This thesis discusses the recent developments to the electric propulsion plume code DRACO as well as a validation and sensitivity analysis of the code using data from an AFRL experiment using a Busek 200 W Hall Thruster. DRACO is a PIC code that models particles kinematically while using finite differences schemes to solve the electric potential and field. The DRACO code has been recently modified to improve simulation results, functionality and performance. A particle source has been added that uses the Hall Thruster device code HPHall as input for a source to model Hall Thrusters. The code is now also capable of using a non-uniform mesh that uses any combination of uniform, linear and exponential stretching schemes in any of the three directions. A stretched mesh can be used to refine simulation results in certain areas, such as the exit of a thruster, or improve performance by reducing the number of cells in a mesh. Finally, DRACO now has the capability of using a DSMC collision scheme as well as performing recombination collisions. A sensitivity analysis of the newly upgraded DRACO code was performed to test the new functionalities of the code as well as validate the code using experimental data gathered at AFRL using a Busek 200 W Hall Thruster. A simulation was created that attempts to numerically recreate the AFRL experiment and the validation is performed by comparing the plasma potential, polytropic temperature, ion number density of the thruster plume as well as Faraday and ExB probe results. The study compares the newly developed HPHall source with older source models and also compares the variations of the HPHall source. The field solver and collision model used are also compared to determine how to achieve the best results using the DRACO code. Finally, both uniform and non-uniform meshes are tested to determine if a non-uniform mesh can be properly implemented to improve simulation results and performance. The results from the validation and sensitivity study show that the DRACO code can be used to recreate a vacuum chamber simulation using a Hall Thruster. The best results occur when the newly developed HPHall source is used with a MCC collision scheme using a projected background neutral density and CEX collision tracking. A stretched mesh was tested and proved results that are as accurate as a uniform mesh, if not more accurate in locations of high mesh refinement. / Master of Science

Depuis une vingtaine d’années, l’engouement pour les propulsions hydrojets n’a fait que croître et elles s’imposent aujourd’hui comme la propulsion marine incontournable pour les hautes vitesses. Dans un même temps, un outil permettant un gain considérable de temps et d’argent s’est lui aussi développé considérablement. En effet, la CFD (Computational Fluid Dynamics) est devenue une pratique courante lorsqu’il s’agit de prévoir le comportement d’un écoulement sans avoir à passer par un modèle réel. Elle sera utilisée tout au long du projet pour simuler le flux au travers de la propulsion. Le design d’une entrée d’eau est capital : une entrée d’eau mal conçue engendrera des zones de cavitation, de la recirculation sur la lèvre ou la rampe, des pertes importantes et un champ de vitesse non uniforme à la face de la pompe. Il en résultera une diminution du rendement de l’entrée, mais aussi une diminution du rendement de la pompe, puisqu’optmisée pour un flux uniforme. L’objectif de ce projet sera d’optimiser l’entrée d’eau pour augmenter le rendement global de la propulsion et ainsi réduire la consommation d’essence de 6 % sur un cycle donné. Actuellement, les conduites d’entrées sont conçues pour optimiser une vitesse de croisière moyenne. Dans ce projet, le but sera d’éviter d’avoir un compromis à faire entre les basses vitesses, la vitesse de croisière et la vitesse de pointe, et d’optimiser la géométrie de l’entrée pour une large plage de fonctionnement. Cela passe par une géométrie variable et donc un mécanisme asservi. Afin de concevoir un tel système, il sera nécessaire de trouver les géométries optimales pour les différents régimes de fonctionnement. Une étude CFD 2D paramétrable permettra de trouver les lignes directrices de ces géométries. Un modèle 3D devra ensuite être validé, puis utilisé pour pouvoir affiner les géométries optimales. Un système sera alors conçu puis testé sur le modèle CFD. Des tests expérimentaux viendront finaliser l’étude.

