Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

Highly ionic beams of several hundred microampere per squared centimeter have been measured from porous glass ionic liquid electrospray sources fabricated using a conventional mill. The thrust output from three prototype devices, two emitting the ionic liquid 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide and one emitting 1-ethyl-3-methylimidazolium-tetrafluoroborate, was measured directly using a precise balance. Thrusts up to 50 μN were measured when emitting 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide in a bipolar, alternating potential configuration at less than 0.8 W input power and with propellant supplied from an internal reservoir. Measurements of mass spectra via time-of-flight spectrometry, angle resolved current distributions, ion fragmentation, and energy deficits have been applied to accurately calculate thrust and mass flow rates indirectly from the same devices. For two of the three cases, calculated and directly measured thrusts were in agreement to within a few micronewtons at input powers from 0.1 to 0.8 W. Emissions of 1-ethyl-3-methylimidazolium-tetrafluoroborate were shown to yield nearly purely ionic beams supporting high propulsive efficiencies and specific impulses of ∼65% and greater than 3200 s, respectively, at 0.5 W. Conversely, greater polydispersity was observed in 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide emissions, contributing to reduced specific performance, ∼50% propulsive efficiency, and ∼1500 s specific impulse at 0.5 W.