A Brief Review of Electrospray Propulsion Diagnostics

We review common experimental techniques used in electrospray propulsion, including the direct measurement of thrust and propellant flow rate, and the measurement of plume properties such as mass flux, mass-to-charge distribution, and stopping potential distribution. We also discuss more complex diagnostics, such as using retarding potential and mass spectrometry in tandem. Plume diagnostics are summarized in a table for convenience. Next, we discuss how thrust and propellant flow rate can be estimated from plume data. Citing recent examples from the literature, we show that indirect ‘measurements’ of thrust and flow rate generally agree with direct measurements within ~15% for capillary electrospray thrusters. For passively fed thrusters, plume-based estimates of thrust are reasonably accurate, but plume-based estimates of propellant flow rate are not reliable, differing from direct measurements by a factor of ~2.5 in one prominent study. These conclusions highlight the need for direct measurements, when possible, and for continued investigation of mass loss mechanisms in passively fed electrospray thrusters.