Levitation — Keeping satellites upright and on target

A spacecraft's attitude and orbit control system (AOCS) maintains the orientation of the vessel as it orbits and keeps it on its orbital path, making it critical to mission success and safety. In addition, the efficiency with which the AOCS does its job directly affects the amount of propellant used, indirectly impacting mission cost and payload size.

In order to improve European independence and competitiveness in the commercial and scientific satellite sectors, investigators executed the EU-funded project ELSA (European levitated spherical actuator). Reducing the mass, size and power requirements of the attitude control subsystem (ACS) while maintaining precision and improving reliability were the key objectives.

An ACS traditionally uses a minimum of three reaction wheels or control moment gyroscopes to sense changes in parameters such as acceleration or torque. ACS then uses actuators to generate a corrective response. ELSA scientists built on a novel idea for an ACS proposed over 20 years ago, a so-called reaction sphere that levitates due to a magnetic field. It has nearly perfect symmetry, an inherent optimal multi-axis reference frame and a significant reduction in mass compared to a three-reaction-wheel configuration. Overcoming technical difficulties has brought the technology closer to commercial readiness.

A proof-of-concept laboratory prototype based on this principle has been designed, built and tested. The selected synchronous concept has been sized to approximately meet rough specifications provided by various prime contractors. The first reaction sphere prototype was designed to obtain a torque of 0.2 newton metres and an angular momentum of 23 newton metre seconds.

The ELSA elegant breadboard will allow exploitation of the potential of the reaction sphere in terms of exported force and torques to show the full potential of the design with respect to reaction wheels or control momentum gyroscopes for space applications.

Bringing the idea of a reaction sphere ACS to technical readiness will be a major achievement for the EU space community. This will relieve dependence on foreign systems by providing a strong alternative that could be exported to other nations and companies. ELSA work and outcomes have paved the way to realising this potential.