Formation Control of the Tethers of an Electrostatic Solar Sail

Electrostatic solar sail (E-sail) is a propellant-less spacecraft propulsion concept that uses the dynamic pressure of the solar wind as a source of continuous thrust. E-sail consists of numerous long conducting tethers that are kept positively biased by an electron gun and centrifugally stretched by the spacecraft spin. The tethers form a circular electric field that deflects solar wind protons and extracts their momentum, acting like a "virtual" sail. The relative positioning of the tethers is essential for optimizing the effective propulsive force applied to the spacecraft. It has been proposed that the tip of each tether host a small remote unit with actuators to control its motion. This paper proposes to tackle the E-sail tether positioning problem using the concept of formation control of multi-agent systems. Specifically, we design a nonlinear controller that ensures that the E-sail tethers behave like an axially-symmetric, planar rigid body while stabilizing the attitude of the remote units. Simulations demonstrate the proposed control in action.