While electrospray devices have been used in a variety of applications for decades, they have recently seen a surge in research within the field of electric propulsion. These research efforts have helped significantly improve the understanding of electrospray thruster operation and optimization, however they have primarily been focused on capillary-based, droplet emitting devices due to the more readily available manufacturing techniques. In contrast, ion emitting, porous-media-based electrospray devices are less developed both theoretically and experimentally. Presented here are fabrication methods and thruster characterization results for an entirely conventionally machined, high performance porous-media electrospray thruster. The goal of this work was to explore the performance capabilities of an ion-mode electrospray thruster which could be fabricated and tested rapidly using techniques readily available to virtually any institution, with the hope of enabling more academic and industrial development of this technology. The thruster described here consisted of 576 emitters conventionally machined out of porous borosilicate glass and is able to maintain stable operation up to ± 700 µA of emitted ion current. The overall thruster design is described, and detailed fabrication steps are presented for this device. Additionally, performance characteristics are discussed for both positive and negative ion emission, including I–V curves and direct thrust measurements, as well as measurements of the emitted ion angular, 2D spatial, mass, and energy distributions. Examples of the performance of this device compared to other devices found in the literature are also discussed.
A dual-axis torsional thrust stand was successfully demonstrated at the Air Force Research Laboratory, enabling direct simultaneous thrust and mass loss measurement for the Air Force Electrospray Thruster Series 2 passively fed electrospray thruster. The dual-axis system is effectively two nulled torsional thrust stands sharing a single dual-axis gimbal with a thrust and mass resolution of ±0.2 µN and ±0.04 mg, respectively. The development of this system was inspired by a need for direct efficiency characterization of electrosprays via in situ mass measurements, and performance was compared to thruster masses measured pre- and post-testing using an analytical balance. Mass consumption data captured via the dual-axis stand, which is calibrated to a traceable uncertainty of 1.6%, varied between −5% and 18% as compared to analytical balance measurements throughout a multi-month testing effort highlighting the limitations in pre/post-weighing as a method for capturing propellant consumption due to absorption of atmospheric moisture when thrusters are removed from vacuum. Thrust stand tests were limited to short term operation with a daily available testing window of ∼5 h due to thrust stand drift following the 24 h cyclic temperature variations of the testing facility. A thorough investigation into the root cause of ambient thermal drift suggests that the thermal response of commercial flex-pivot bearings is directly producing spurious torques on the order of 10 μN m/°C. Additionally, unresolved charging effects on thrust stand hardware currently limit thrust stand operation to tests operating with a positive thruster polarity. Further development and long duration test stability require both a targeted investigation into flex-pivot thermal response and minimization of facility effects.
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