Propulsion Requirements for Drag-Free Operation of Spacecraft in Low Earth Orbit

The use of drag-free spacecraft in low Earth orbit has the potential for enabling gravitational surveys with substantial improvement in sensitivity over the Gravity Recovery and Climate Experiment (GRACE) mission. We evaluate the required thrust envelope, maximum thruster dynamic response, and Delta*DV for a 5-year drag-free mission over a range of low-Earth-orbit altitudes (150-450 km). The analysis uses the Jacchia-Roberts atmospheric density model and an adaptive, proportional-derivative control algorithm in which full-state error feedback is used to command compensation of disturbing forces by the thrusters. The actuator (thruster) control resolution and control loop frequency are limited to 0.1 mN and approximately 0.8-31 Hz, respectively. Detailed results at 450 km show that the thrust ranges from 10 to 145 Delta*mN, the maximum thruster dynamic response is 5.9 mN/s, and the Delta*DV for a 5-year mission is approximately 20 m/s. Although drag is the dominant disturbance force, the disturbance due to solar radiation pressure is on the same order at 450 km. At 250 km, the thrust ranges from 1.3 to 5.4 mN, the maximum thruster dynamic response is 6.9 mN/s, and the Delta*DV for a 5-year mission increases to 865 m/s. We consider seven different electric propulsion options as possible candidates for this mission. The selection will be strongly dependent on the altitude, which for the altitudes considered, increases the required thrust (and power) through several orders of magnitude. We conclude that existing thruster technology should be adequate for altitudes above 250 km, but below this the required power and Delta*DV for a 5-year mission may be prohibitive.