Design and Experimental Investigation of a Hydroxyl Ammonium Nitrate Based Workhorse Microthruster
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Hydroxylammonium nitrate-based (HAN-based) monopropellant is being investigated as an alternative to hydrazine due to its lower inherit hazard of handling and higher energy density. It is hypothesized the light-off temperature and corresponding preheat power required for microthrusters using HAN-based monopropellants decrease with decreasing droplet size. The viscous nature of this propellant, however, creates significant challenges in its atomization. Having developed a means to control the droplet size down to 50 micrometers in diameter, experiments are proceeding with a laboratory-scale workhorse thruster apparatus designed for the operation in the 0.1 to 1 Newton thrust range. As part of this program, the injector atomization capabilities in a pressurized test cell were examined with high speed photography, flow-through light-off experiments were carried out at ambient pressure using interchangeable steel and quartz heated catalyst housings, and thruster tests were performed at elevated chamber pressures. Results from these experiments show discrepancies between ambient pressure ignition testing of the HAN based monopropellant and those conducted in the pressure building workhorse thruster. This may suggest a link between the complete combustion of the propellant and chamber pressure. In the workhorse thruster, successful ignition of the HAN-based monopropellant was obtained with a combustion efficiency of approximately 75 percent.