Mechanically Amplified Milli-Newton Thrust Balance for Direct Thrust Measurements of Electric Thrusters for Space Propulsion

Abstract

Direct thrust measurements by means of a thrust balance (TB) are the golden standard for measuring thrust and, concurrently, the specific impulse in electric thrusters. To measure these properties in the novel class of electrodeless plasma thrusters, a new TB based on the Variable Amplitude Hanging Pendulum with Extended Range (VAHPER) concept has been developed. The TB has a mechanical amplification mechanism with an angular magnification of $31^\circ /^\circ $ . Using Lagrangian mechanics, we show that the TB loaded with a 5.2-kg thruster prototype behaves like a damped harmonic oscillator with a natural frequency of 0.37 Hz. A variable damping system provides damping with an optimal damping ratio of 0.78, which corresponds to a settling time of only 1.8 s. Both the model and the damping and calibration system have been validated. To accommodate the particularities of medium power electrodeless plasma thrusters, the TB design includes the following features: an optical displacement sensor, water cooling, liquid metal connectors, and dedicated vacuum-rated electronics for autoleveling, remote (in-vacuum) calibration, and temperature monitoring. To test the TB, measurements were performed on a 500-W Helicon Plasma Thruster breadboard model. When loaded with this thruster, the measured stiffness of the system was 12.67 ± 0.01 mN/mm. For this stiffness, the thrust range is 150 mN with a 0.1-mN resolution. The relative uncertainty on the thrust measurements is found to be on the order of 2%.