Advanced propulsion for microsatellites

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2000.Includes bibliographical references (leaves 162-166).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.by Vadim Khayms.Ph.D

Modellierung und Analyse von Wellen-Bauwerk-Boden Interaktion für monolithische Wellenbrecher

Monolithic breakwaters are preferred to other types of structures in terms of economical and environmental aspects. Nevertheless, they are more vulnerable to foundation failures, especially to stepwise failures. Due to the highly complex processes involved in wave-structure-foundation interaction, no reliable model yet exists for this failure mechanism. Therefore, a semi-coupled CFD-CSD model system and a simplified model are developed in OpenFOAM to describe wave-structure-foundation interaction for monolithic breakwaters, and particularly stepwise failures. The CFD model is an extension of the incompressible multiphase Eulerian solver of OpenFOAM by introducing different seepage laws and a simplified fluid compressibility model. The CFD model is successful in reproducing breaking wave impact including effect of entrapped air. A new CSD model is developed to solve the fully dynamic, coupled Biot equations with a new approach taking advantage of the PISO algorithm to resolve pore fluid velocity-pressure coupling. Soil-structure interaction is introduced via a frictional contact model and for soil behaviour, a multi-surface plasticity model is implemented. The model is validated against analytical models and physical tests. The model succeeds to reproduce wave-induced residual pore pressure buildup and soil densification followed by pore pressure dissipation. A one-way coupling of both models is achieved by transforming the CFD model output into input for the CSD model. The semi-coupled model system is applied successfully to reproduce selected results of a caisson breakwater subject to breaking wave impact in the Large Wave Flume (GWK). The model system is applied to expand the range of conditions tested in GWK for response of the soil foundation. A new load eccentricity concept, is proposed to classify response of the foundation in four load eccentricity regimes. Load eccentricity carries all significant information related to wave loads (horizontal and uplift) and to properties of the structure (mass and geometry). Using this concept, recommendations are drawn for design of monolithic breakwaters, and a new simplified nonlinear 3-DOF model is developed with elastoplastic springs. Model parameters are calibrated using results from the CFD-CSD model for different sand relative densities and different load eccentricities. The simplified model can simulate the stepwise failure (sliding, settlement and tilt) as well as the overall failure (overturning).Caisson-Wellenbrecher werden aufgrund ökonomischer und Umweltaspekte bevorzugt. Jedoch sind sie empfindlicher gegen das Versagen des Baugrundes insbesondere gegen schrittweises Versagen. Aufgrund der Komplexität der Wellen-Bauwerk-Boden Interaktion liegt noch kein verlässliches Modell für diesen Versagensmechanismus vor. Deswegen werden ein semi-gekoppeltes CFD-CSD Modellsystem und ein vereinfachtes Modell in OpenFOAM entwickelt. Das CFD-Modell stellt eine durch Sickerströmungsgesetze und ein vereinfachtes Modell der Fluidkompressibilität erweiterte Version des mehrphasigen Strömingslösers von OpenFOAM dar. Das CFD-Modell wurde erfolgreich eingesetzt, um Druckschlagbelastungen durch brechende Wellen mit Lufteinschlüssen zu reproduzieren. Ein neues CSD-Modell wurde für die Lösung der voll dynamischen, gekoppelten Biot-Gleichungen mit einem neuen Ansatz entwickelt. Dabei wird der PISO-Algorithmus genutzt, um die Kopplung von Geschwindigkeit und Druck des Porenfluids zu lösen. Die Bauwerk-Boden Interaktion wird über ein Reibungs-Kontaktmodell eingeführt und für die Plastizität des Bodens ein Mehrflächenmodell implementiert. Die Validierung des CSD-Modells erfolgte durch analytische Modelle und Laborversuche. Mit dem Modell ist es gelungen, den Porenwasserdruckaufbau, die Bodenverdichtung und die Dissipation des Porenwasserdruckes zu reproduzieren. Es wurde eine Einweg-Kopplung der Modelle implementiert, in dem der Output des CFD-Modells als Input für das CSD-Modell aufbereitet wird. Mit dem validierten semi-gekoppelten Modellsystem ist es gelungen die Experimente im Großen Wellenkanal (GWK) zu reproduzieren. Darüber hinaus wurde das Modellsystem eingesetzt, um die getesteten Bedingungen zu erweitern. Ein neues Lastexzentrizitätskonzept wurde eingeführt, um die Gründungsverhaltens in vier Regime zu klassifizieren. Die Lastexzentrizität fasst alle relevanten Informationen der Wellenbelastung (Horizontal und Auftrieb) und der Bauwerkseigenschaften (Masse und Geometrie) zusammen. Unter Anwendung dieses Konzepts werden Empfehlungen für die Bemessung monolithisches Wellenbrechers ausgesprochen. Darüber hinaus wurde ein vereinfachtes nichtlineares 3-DOF Modell mit elasto-plastischen Federn entwickelt. Die Modellparameter wurden für unterschiedliche relative Dichte des Bodens und Lastexzentrizität kalibriert. Das vereinfachte Modell kann das schrittweise Versagen (Gleiten, Setzung und Kippen) sowie das Gesamtversagen (Umkippen) simulieren

Bibliography of Lewis Research Center technical publications announced in 1985

This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1985. All the publications were announced in the 1985 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses

Bibliography of Lewis Research Center technical publications announced in 1984

This compilation of abstracts describes and indexes the technical reporting that resulted from the scientific and engineering work performed and managed by the Lewis Research Center in 1984. All the publications were announced in the 1984 issues of STAR (Scientific and Technical Aerospace Reports) and/or IAA (International Aerospace Abstracts). Included are research reports, journal articles, conference presentations, patents and patent applications, and theses