Robust spacecraft attitude control under multi-tank sloshing disturbances during orbit correction maneuvers

Spacecraft equipped with liquid propulsion systems are susceptible to propellant sloshing during orbit correction maneuvers. During a maneuver, the liquid propellant is pushed towards the bottom of the tanks due to the effective gravity of the thrusters, resulting in lateral liquid motion and a sloshing phenomenon. The sloshing effect, if not mitigated by the attitude control system, may cause dynamic instabilities and reduce maneuver performance. It is difficult to synthesize fluid mechanics models in a control design process, and spacecraft dynamics under sloshing remains a great challenge in terms of attitude control design. Mechanical analogies, such as pendulums or spring-mass systems, have been found to capture the main effects of liquid sloshing and can serve as a control design model; however, such mechanical analogies are inherently uncertain and a robust control methodology is required for assuring the spacecraft stability and performance during a maneuver. In this work, the quantitative frequency design approach is applied for the first time to provide robust controllers for spacecraft with multi propellant tanks and an attitude dynamics model incorporating an uncertain sloshing mechanical analogy. This methodology has been used to design the attitude control of SpaceIL's lunar lander Beresheet and was proved successful in 10 orbit correction maneuvers. A case study of one of these maneuvers is presented as well as telemetry results from the mission.