. From testing, it was found that the ignition voltages of the propellants were 225V, 300V, 325V, 375V, and 400V, respectively. The trend seen was linear, with the ignition voltage increasing with the fuel-to-oxidizer ratio. From the CEA analysis, there were potential benefits seen from fuel-rich mixtures, which were determined to be accurate from RGA measurements. Electrical response testing found that as the fuel-to-oxidizer ratio increased, the ignition voltage of the propellants also increased.