Assessment of Electrical Thruster Configurations for Lunar CubeSat Attitude Control

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Abstract

This research investigated the application of electrical thrusters within the Attitude Determination and Control System (ADCS) of the European Space Agency’s LUMIO mission, a 12U CubeSat set to op- erate in a quasi-periodic halo orbit around the Earth-Moon L2 point. While CubeSats have become pivotal tools for lunar exploration, the use of electrical thrusters for ADCS, particularly in the lunar en- vironment, remains under-explored. This study aimed to evaluate the impact of replacing LUMIO’s current ADCS design by electrical thruster configurations on attitude control performance, robustness, and connectivity, bridging a critical gap in current literature.
A simulation framework was developed to assess spacecraft performance over a two-week period, ex- amining the LUMIO mission reaction wheel set-up, as well as four distinct thruster configurations. Key metrics included thrust output, angular velocity, and pointing accuracy, represented by the half-cone offset angle. In addition, power and energy requirements were investigated. Robustness tests eval- uated system adaptability to single thruster failures, solar array deployment, and high initial angular velocities. Physical validation was achieved by porting the control algorithm to an embedded STM32 Nucleo board, which was connected to a solenoid valve representing a dummy thruster. This setup allowed for real-time testing of execution accuracy, signal fidelity, and system integration.
The results demonstrated that the control algorithm consistently maintained high pointing accuracy, keeping the half-cone offset angle within the mission requirement of 0.18 [◦]. Reaction wheels and electrical thrusters effectively generated the required control torques, with negligible angular momen- tum build-up in the nominal simulation. Overdetermined thruster configurations showcased enhanced energy efficiency, reducing total energy consumption by approximately 29% compared to determinate setups. However, the study also highlighted challenges, such as the minimum impulse bit constraints of the Pocket Rocket thrusters, which limited precise low-thrust operations.
Robustness tests revealed that determinate configurations failed to accommodate single thruster fail- ures, while overdetermined systems adapted effectively, albeit with increased power demands. From the results, it is recommended for any space mission to be certain that the on-board computer is aware of any thruster fail occurring. The undeployed solar array configuration significantly reduced energy re- quirements, supporting its feasibility for early mission phases. The thruster configurations showed not to be able to realistically counteract de-tumbling manoeuvres, significantly degrading their feasibility in the actual LUMIO mission. Additionally, the embedded system validation confirmed accurate command signal generation and execution, bridging the gap between simulation and real-world implementation.
Although the electrical thruster configurations adhered to pointing accuracy requirements in nominal simulation conditions, they impose significantly higher power and energy demands and are unable to counteract high de-tumbling manoeuvrers compared to the current reaction wheel configuration in the LUMIO ADCS design. Addressing main engine parasitic torques and exploring alternative, higher- performance thrusters may offer a feasible solution for integrating electrical propulsion into lunar Cube- Sat ADCS. While the reaction wheel and non-electrical thruster combination remains the preferable option for now, this research demonstrates the potential for electrical thrusters in advancing CubeSat capabilities. Future studies should build upon this foundation by addressing current limitations, extend- ing simulation durations, and exploring innovative propulsion technologies to unlock their full potential in deep-space missions.

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