Spin Plane Control and Thrust Vectoring of Electric Solar Wind Sail

The electric solar wind sail is a propulsion system that uses long centrifugally spanned and electrically charged tethers to extract the solar wind momentum for spacecraft thrust. The sail angle with respect to the sun direction can be controlled by modulating the voltage of each tether separately to produce net torque for attitude control and thrust vectoring. A solution for the voltage modulation that maintains any realistic sail angle under constant solar wind is obtained. Together with the adiabatic invariance of the angular momentum, the tether spin rate and coning angle are solved as functions of temporal changes in the solar wind dynamic pressure, the tether length, or the sail angle. The obtained modulation also gives an estimate for the fraction of sail performance (electron gun power) to be reserved for sail control. We also show that orbiting around the sun with a fixed sail angle leads to a gradual increase (decrease) in the sail spin rate when spiraling outward (inward). This effect arises from the fact that the modulation of the electric sail force can only partially cancel the Coriolis effect, and the remaining component lays in the spin plane having a cumulative effect on the spin rate.