A water resistojet propulsion system on a 6U CubeSat EQUULEUS: Demonstration of reaction control in deep space

AQUARIUS (AQUA ResIstojet propUlsion System), a water resistojet propulsion system installed on a 6U CubeSat EQUULEUS (EQUilibriUm Lunar–Earth point 6U Spacecraft), successfully conducted multiple orbital transfers including the world’s first water-fueled propulsion system operation in deep space. We present the on-orbit performance analysis of AQUARIUS with a focus on reaction control thrusters responsible for the spacecraft’s angular momentum management. The reaction control thrusters were capable of producing torques about all axes for angular momentum management as intended. Additionally, they provided fine translational delta-V, demonstrating the thrusters’ dual functionality and reliability in critical maneuvers. The impulse vector of each reaction control thruster was estimated by considering the thrust vector direction. The calculated impulse in the translational axis was about 0.10 N$\cdot$s per shot, which was validated by comparing the impulse obtained from the Doppler shift of the communication waves. The estimated on-orbit specific impulse of the reaction control thrusters was approximately 78 s, 1.3 times larger than the measured value in the ground test. This improvement on orbit could be due to the degradation of thrust coefficient efficiency in the ground test caused by low Reynolds number flows and high background pressure lowering the apparent pressure thrust. The consumed propellant mass on orbit was estimated but deviated from the expected value under high-temperature conditions, possibly due to temperature measurement uncertainties or the influence of the on-orbit environment.