Recent interest in the replacement of hydrazine as a liquid monopropellant has focused on the use of hydroxylammonium nitrate (HAN)-based monopropellants. HAN-based monopropellants offer benefits of improved performance and safer handling when compared to hydrazine, though its combustion properties are not as well understood as hydrazine. The work presented in this thesis seeks to improve the understanding of the combustion properties of HAN by conducting fundamental experiments to measure the burning rate of 82.4 weight percent aqueous HAN solutions. To verify the accuracy of these burning rate measurements, two methods were used to simultaneously measure the burning rate of aqueous HAN inside of a constant-volume pressurized strand burner for test pressures ranging from 2.37 to 15.10 MPa. This required the development of a method to measure the burning rates by use of a high-speed video camera and comparison of these results against those measured by monitoring the change in pressure of the strand burner. The use of simultaneous burning rate measurements also compared the burning rate results for two different sources of aqueous HAN to further validate the accuracy of these measurements. Results of this investigation showed that aqueous HAN combustion results in two-phase burning where the liquid and gas phases are separated in the pressure trace by an inflection point in the rate of increasing pressure produced by the burning propellant. The method of burning rate measurements by the change in pressure was modified to use this inflection point as a point of propellant burnout, rather than the point of maximum pressure. By doing this, it was verified that either this inflection point method or the high-speed video method could be used to measure the liquid phase burning rate of aqueous HAN, but also indicated that the gas phase produces significant exothermic reactions. A second burning rate measurement for HAN is defined by use of the peak pressure to capture the complete combustion of aqueous HAN and its two-phase burning behavior. An experiment to detect the presence of the hydroxyl radical as an intermediate combustion species in this gas phase reaction was also performed, which resulted in no evidence of its presence in either phase of aqueous HAN combustion.
