Measurement of SPT-140 Hall Thruster Swirl Torque during the Psyche Mission

Hall-thruster swirl torque is small enough that it is difficult to measure in ground-based facilities, but large enough that its effects on interplanetary spacecraft can be significant and must be actively managed. NASA’s Psyche mission has provided the first opportunity to directly measure the swirl torque produced by a Hall thruster operating at powers greater than 1.5 kW. When the electric propulsion system is operating, the spacecraft attitude control methodology forces the net torque on the spacecraft to align with a pre-determined direction in the body frame, after which the thruster-induced swirl torque can be separated from the other torques acting on the spacecraft. The swirl torques produced by three of the four thrusters on the Psyche spacecraft were determined over the full operating discharge power range of 1.0 to 4.5 kW, and the maximum value of that torque was 0.86 mN-m at full power. It is shown that the SPT-140 swirl torque results can be scaled from full power to lower powers by the product of the discharge current and the magnet current, and evaluation of cruise thrusting data as the thruster is throttled down in power suggests that swirl torque is not changing significantly with time as a function of operating condition. Finally, calculations of swirl torque using updated numerical simulations are shown to agree reasonably well with the flight data, thus providing confidence in their use for other thrusters prior to flight.