Advanced propulsion for microsatellites

Abstract

Microsatellites have become increasingly popular in recent years as they offer significant cost savings, higher reliability, and are generally more affordable for a large variety of commercial applications. Since many microsatellite missions require considerable propulsion capabilities, miniaturization of the propulsion subsystem is critical in the design of most miniature spacecraft. A broad range of existing propulsion technologies have been considered for the purpose of identifying those devices which maintain high performance at small scale. Scaling laws were developed for each of the selected devices so as to preserve, whenever possible, the basic non-dimensional quantities which ultimately determine the performance of the individual thrusters at small scale. Hall thrusters were initially identified as most promising. In an effort to miniaturize the Hall thruster, a number of complications have been encountered. Some of the most troublesome were higher magnetic field requirements, larger internal heat fluxes and temperatures, and difficulties associated with the manufacturing of the various miniaturized components. In order to validate the proposed scaling laws, a 50 Watt Hall thruster has been designed, manufactured, and tested in a vacuum tank. Results of the experimental testing indicate that, although the maximum thrust levels obtained were on the order of 1.8 mN, about two thirds of the nominal design value, the propellant utilization efficiencies were unexpectedly low at approximately 40%. Close examination of the magnetic assembly has shown that the tip of the iron center pole was overheating during operation due to the insufficient heat conduction. The tip temperatures were estimated to reach 900°C, exceeding the Curie point of iron. As a consequence of the change in the magnetic field profile and the resultant leakage of electrons, the observed ionization fraction and, therefore, the utilization efficiency were lower than expected. Despite the low efficiencies, which were most likely caused by the design imperfections rather than physical limitations, the effort to miniaturize a Hall thruster has provided a number of useful insights for any such attempts in the future. Most importantly, this work has highlighted the generic difficulty, common to all plasma thrusters, associated with the increase of the plasma density as the scale of the device is reduced. The consequences of strict scaling, most notably the higher particle fluxes which cause an increase in the erosion rates and significant loss of operating life at small scale, created a strong incentive to search for propulsion schemes which avoid ionization by electron bombardment. In the quest for a more durable device that could operate at low power, yet provide sufficient operating life to be of practical interest, colloidal thrusters were considered for miniaturization. These are representatives of a technology of electrostatic accelerators which does not rely on ionization in the gas phase and, hence, their operating life is not compromised at small scale. In addition to their intrinsically small dimensions and extremely low operating power levels, eliminating the need for further "miniaturization", colloidal thrusters possess a number of desirable characteristics which make them ideal for many microsatellite missions. Although the physics of electrospray emitters has been studied for decades, many of the mechanisms responsible for the formation of charged jets are still poorly understood. In order to gain further insight, a semi-analytical fluid model was developed to predict the effects of fluid's viscosity on the flow pattern. Results of the analysis indicate that over a broad range of operating conditions viscous shear flow is insignificant in the vicinity of the jet irrespective of the fluid's viscosity. In an attempt to further understand the physics of colloidal thrusters, specifically the effects of internal pressure, electrode geometry, and the internal electrostatic fields on the processes involved in the formation of charged jets, a detailed electrohydrodynamic model was formulated. A numerical scheme was developed to solve for the shape of the fluid meniscus given a prescribed set of operating conditions, fluid properties, and electrode configurations. Intermediate solutions for the conical region have already been obtained, however, convergence in the vicinity of the jet requires further studies. A fully developed model promises to provide valuable information and guidance in the design of colloidal thrusters.