Comparison between direct and indirect measurements for externally wetted electrospray thruster with adapted analytical balance

Abstract

Ionic liquid electrospray propulsion systems are a promising technology for the rapidly growing small satellite market, potentially exhibiting high specific impulse and efficiency even at very low power. Direct performance characterization of electrospray thrusters is a challenging issue, given the low thrust and mass flow rate values. In this work, an analytical balance was modified for direct performance characterization of electrospray thrusters with sub-micronewton thrust uncertainty and sub-milligram-per-hour mass loss rate measurements. An electrostatic calibration device was built with commercial-off-the-shelf heat sinks as electrostatic combs, and characterized with high-precision calibration weights to correct the measurements of the balance in vacuum. The performance of an externally wetted electrospray thruster was obtained with simultaneous direct and indirect measurements by setting up the modified balance and a time-of-flight mass spectrometer in tandem. Direct thrust values of the order of $\mu$N and in situ mass loss rates $<$ mg/h were consistently measured simultaneously. Thrust results concurred with indirect measurements with an average relative difference of 10.5%, showing a consistent ratio to the emitted current of $\approx 0.1\mu$N/$\mu$A. In contrast, the mass flow rate estimated from time-of-flight data was notably lower than the mass loss rate measured with the balance, with relative differences from −23.1% to −64.7%. Possible causes for this discrepancy and methods for their diagnosis are proposed.