Performance evaluation and thermography of solid-propellant microthrusters with laser-based throttling

Among propulsion devices, conventional solid-propellant thrusters require neither a tank nor a valve and therefore have relatively high reliability and simple structures. However, the interruption and restarting of thrust production in solid-propellant thrusters is difficult because combustion is autonomously sustained after ignition. Hence, we propose a new solid-propellant microthruster that can be throttled through laser heating. The proposed thruster uses a combustion-controllable hydroxyl-terminated poly butadiene/ammonium perchlorate-based solid propellant, wherein combustion is maintained only while the burning surface is heated using a semiconductor laser. In this study, thrust was measured at various laser-head traverse velocities to enhance the performance of the prototype. Moreover, propellant holders were fabricated from polycarbonate (PC) and polymethyl methacrylate (PMMA) because the melting point of PC is higher than that of PMMA, which could increase the local pressure in the interface between the propellant holder and the solid propellant and consequently reduce ignition delay. The prototype microthruster with the PMMA propellant holder yielded a thrust of 0.06 N and Isp efficiency of 70% at a laser power density of 0.83 W/mm2 and produced stable thrust at laser-head traverse velocities ranging from 0.85 to 0.95 mm/s, whereas that with the PC propellant holder exhibited unstable thrust in all tests. Firing tests showed an ignition delay of approximately 3 s and yielded a peak in Pc after ignition. Temperature profiles measured through thermography revealed that the burning-surface temperature was dependent on the laser-head traverse velocity and propellant-holder material. For stable combustion, the burning-surface temperature would be kept constant at 1040 K, an ignition temperature was 1250 K in vacuum, and one-half the ignition delay (1.5 s) is the time required for heating the burning surface to the ignition temperature.