Thermal Design and Validation for a 6U Deep Space CubeSat EQUULEUS under Constraints Tightly Coupled with Orbital Design and Water Propulsion System

EQUULEUS, a 6U CubeSat co-developed by JAXA and the University of Tokyo, will fly to a libration orbit around Earth-Moon Lagrange point L2 (EML2), and conduct scientific observations. One of the scientific missions is to observe the impact flashes at the far side of the moon from EML2 for the first time. To realize these missions, thermal design of EQUULEUS faces many constraints coupled with the design of other subsystems. This paper describes the thermal design concept and its validation results. First, in the observation phase from EML2, the relationship between the sun direction and the observation direction changes periodically. To continuously generate adequate power, the solar array paddles were configured to be rotatable using a gimbal mechanism along the axis perpendicular to the lunar orbit plane. With this configuration, EQUULEUS experiences sunlight on only four panels out of six during its mission period. The product of the optical property (α or ε) and the panel area was designed to be thermally equal for these four panels, which helped the thermal design in averaging the heat input to the spacecraft. Second, since EQUULEUS adopts a water resistojet propulsion system for trajectory control and angular momentum unloading, heat flow design becomes complicated. This propulsion system requires a large amount of heat for vaporization. To save the heater wattage supplied to the vaporization chamber under severe power constraint, we found an optimal solution by coupling the vaporization chamber and the communication module, which consumes a large amount of power. To validate this thermal design concept, one-node analysis, three-node analysis, and multiple-node analysis with Thermal Desktop were sequentially conducted. Finally, after thermal correlation using the thermal vacuum test results, power consumption analysis, especially detailed in thrust operation phase, was conducted, which showed the feasibility for the entire EQUULEUS mission phases.