Single-Crystal Molybdenum Solar Thermal Propulsion Thruster

We detail the design, fabrication, and experimental and calculation results for three cavity solar thermal propulsion (STP) thrusters — medium, small, and very small — made of single-crystal molybdenum (SC-Mo), developed in the National Aerospace Laboratory of Japan (NAL). We obtained very high temperatures — 2,300 K for the propellant gas and 2,200 K on the outer surface of the thruster — at an appropriate propellant flow with the small thruster by solar-heating with a suitable concentrator of 1.05 m in diameter, which corresponds to an 800-second-class specific impulse thruster. Temperatures obtained in experiments were much lower than expected, however. To find points for improvement and evaluate thruster performance in space, we conducted a model calculation that confirmed the thruster could achieve an 800 to 900-second-class specific impulse with a 0.1–2 N class thrust magnitude in space. STP is thus a candidate for near-future propulsion in applications such as upper stages of orbit transfer vehicles.