TI tether rig for solving secular spinrate change problem of electric sail

Abstract: The electric solar wind sail (E-sail) is a way to propel a spacecraft by using the natural solar wind as a thrust source. The problem of secular spinrate change was identified earlier which is due to the orbital Coriolis effect and tends to slowly increase or decrease the sail's spinrate, depending on which way the sail is inclined with respect to the solar wind. Here we present an E-sail design and its associated control algorithm which enable spinrate control during propulsive flight by the E-sail effect itself. In the design, every other maintether ("T-tether") is galvanically connected through the remote unit with the two adjacent auxtethers, while the other maintethers ("I-tethers") are insulated from the tethers. This enables one to effectively control the maintether and auxtether voltages separately, which in turn enables spinrate control. We use a detailed numerical simulation to show that the algorithm can fully control the E-sail's spin state in real solar wind. The simulation includes a simple and realistic set of controller sensors: an imager to detect remote unit angular positions and a vector accelerometer. The imager resolution requirement is modest and the accelerometer noise requirement is feasible to achieve. The TI tether rig enables building E-sails that are able to control their spin state fully and yet are actuated by pure tether voltage modulation from the main spacecraft and requiring no functionalities from the remote units during flight.