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

Highly ionic beams of several hundred μA per cm 2 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 EMI-Im and one emitting EMI-BF 4, was measured directly using a precise balance. Thrusts up to 50 μN were measured when emitting EMI-Im 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 three cases, calculated and directly measured thrusts were in agreement to within a few μN at input powers from 0.1 W to 0.8 W. Emissions of EMI-BF4 are shown to yield nearly purely ionic beams supporting high propulsive efficiencies and specific impulses, ∼ 65 % and > 3200 s respectively at 0.5 W. Conversely, greater polydispersity was observed in EMI-Im emissions, contributing to reduced specific performance, ∼ 50 % propulsive efficiency and ∼ 1500 s specific impulse at 0.5 W. Nomenclature α = alternation duty cycle γ = liquid surface tension (N/m) ΔV = energy deficit (V) ηprop. = propulsive efficiency θ = beam angle measured θ eff = effective beam angle θ0 = beam angle offset κ = ToF thrust factor φ = ToF mass flow factor A = collector plate area (m 2) f n = fraction fraction due to species with solvation n h = emitter height (μm) I(t) = ToF collector current (A) I b = beam current (A) I em = emitted current (A) I ex = extractor grid current (A) Ic = un-gated collected current (A) Isp = specific impulse (s) F T /m = piecewise fragmentation modifiers to ToF calculations j(θ) = collected current density (A/m 2) K ξ = kinetic energy of species ξ L = ToF flight distance L c = distance from the source to the collector (m) m ξ = mass of species ξ (kg) ˙ m = mass flow rate (kg/s) q = species charge (C) t ξ = flight time of species ξ R c = emitter apex radius of curvature (m) s = emitter apex to extractor grid separation (μm) t = flight time (s) T = calculated …