MicroThrust MEMS electrospray emitters - integrated microfabrication and test results

Ryan, Charles, Stark, J.P.W., Ataman, Caglar, Dandavino, Simon, Chakraborty, Subha and Shea, Herbert (2012) MicroThrust MEMS electrospray emitters - integrated microfabrication and test results. Space Propulsion Conference, , Bordeaux, France. 07 - 10 May 2012. 8 pp . (In Press)

Abstract

With the growth of interest in small satellites (<10kg), there is a particular need to provide a propulsion element for this class of spacecraft. Microfabricated electrospray thrusters offer a solution to this problem. By using ionic liquids as the propellant solely ions can be emitted, resulting in a large specific impulse[1] . The thrust from an individual emitter is though a fraction of a N. However by using well-established MEMS technologies thousands of capillary emitters can be manufactured within an area of a few cm2, increasing the thrust to the mN level. We report on results from the Microthrust FP7 Project1,where the aims are to manufacture and test a complete breadboard thruster system based upon microfabricated thruster chips, alongside the design of a flight system that could enable a CubeSat to leave earth orbit. Prior to this project we had developed a number of manufacturing processes for specific thruster elements[2,3] . We report here on a new generation of microfabricated emitters, and their relative performance. The emitters consist of 70μm high etched-Silicon capillaries with outer diameters tapering to less than 10μm. Previous designs included 5μm silica microspheres within the 18 to 24μm internal diameter of the emitter to increase the hydraulic impedance[4]. However the filling factor of these microspheres in individual emitters differed; therefore a new generation of emitters having more similar impedance and with 5 -10μm internal diameters and hole depths of 100μm have been manufactured. Previously the etched-Silicon extractor chip was aligned to the emitter chip using 200μm ruby spheres [2] . Due to assembly difficulties this has been replaced with a polymer- based wafer bonding interface, allowing for simplified assembly and a wafer -scale fabrication process. These emitters have been tested in both uni-polar and bi-polar mode, using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4). The tests herein have been achieved without an acceleration stage. The Time-of-Flight data shows a mixed ion-droplet regime, approaching a Purely Ionic Regime (PIR) at low flow rates

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