Effect of LiF Particle Diameter on Combustion Controllable Solid Propellant Microthruster Using Laser Heating

Abstract

This paper describes the effect of adding lithium fluoride (LiF), a negative combustion catalyst, into a 0.1N class combustion-controllable solid-propellant microthruster using laser heating. In our previous studies, self-sustaining combustion has been suppressed by using fuel-rich propellant, which decreased theoretical specific impulse. Hence, we attempted to enable combustion-interrupt capability and enlarge the specific impulse by applying LiF to a propellant with a higher theoretical specific impulse. Using HTPB/AP composite propellants with 1.0 wt% LiF, the thrust was measured using a prototype thruster, and the burning surface temperature before ignition was evaluated using thermography. The distribution of LiF particles was varied using sieving; tests were conducted using non-sieved, 50-µm, and 100-µm sieved particles. Thrust measurement revealed that combustion was interrupted by pausing laser irradiation at success rates of 3/5 and 1/5 for non-sieved and 50 µm-sieved LiF, respectively. The thruster using the non-sieved LiF propellant yielded the highest specific impulse of 88.3 ± 10.0 s, and that with 100 µm-sieved LiF presented the shortest ignition delay of 0.6 ± 0.3 s. The thermography on the burning surface showed that the 100 µm-sieved LiF propellant yielded the smallest time variations in temperature and the slowest rise in temperature.