Design and characterisation of electrospray thrusters with high emission density

Abstract

This thesis investigates high-performance electrospray thrusters for nanosatellite applications. The thruster, termed Porous-emitter Electrospray Thruster (PET), consists of a porous emitter and a porous propellant reservoir made, allowing propellant to be transported passively. In order to increase the electrospray emission density and thrust density, different PET thruster emitters manufactured using low-cost techniques were studied using numerical simulations and by experiments. The electric field simulations suggest that the onset voltage of a porous emitter, contrary to capillary emitters, decreases with the size of the Taylor cone on the emitter tip. Based on multiple thruster tests in a vacuum, the onset voltages were between ±1800 to ±2200 V. The PET electrospray thrusters demonstrated high emission current, with the maximum values around 3 mA near ±3500 V. The per-tip emission current up to around 100 µN was high compared to most state-of-art development and was found mainly a result of the large emission tips that introduce more emission sites. Time-of-flight characterisations suggest that the emitted particles were mainly monomer ions and dimer ions with high specific impulse ranging from 3891 to 7459 s. The estimated thrust ranges from approximately 0.8 to higher than 263.5 µN, higher than most similar designs. The polydispersive efficiency due to the presence of multiple particles was calculated, ranging from 86.1% to 94.5%. The voltage losses of the thruster were measured using a retarding potential analyser, suggesting a non-kinetic efficiency from 92% at -2100 V to 86.7% at 2600 V. Evidence of the fragmentation from dimer ions to monomer ions was found in retarding potential analysis results, and their energy variations agree with theoretical calculations. The thruster plume angle increased from 12.4 degrees at low voltages of ±1800 V to approximately 60.8 degrees at high voltages of ±3500 V, with the angular efficiency decreased from 99.2% to 88.8%, correspondingly. The trajectory of charged particles in the plume is studied using numerical simulations, and the results suggest that the increase of the plume area is mostly a result of the expanded emission area at high voltages. By estimation, the thruster power efficiency ranges from 50% to 60%. The PET thrusters demonstrated a relatively high emission current and thrust at the cost of higher operating voltages and power. A PET thruster was tested continuously for 18 hours with relatively stable emission current and minor variations. The thruster performance was evaluated in two nanosatellite mission scenarios requiring high velocity-changes and precise thrust, and they show adequate manoeuvre performance. The preliminary studies in this thesis demonstrated that the PET electrospray thrusters have promising propulsive performance, and further development is recommended.

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