Homan-Cruz, Gabriel Dylan (2015-12). Experimental Techniques to Study the Combustion of Aqueous Solutions of Hydroxylammonium Nitrate with Additives. Master's Thesis.
Aqueous solutions of hydroxylammonium nitrate (HAN) are promising as possible replacements for hydrazine as liquid rocket propellants. HAN-based propellants offer lower toxicity, higher density, and possibly higher specific impulse compared to hydrazine, but further research is needed to fully understand the combustion of HAN mixtures and to implement them in actual applications. The thesis presented here studies the combustion of HAN mixtures in a constant volume strand-burner over a pressure range between 3 and 20 MPa. A solution of 82.4 wt% HAN in water was used as a baseline from which other solutions were created. Silica and titania nanoparticles were added separately at concentrations of 1 wt% and 3 wt% in the baseline solution based on previous studies that have shown their effects on burning rates of other types of liquid propellants. A 14.9-wt% methanol mixture was examined based on previous studies that showed methanol's effectiveness as a reducing agent in HAN-based propellants. A combination of 14.9 wt% methanol and 1 wt% silica was also studied. The preparation of these formulations is discussed in detail. Two different methods to measure the burning rates of HAN-based propellants were used in this study. First, a peak-pressure method used the point of highest pressure in the pressure trace to mark the end of the sample combustion. This method has been used successfully with other liquid and solid propellants, but for HAN-based propellants it showed large discrepancies in burning rates when compared to similar formulations studied by other research groups. An inflection-point method was developed using the results of high-speed video to identify an inflection in the pressure trace as the end of visual burning. This method was applied retroactively to burns that were initially measured with the peak-pressure method. The peak-pressure method shows an increase in burning rates with the addition of the nanoparticle additives, especially at lower pressures. This method also shows very complex pressure-dependent burning regimes for mixtures containing methanol. The baseline solution with only added methanol increased burning rates at low pressures and a plateau of increased burning rates between 11 and 19 MPa. On the other hand, the methanol solution with the addition of silica has burning rates more similar to the baseline. As expected, the inflection-point method produces much higher burning rates compared to the peak-pressure method for HAN-based propellant mixtures. The results are in better agreement with similar formulations studied by other groups, although it is unclear whether the resulting burning rate best represents the entire burning of the HAN-based mixture. The inflection-point method shows almost no effect of nanoparticle additives on the burning rates of the baseline HAN mixture. Also, the inflection-point method produces burning rates for the methanol mixtures that are lower than the baseline across most of the tested pressure range. The methanol mixtures also maintain the pressure-dependent burning regimes found with the peak-pressure method.