Thrust Density in Porous Electrospray Thrusters

Abstract

A path for increasing thrust density in electrospray thrusters is through fabrication of denser arrays of emitters. Conventional arguments assume thrust to scale linearly with the emitter number, but there has not been a critical analysis to examine the behavior of this trend at very high densities. Here, we describe a model for thruster current as a function of array density which considers how hydraulic losses change as density increases, and we find that the ideal scaling is a poor approximation. In the optimistic cases, the current increases monotonically with density but with diminishing returns. In the worst cases, packing more emitters into the same space is detrimental as hydraulic losses dominate over gains in the number of emitters. Under certain conditions there is an optimum density which maximizes the net output. We also describe the fabrication and testing of a family of porous electrospray emitters featuring pore sizes in the 10 nm to 100 nm range, with the purpose of leveraging the high precision and uniformity afforded by these materials to develop a platform suitable for experimentally validating the density models. A set of test results from two of these thrusters is presented, both having a 450 µm pitch but with different pore sizes. The 100 nm pore thruster shows characteristics similar to other porous electrosprays, emitting in the pure-ion mode at currents up to 400 µA and exhibiting current-temperature behavior commensurate with the liquid viscosity. The 10 nm pore thruster appears to be greatly flow-restricted, producing about an order of magnitude less current at analogous conditions and showing negligible response to changes in temperature.