  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.
