Electric Propulsion and Controller Design for Drag-Free Spacecraft Operation in Low Earth Orbit

Results are presented of a study evaluating controller performance, as well as thrust, power, and propellant mass requirements for drag-free spacecraft operation at orbital altitudes of 160 – 200 km. A MSISE-90 atmospheric model is used in conjunction with a control algorithm developed in previous work. This controller is designed around an onboard inertial sensor which uses a freely floating reference mass to measure deviations in the spacecraft position, resulting from non-gravitational forces, from a desired target orbit. Thruster (control actuator) models are based on a Hall thruster to provide the orbital along-track acceleration and colloid thrusters for the normal and cross-track acceleration. This study extends previous work by incorporating 1) a time delay in the spacecraft position and velocity updates, 2) injection of a measurement uncertainty into the spacecraft state and inertial sensor data, and 3) a parametric thruster model for the Hall and Colloid thrusters. The most demanding propulsion requirements correspond to the lowest altitude considered, 160 km. At this altitude the maximum along-track thrust component is calculated to be 111 mN with a required dynamic (throttling) response of 103 μN/s. The ability of a colloid thruster to control the normal and cross-track drift is found to be dependent on how frequently the spacecraft state data is updated. Reducing the period between updates from 10s to 1s reduces the maximum normal thrust component from 1306 μN to less than 100 μN, suggesting that spacecraft state update frequency could be a major driver in the propulsion technology selection. Sensitivity of maximum required thrust and accumulated sensor error to measurement uncertainty is found to be less of a driver than state update frequency. For the spacecraft point design considered with a propellant mass fraction of 0.18, the mission lifetime for the 160km case was calculated to be 0.7 yrs. This increases to 1.3 yr at an altitude of 200 km.