<div>For decades nuclear fusion space propulsion has been studied but due to technological set backs for self-sustaining fusion, it has been repeatedly abandoned in favor of more near-term or present day solutions. While these present day solutions of chemical and electric propulsion have been able to accomplish their missions, as the human race looks to explore Mars, a near term solution utilizing nuclear fusion propulsion must be sought as the fusion powered thruster case currently does not meet the minimum 0.2 thrust-to-weight ratio requirement. The current work seeks to investigate the use of a ssion powered magnetic fusion thruster for a manned Mars mission with an emphasis on creating a very near-term propulsion system. This will be accomplished by utilizing present day readily available technology and adapting methods of nuclear electric and nuclear fusion propulsion to build this ssion assisted propulsion system. Near term solutions have been demonstrated utilizing both DT and D-He3 fuels for a ssion powered and ssion assisted Dense Plasma Focus fusion device capable of achieving thrust-to-weight ratios greater than 0.2 for V's of 20 km/s. The Dense Plasma Focus can achieve thrust-to-weight ratios of 0.34 and 0.4 for ssion assisted and ssion powered cases, respectively, however, the Gasdynamic Mirror device proved to be an infeasible design as a ssion powered thruster due to the increased weight of a ssion reactor.</div>

<p>The purpose of the project is to replicate conditions found inside the reaction chamber of a nuclear thermal propulsion (NTP) rocket engine, thereby evaluating robust materials and construction techniques for future NTPs. The need to test materials exposed to hydrogen under combined high temperatures and pressures is crucial to determine their resistance to hydrogen attack.</p> <p>The proposed test article is a SiC resistive heating element which would heat the hydrogen gas flowing at 5.6 g/s from 300 to 2400 K at nominal pressure of 1000 psia then cool it to below its auto-ignition temperature before it is vented to the ambient air. The experimental evaluation of the test article should validate the reliability of materials used in the construction of the pressure vessel. The pressure vessel houses a resistive heating element made from open-cell refractory carbide foam which pairs well with hot hydrogen gas due to its resistance to thermal shock. The enclosure to encapsulate the heating element is lined with an oxide coated rhenium tube capable of sustaining high thermal and structural loads, and the outer shell is made from Inconel 718. Rhenium is a robust material with excellent ductility, is non-reactive with hydrogen, and is creep-resistant at high temperatures. Inconel 718 has a high yield strength capable of handling high temperature applications. </p> <p>Cooling the hydrogen gas requires designing a water-cooled nozzle to transport the gas to a heat exchanger. The design of the nozzle and its mechanical components involved analyzing the heat transfer through materials, predicting their structural integrity, and examining potential failure points. The 1-D steady-state heat transfer analysis is conducted to predict the inner and outer surface temperatures, heat flux, and fluid heat transfer coefficients. These parameters are considered in selecting the best candidate materials, copper and Inconel 718, to make the nozzle. To prevent gas leakage between interfaces of multiple components and joints, a careful selection of sealing techniques are implemented, including the use of bimetallic weldments and pressure-energized metal seals. </p> <p>Although the proposed test article was never tested due to schedule and budget limitations, the documentation of its design and analysis is complete and the system is ready for manufacturing and testing. The long lead times to manufacture, to inspect, and to validate the vessel were underestimated in the project scheduling. The rental cost of the electrical equipment required to run the test under initial design conditions exceeded budget. As a solution to satisfy the temperature and budget requirements, halving the flow rate and decreasing the delivered electrical power by 48% are proposed. </p> <p> The success of testing the pressure vessel at operating conditions would provide a physical and quantitative study on potential materials used on future NTP ground tests. The test would run for 5 minutes during which the strength of the materials weaken as a result of the diffusion of free carbon from their surfaces. Upon completion of the test, the performance of these materials would be evaluated for signs of macroscopic and microscopic surface effects on the test article. </p> <p> </p>

