Spacecraft charging and attitude control characterization of electrospray thrusters on a magnetically levitated testbed

Abstract

Electrospray thrusters are an attractive technology for small satellite propulsion. A thorough study of the spacecraft charging and attitude control performance of electrospray thrusters acting on a small satellite is presented. The experimental portion of the study employs a new type of magnetically levitated testbed to measure thrust, characterize spacecraft charging phenomena, and demonstrate precision attitude control maneuvers. The testbed magnetically levitates a mockup-satellite in vacuum conditions. The satellite carries thrusters, batteries, radio, and high voltage thruster electronics. The magnetic levitation and high vacuum provide a zero-friction environment for the thrusters to actuate. The thrusters are placed on the satellite in such a way to produce a net torque when fired. The thrusters are fired and the corresponding rotational movement of the satellite is analyzed to calculate thrust. The uncertainty of the thrust measurement is estimated to be ±0.35 [mu]tN (3[sigma]). Theoretical and experimental methods were developed to investigate the spacecraft charging characteristics anticipated to be observed on spacecraft during the operation of electrospray thrusters. These devices can produce positively and negatively charged ion-beams. An electrical model of this configuration was created to predict the charging properties of electrically isolated systems. This model simulates a bipolar electrospray thruster system. Experiments were conducted on the magnetic levitation testbed. The results from the experimental tests demonstrate that neutralization with heavy ionic species is indeed possible. The thrusters are able to fire in a bipolar configuration for long periods of time inducing bounded spacecraft charging in the range from -400V to +600V when emitting currents of about 20 [mu]A. It was found that the presence of low-energy ions produced by the fragmentation of large clusters and the external space plasma play a significant role in the neutralization characteristics. This electrical model was verified by reproducing the experimental results, thus validating its use to estimate bulk spacecraft charging properties. Electrospray thrusters are capable of producing impulse bits in the 10-⁶ N-s or lower, depending on the configuration. These characteristics have the potential to allow for long term pointing in the arcsecond range or better with practically no jitter. Experimental work is performed on the magnetic levitation testbed to demonstrate the actuation characteristics under non-optimal control. It is found that subject to a noisy attitude sensor and external perturbations, electrospray thrusters are capable of producing pointing accuracies of 22 arcseconds 3[sigma] error during 10 hours on a platform similar in size and mass to a 1U CubeSat. The implementation of such capabilities could complement or eliminate the need of reaction wheels and magnetorquers, especially in missions beyond low earth orbit, while including propulsive capabilities for additional maneuverability in technology development or scientific missions.