Electrospray Thrusters for Small Spacecraft Control: Pulsed and Steady State Operation

摘要

Busek's BET-300-P passively fed electrospray thruster has been experimentally evaluated as a precision control actuator for small spacecraft. Electrospray thrusters have been shown to provide high resolution and low-noise thrust aboard the ESA LISA Pathfinder mission. If successfully miniaturized, the technology could enable new mission concepts for small space-craft including CubeSats. Experimental results from two thrusters are presented including direct measurements of thrust and in-situ mass flow rate measurements. The thrusters were operated in both continuous-output and pulse-width-modulated states. In both cases, direct measurements confirmed an ability to ramp thrust from a few to 10's of µN. Under continuous control, thrust was ramped from less than 5µN to over 150 µN and 400 nN thrust command steps were demonstrated over the full range. Calculated thrust noise was shown to be less than 0.1µN/Hz~(1/2) from 10mHz to 10Hz under continuous thrust control set-points from 15µN to 116µN. An ability to completely stop emission when using passive propellant feeding was applied to generate 50ms wide current pulses; corresponding to minimum impulse bits as low as ~2µNs. In-situ mass flow rate measurements enabled characterization of specific impulse and efficiency over the thrust range. At over 35 µN, the specific impulse ranged from 840 to 1050s and thrust efficiency from 20 to 30%. Higher efficiency and specifìc impulse were observed at low thrust. Necessary developments are discussed in the context of a high performance reaction control system for CubeSats and other small spacecraft.