Effect of fuel–to–oxidizer ratio on HAN–based electric solid propellants

This thesis will explore the effects of fuel – to – oxidizer ratio on the performance of HAN-based electrically activated solid rocket propellants. The study has three sections, a theoretical section and two experimental sections. The first section of the study will use software and equation-based modelling to calculate performance changes across different fuel – to – oxidizer ratios. The equilibrium stoichiometric mixture ratios for electric solid propellants are an 80:20 HAN: PVA mixture, however mixtures as high as 65:35 PVA:HAN were found to exhibit potentially desired behaviors. As a mixture becomes fuel rich, the performance will generally decrease, however the combustion temperature will also decrease, having potential benefits for the propulsion system. The highest theoretical performance was at a 70:30 HAN: PVA mixture, with performance being 1.8% higher than a stoichiometric mixture at a 2.94% lower combustion temperature. At a 60:40 HAN: PVA mixture, performance is equivalent to a stoichiometric fuel – to – oxidizer ratio, however the combustion temperature is 12.1% lower. This trend continues, with a 60PVA:40HAN mixture exhibiting a 15% decrease in performance and a 40% decrease in combustion temperature. Lower combustion temperatures will open the possibility of lighter, simpler, and cheaper thrusters using electric solid propellants. Theoretical calculations also showed benefits to fuel rich propellants in usage cases such as a gas generator. The amounts of CO and H2 in the exhaust was calculated to be present at higher fuel – to – oxidizer ratios, which previous work has determined would be useful for applications in multimode propulsion. At higher fuel – to – oxidizer ratios, equation-based modelling showed that the initial electrical and heat conductivity, as well as the specific heat capacity of the propellant would be lower. This affects the initial conditions of the propellant, which affects both the ignition as well as the combustion behaviors of the propellant. The second section of the project determined the accuracy of the theoretical calculations by measuring the combustion products of various propellant mixtures and comparing them to the predicted values. From measured combustion products, the molecular weight could be calculated and compared to the predicted values. Mixtures tested include HAN: PVA ratios of 80:20, 70:30, 60:40, 50:50, and 40:60. The error between theoretical and experimental molecular weights was determined to be -6.99%, -4.89%, -4.97%, 6.64%, and 17.81% respectively. There are several potential sources for this error, primarily the mixing process and the combustion processes. Water from the mixing process is likely found in the propellant, becoming part of the exhaust products. Some of this water could also undergo electrolysis, increasing the H2 content of the exhaust. At higher fuel – to – oxidizer ratios, a steep temperature gradient will exist during combustion, differing from assumptions used in CEA and kinetic modelling. This likely causes differences in the exhaust composition of the propellant as some reactants don’t have the required energy to proceed as CEA predicts. Reaction rates also influence exhaust composition, as complex molecules are likely to move away from the heat of the reaction before fully forming into simpler molecules. These factors lead to higher fuel – to – oxidizer ratios having higher molecular weights than predicted, while oxidizer rich propellants tended to have lower molecular weights than perdicted. Finally, the third section of this project aimed to determine how the electrical performance of the propellant changed as the fuel – to – oxidizer ratio was increased. One aspect of this section was the ignition voltage of the propellant, as this will be important to understand for real-world operations of electric solid propellants. Another aspect is with burn progression, as any differences across different fuel – to – oxidizer ratios will as be important to note for real-world operations. It was found that as the fuel – to – oxidizer ratio increased, the power requirement from the propellant as increased. Ignition voltages increased at a slightly exponential rate, with coinciding with the conductivity of the propellant decreasing as the fuel content increases. Propellants required a “build-up” time between power being applied and the full combustion of the propellant, which was observed to increase as the fuel content increased. The ignition voltage for fuel weight percentages per mm was found to be 45, 51.92, 57.47, 59.1, 63.3, 68.43, 74.52, and 76.92 V/mm, for fuel weight percents of 20, 30, 40, 45, 50, 55, 60, and 65 respectively. The build-up time per mm of propellant was found to be 0.38, 0462, 0.498, 0.545, 0.788, 0.98, and 1.186 s/mm for fuel weight percentages of 20, 40, 45, 50, 55, 60, 65 respectively. The mechanics of electric solid propellants were studied for high fuel content mixtures, with it being determined that combustion location and on-off capability is consistent with what is seen for oxidizer rich mixtures.Fuel rich electric solid propellants were shown to have potential benefits in both a propulsive and systematic sense. However, there are considerations to be made for the usage of propellants with higher fuel – to – oxidizer ratios, such an increased power requirement as well as increased delay between power application and ignition.