Microfabricated Electrospray Thrusters for a Modular Spacecraft Propulsion System

With the growing capabilities of miniature components and systems, scientific and exploration missions making use of small spacecraft (<100kg) are becoming increasingly promising. The realization of these missions would launch a new era of space exploration, no longer the preserve of large space agencies. Unfortunately, such missions are held back by the lack of a critical technology, a small and efficient propulsion system capable of delivering sufficient velocity changes (ΔV) to complete orbital manoeuvres. This thesis describes the development of such a system, focusing on the microfabricated electrospray emitter arrays at the core of the thruster. Each emitter aims to operate in the Purely Ionic Regime (PIR), electrically generating a spray of charged molecules and optimizing the propellant usage (specific impulse). Micromachined in silicon, the thrusters boast unmatched precision and integration, with up to 1527 emitters/cm2 achieved. For the first time, two-level electrodes are integrated at wafer level, individually providing extraction and post-acceleration to each emission site. In addition to significantly increased performance, the post-acceleration electrodes enable several new system features (tunable power consumption, thrust and specific impulse) essential to the system. The emitters themselves, 100µm tall capillaries fabricated by Deep Reactive Ion Etch, have inner diameters as low as 6.9 ± 0.19µm, the smallest to date. The thrusters were characterized with specific attention to the sprayed beam composition and the effect of the accelerator electrodes. Upwards of 90% ionic content was measured, a record for this type of devices. Operation of the accelerator electrodes was also validated, showing that, as expected, the accelerators did not affect the emission process but successfully improved the performance of the thruster by focusing the beam and increasing the energy of the particles. In addition to the thruster development, several fundamental aspects of the electrospray emitters were studied. In-situ Scanning Electron Microscope experiments were carried out to understand the spray formation mechanisms and a detailed study of the propellant transport in the capillaries was presented. Finally, a new type of emitters, fabricated entirely of insulating materials and applied to mass spectrometry was investigated, providing an Earth-based application to the thruster technology.

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