Emission characterization of porous electrospray thrusters with actively controlled flow rate

Abstract

A pressurized propellant feeding system was designed and tested for a porous-emitter electrospray thruster. The system provides a preliminary design for an electrospray-monopropellant dual-mode propulsion system that uses a HAN-based ionic liquid propellant. For ease of initial experimental validation, a conventional ionic liquid propellant, EMI-BF₄, was used in this pressurize-feeding electrospray system. The dry thruster was fed from a pressurized propellant storage beaker via a tubing. The dimensions of the feeding tube were calculated to deliver the nominal flow rate. In order to reduce the time of filling the thruster, a bypass feeding line with a high flow rate was integrated into the system. The transient behavior of the fluidic impedance and flow rate during the initial propellant filling stage were computed for analysis. The flow rates and propellant feeding performance using both the nominal and bypass feeding lines were tested and proved effective. The current-voltage characteristics of the electrospray thruster were measured in vacuum tests, with the propellant delivery pressure changed from 1 bar to 0 bar. The current values at the negative polarity significantly increased with higher feeding pressure, whilst the positive currents only experienced minor variations. With the pressure changing from 1 bar to 0 bar, the current at -3000 V changed from -807.54 μA to -217.18 μA,whilst in comparison, the current at +3000 V only varied from +216.52 μA to +300.29 μA. This preliminary design and test demonstrated that using a pressurized propellant feeding system on an electrospray thruster with a porous emitter is feasible. This opens up the possibility that the electrospray emission performance can be controlled not only by the thruster voltage but also by the propellant delivery pressure.

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