Numerical Study of a Low-Cost Micro-Electrospray Thruster with Hyper-Multiplexed Emitters

Ma, Chengyu and Ryan, Charles (2017) Numerical Study of a Low-Cost Micro-Electrospray Thruster with Hyper-Multiplexed Emitters. In Re-inventing Space Conference, London UK, 24–27th October 2017. (In Press)

Abstract

Due to lower mass and volume, CubeSats have been developed worldwide as a promising micro-platform for future space applications. CubeSats are cost-effective because of their size, but also the use of modular components and manufacturing methods. A propulsion subsystem can provide a CubeSat with more ability to do orbit control, attitude manoeuvre, after-mission disposal and mission time enhancement for more practical applications. An electrospray thruster has relatively small mass and dimension, whilst consuming small amounts of power during operation, making it a promising propulsion type for CubeSat applications. Electrospray thrusters operate by the creation of charged droplets and/or ions from the propellant thanks to an applied electric field. The charged species are directly solvated from the propellant surface at emission points. To increase the overall thrust, they are often manufactured as a multiplexed array rather than a single emission point. This paper presents initial work investigating utilizing a macroporous silicon (MPS) membrane as the emitterarray. An MPS membrane consists of a 350 µm thick sheet of Silicon, through which have been manufactured 1-5 µm holes, situated 5-20 µm pitch distance from each other. Due to the high density of pores, the overall thrust from an MPS membrane could be very high. But the operation of such an array may be complicated by the interaction of emission sites, through the interaction of the applied electric fields and also space charge effects due to the downstream charged plume. Presented is a feasibility analysis of Macro-Porous Silicon emitter Array (MPSEA), investigating the electrostatic requirements for Taylorcone formation and ions emission . A mathematical model of the equilibrium condition between liquid surface tension and surface electrical stress is studied,from which the onset voltage required for charged species emission is estimated. The effect of having close possible emission sites is investigated, whilst having one simple extractor electrode to apply the electric field. A simplified model with 5 by 5 emitters array in the centre of the silicon membrane is studied in this paper. Through gradually reducing the simulated apex curvature applied to the meniscus, a converging potential distribution can be reached, leading to an estimate of the onset voltage variation with different geometric parameters. Ions emission trajectories through electric field are simulated and optimised for a smaller plume divergence angle. Finally good propellant wetting properties through these silicon micro-poress are demonstrated. The next stage is to manufacture a prototype electrospray thruster based upon the macroporous Silicon membrane material.

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