Multiplexed Electrospray for Space Propulsion Applications

Abstract

Electrospray propulsion (ESP) offers the possibility to change widely the specific impulse Isp (from hundreds to thousands of seconds and at fixed power) at higher propulsion efficiency than most electrostic thrusters operating at fixed Isp. This critical advantage is offset by the sub-N thrust level offered by single ESP emitters, that is much too low for most missions other than those requiring extremely precise thrust control. Accordingly, here we aim at: a) multiplexing the electrospray source using silicon-based microfabrication and b) identifying a suitable propellant offering wide Isp control. We demonstrated continuous operation of a multiplexed electrospray (MES) microfabricated device consisting of an array of 7 identical cylindrical nozzles, each measuring 10 m in inner dimater and 30 m in outer dimater, with nozzle-to-nozzle distance of one mm, resulting in a packing density of 150 emitters per square centimeter. The propellant was a solution of methylammonium formate dissolved in formamide at a concentration of 21.5% by volume, with a high electric conductivity K=1.8 S/m. Total propellant flow rate was varied in the 0.0024-4.9 L/h range, yielding a specific impulse in the 1300-130 s range, a thrust T in the 1-2000 nN range, and an efficiency of 70-97%. Post-acceleration to 10 kV would quadruple T and Isp. The MES was successfully tested in a vacuum system for nearly 7 hours without any noticeable electrode degradation, while propulsive parameters were measured by the “time of flight” technique. The success of the demonstration relied on two critical design features: a) the realization of a microfabricated multi-hole extractor electrode carefully positioned and aligned with respect to the nozzle array; and b) an increase in the device hydraulic impedance through the insertion of 2 m silica microbeads in the nozzles to ensure uniform electrospray dispersion from all nozzles at the smallest propellant flow rates. Further scaleup by multiplexing is being pursued to fulfill mission requirements.