Micromachined electrospray thrusters for spacecraft propulsion

Micromachining has enabled the downscaling of large, massive and power hungry systems into small batch-produced integrated devices. Recent progress in electrospray thruster technology, in particular the discovery of an ionic emission mode using the ionic liquid EMI-BF4 as fuel has sparked interest in miniaturizing this thruster technology, initially developed in the 1950's and lying dormant for several decades. Electrospray thrusters operate by applying a potential difference between a conductive liquid, usually on the tip of a needle or capillary, and an extractor electrode. Once a threshold voltage is reached the electric stress at the apex of the liquid surface overcomes surface tension and a spray of particles is ejected toward a counter electrode. The purely electrostatic nature of this type of thruster makes it an ideal candidate for miniaturization and the use of ionic liquids, also known as molten salts, as fuel allows operating the thruster in bipolar mode eliminating the need for an additional neutralizer. This thesis describes a process flow to fabricate planar arrays of silicon capillaries with integrated individual extractor electrodes. The developed process flow offers the possibility to manufacture arrays on the wafer scale allowing, in principle, to increase thrust from a fraction of micronewton for a single capillary to the millinewton level for large arrays. This process flow has been validated by microfabricating several thruster prototypes, which were packaged using Low Temperature Co-Fired Ceramic (LTCC) technology. In conjunction with this microfabrication process an onset voltage model was developed intended as design tool during thruster layout. This model allows to predict the voltage at which particle emissions initiate for complex geometries and to estimate the effect of dimensional variations on parameters such as crosstalk in large arrays. Tests carried out with thruster prototypes using single capillaries show a well defined energy distribution of the particles and the possibility to modulate spray current by changing the voltage. Controlled variations in the fluidic impedance of the emitters allow to spray in either ionic or droplet mode. Time-of-flight measurements with arrays show a beam composed of ions (monomers, dimers), thus yielding a high specific impulse. A first life test finally shows thruster operation for several hours with stable beam properties.