Ultra-High Density Ion Propulsion From Ionic Liquids

ABSTRACT: Busek Co. Inc. and Massachusetts Institute of Technology (MIT) propose to explore the physical limits of ionic liquid propulsion via development of new theory to explain effects of close packing density and to predict performance limits. The research is motivated by observations and tests demonstrating that emission from 2-dimensional porous surfaces yields order of magnitude greater emission densities than state-of-the-art MEMS approaches, and also that the emission densities, specific impulse, thrust, and charge/mass do not behave as expected or would be predicted by current electrospray theory. It is postulated that this is due to the changing nature of the emitter surface itself as propellant flowrates vary, and that emission unconstrained by fixed individual extractor apertures promotes more natural and free formation of emission sites at varying densities dictated by instantaneous operating conditions. For the Phase I effort, Busek and MIT shall develop theory describing porous emission from large 2-dimension surfaces and use available data and additional testing to confirm validity of the theory. These findings shall predict performance limits of the 2-D surface emission phenomena. The Phase II shall subsequently use the theoretical predictions to develop novel prototype emitters to confirm the theory and attain ultra high-density ion emission. BENEFIT: The continual challenge of realizing the benefit of electrospray-based propulsion has been scale-up of thrust to the levels relevant and desired for most missions. With individual emitters producing thrusts commonly less than several microNewtons, and more usually only at the nanoNewton level, large-scale multiplexing is required to achieve milliNewton thrust levels and beyond. Because of this, MEMS emitter architectures have been assiduously investigated owing to their capacity to generate large and densely-packed arrays of emitters. To-date, the MEMS approach has demonstrated some significant outcomes, but faces two difficulties for practical implementation: 1) The individual ballasting of emitters to dedicated extractor apertures greatly increases propensity for thruster failure due to shorting, and 2) MEMS process itself may limit emitter density due to microfabrication resolution constraints. 2-dimension surface emission has already demonstrated significantly superior emission densities and operating lifetimes than MEMS approaches based upon the strength of 2 basic concepts- that emitting from a 2-D surface dispenses with the failure-prone extractor that essentially masks much of the emission area, and that emission from a free surface, rather than predetermined sites, promotes natural formation of much greater number of emission sites. Preliminary data have already demonstrated the advantages of the 2-D approach, and the improved understanding enabled by the proposed research is expected to advance the understanding and develop predictive theory for development of extremely dense ion generation.