Feasibility Study of a Micro-Electrospray Thruster Based on a Porous Glass Emitter Substrate

Electrospray thrusters utilize a strong electric field to extract and accelerate charged particles contained at a needle tip in a liquid state [1]. Electrospray thrusters are one of the few types of electric propulsion systems that can offer high specific impulses (i.e. >1000 seconds) whilst still being scalable down to fit within a 1U CubeSat form structure. These small electrospray thrusters, here termed micro-electrospray thrusters, are attracting considerable interest due to this Nanosatellite system suitability [2]. The thrust though from an individual downscaled micro-electrospray emitter is relatively small, of the order of 0.1 μN [3]. Given this small value, it is necessary to ‘multiplex’ the thrust by having an array of hundreds or indeed thousands of emitters [4–7]. To create these large arrays many different manufacturing methods have been investigated, including microelectromechanical system (MEMS) methods [8,9], electrochemical etching [10,11], laser ablation [12–14], computer numerical control (CNC) machining [3,15,16] and additive manufacturing [17]. Many of these techniques result in an electrospray emitter design with an emitter tens of micrometres in height and with emitter diameters of the order of ten micrometres if not smaller. This results in a ‘postage stamp’ sized emitter array, which in itself is considerably smaller than perhaps what is required onboard a CubeSat. Alternatively somewhat larger emitters may produce greater charged particle current per emitter, and therefore greater thrust. To investigate the feasibility of a larger emitter design, this paper studies a thruster using an emitter manufactured from sintered porous glass using a CNC machining technique. The emitter manufactured is of a pyramidal shaped design, with an emitter height of 5 mm and side surface angles ranging from 10 ̊ to 30 ̊. Five different prototype emitter shapes are manufactured, investigating the feasibility of manufacturing single emitters using the method. Initial experimental results are illustrated, demonstrating good operation particularly in a bipolar, alternating voltage, mode. Initial Time-of-flight measurements would seem to indicate that the emitter was operating in a purely ionic regime, with a thrust of the order of 9.25 μN per emitter and a specific impulse of the order of 4550 seconds at 3443 V with 60 μA emission current from the one emitter. This value of thrust is particularly high from one emitter, with one reason suspected being the occurrence of multiple emission sites at the emitter tips during the electrospray process. Prototype designs of array of emitters, and also more spike like emitters, are presented with the manufacturing demonstrating the reasonable promise of this very low cost, simple and quick to iterate manufacturing process.