On-orbit Performance Evaluation of AQUARIUS: a Water Resistojet Propulsion System during Initial Flight Operation of a 6U CubeSat EQUULEUS

Abstract

A 6U CubeSat EQUULEUS (EQUilibriUm Lunar-Earth point 6U Spacecraft) successfully executed an orbit transfer using a water-propelled propulsion system in deep space for the first time using AQUARIUS (AQUA ResIstojet propUlsion System), a water resistojet propulsion system. We report on-orbit performance analysis of AQUARIUS in its initial flight operation. AQUARIUS employed a distinct water droplet vaporization structure for efficient gas-liquid separation and a power-saving strategy for the large latent heat of water. The time-averaged thrust observed in two on-orbit operations using two thruster heads was approximately 6.0 mN, with a specific impulse of 91.0 s and a power consumption of less than 14 W. The thrust-to-power ratio increased with the temperature difference between the vaporization chamber and the adjacent communication device. This indicates that the necessary power for the vaporization of the water was effectively compensated for by the heat from the surroundings owing to the appropriate thermal design of EQUULEUS. The thrust coefficient efficiency was found to be approximately 0.86, which was higher than in the ground test (∼0.66). This is possibly due to the extremely low back pressure on orbit that mitigates the nozzle efficiency reduction due to low Reynolds number flows.