COMBUSTION CHARACTERISTICS OF HAN-BASED LIQUID MONOPROPELLANT

A new composition of the hydroxyl ammonium nitrate (HAN)-based solution containing ammonium nitrate, methanol, and water had been developed for monopropellant in a reaction control system (RCS) as an alternative to conventional hydrazine. Comparing this solution with hydrazine, Isp is 20% higher, density is 1.4 times, the freezing point is much lower, and the toxicity is low, which makes this solution promising as a RCS propellant. The linear burning rate of the solution is moderate at the operating pressures of an RCS thruster. However, it was found that the linear burning rate had some characteristics whose mechanisms had not been understood. The combustion mechanism of this solution was investigated, the burning behavior was observed using a medium speed camera, and a temperature profile for the combustion wave was measured with a fine 2.5 mu m-diameter thermocouple. From these results, the instability of the liquid interface may trigger a sudden increase in the burning rate to a violently high region, and methanol was found to be effective in reducing the bubble growth rate in the solution. For RCS thruster use, reactivity with several catalysts was evaluated in an open-cup test. Consequently, the S405 catalyst for hydrazine showed the best performance among Pt-based, Pd-based, Ru-based, Ir-based, and S405 catalysts. Thruster tests were conducted successfully using S405 in both the pulsing and continuous modes. As a result, it was found that S405 has little effect in activation of the reaction of methanol contained in the propellant. High-concentration Pd catalyst was found to improve the decomposition characteristic of the solution.