MicroThrust MEMS electrospray emitters – integrated microfabrication and test results

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. The thrust from an individual emitter is though a fraction of a μN. 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 Project 1, 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. We report here on a new generation of microfabricated emitters, and their relative performance. The emitters consist of 70 μm high etched-Silicon capillaries with outer diameters tapering to less than 10 μm. Previous designs included 5 μm silica microspheres within the 18 to 24 μm internal diameter of the emitter to increase the hydraulic impedance. However the filling factor of these microspheres in individual emitters differed; therefore a new generation of emitters having more similar impedance and with 5 - 10 μm internal diameters and hole depths of 100 μm have been manufactured. Previously the etched-Silicon extractor chip was aligned to the emitter chip using 200 μm ruby spheres. 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.

Presented at:
Space Propulsion 2012, Bordeaux, France, May 7-10, 2012

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