"A study is presented detailing the simulation of a drag-free follow-on mission to NASA’s Gravity Recovery and Climate Experiment (GRACE). This work evaluates controller performance, as well as thrust, power, and propellant mass requirements for drag-free spacecraft operation at orbital altitudes of 160 - 225 kilometers. In addition, sensitivities to thermospheric wind, GPS signal accuracy and availability of ephemeris data are studied. Orbital dynamics were modeled in Matlab and take into account 2 body gravity effects, J2-J6 non-spherical Earth effects, atmospheric drag and control thrust. A drag model is used in which the drag acceleration is a function of the spacecraft’s relative velocity to the atmosphere, and a “drag parameter,” which includes the spacecraft’s drag coefficient and local mass density of the atmosphere. A MSISE-90 atmospheric model is used to provide local mass densities as well as free stream flow conditions for a Direct Simulation Monte Carlo drag analysis used to validate the spacecraft drag coefficient. The controller is designed around an onboard inertial sensor which uses a freely floating reference mass to measure deviations in the spacecraft position, resulting from non-gravitational forces, from a desired target orbit. Thruster (control actuator) models are based on two different Hall thrusters for providing the orbital along-track acceleration, colloid thrusters for the normal acceleration, and a miniature xenon ion thruster (MiXI) for the cross-track acceleration. The most demanding propulsion requirements correspond to the lowest altitude considered, 160 kilometers. At this altitude the maximum along-track thrust component is calculated to be 98 millinewtons with a required dynamic (throttling) response of 41 mN/s. The maximum position error at this altitude was shown to be in the along-track direction with a magnitude of 3314.9 nanometers and a peak spectral content of 1800 nm/sqrt(Hz) at about 0.1 Hz. At 225 kilometers, the maximum along-track thrust component reduces to 10.3 millinewtons. The maximum dynamic response at this altitude is 4.23 mN/s. The maximum along-track position error is reduced to 367.9 nanometers with a spectral content peak of 40 nm/sqrt(Hz) at 0.1 Hz. For all altitudes, the maximum state errors increase as the mission length increases, however, higher altitude missions show less of a maximum displacement error increase over time than those of lower orbits. The ability of a colloid thruster to control the normal drift is found to be dependent on how frequently the spacecraft state data is updated. Reducing the period between updates from 10 seconds to 1 second reduces the maximum normal state error component from 199 nanometers to less than 32 nanometers, suggesting that spacecraft state update frequency could be a major driver in keeping the spacecraft on the target trajectory. Sensitivity of maximum required thrust and accumulated sensor error to measurement uncertainty is found to be less of a driver than state update frequency. A ‘worst case” thermospheric wind gust was modeled to show the increase on propulsion requirements if such an event were to occur. At 200 kilometers, maximum winds have been measured to be in increase of 650 m/s in the westward direction in the southern pole region. Assuming the majority of the 650 m/s gust occurs over a 4 second time span, the maximum required cross-track thrust at 200 kilometers increases from 1.12 to 2.01 millinewtons. This large increase may drive the thruster choice for a drag-free mission at a similar altitude. For the spacecraft point design considered with a propellant mass fraction of 0.18, the mission lifetime for the 160 km case was calculated to be 0.76 years. This increases 2.27 years at an altitude of 225 km."

"The development of Retarding Potential Analyzers (RPAs) capable of measuring high-density stationary and flowing plasmas is presented. These new plasma diagnostics address the limitations of existing RPAs and can operate in plasmas with electron densities in excess of 1x1018 m-3. Such plasmas can be produced by high-powered Hall Thrusters, Pulsed Plasma Thrusters (PPTs), and other plasma sources. The Single-Channel micro-Retarding Potential Analyzer (SC-microRPA) developed has a minimum channel diameter of 200 microns, electrode spacing on the sub-millimeter scale and can operate in plasmas with densities of up to 1x1017 m-3. The electrode series consists of 100 micron thick molybdenum electrodes and Teflon insulating spacers. The alignment process of the channel, as well as the design and fabrication of the stainless steel outer housing, the Delrin insulating tube, and all other microRPA components are detailed. To expand the applicability of the SC-microRPA to densities above 1x1018 m-3 a low transparency Microchannel Plate (MCP) has been incorporated in the design of a Multi-Channel micro-Retarding Potential Analyzer (MC-microRPA). The current collection theory for the SC-microRPA and the MC-microRPA is also derived. The theory is applicable to microRPAs with arbitrary channel length to diameter ratios and accounts for the reduction of ion flux due to the microchannel plate in the case of the MC-microRPA, due to absorption of ions by channel walls, and due to the applied potential. Current-voltage curves are obtained for incoming plasma flows that range from near-stationary to hypersonic, with temperatures in the range of 0.1 to 10 eV, and densities in the range of 1x1015 m-3 to 1x1021 m-3. The SC-microRPA current collection theory is validated by comparisons with the classical RPA theory and particle-in-cell simulations. Determination of unknown plasma properties is based on a fuzzy-logic approach that uses the generated current-voltage curves as lookup tables."

Plusieurs problèmes d’optimisation — dans l’eau ou à l’interface avec l’air — sont abordés, allant de l’optimisation de la forme des coques de bateaux à celle de la propulsion en aviron et dans la nage avec palmes. Des approches théorique, expérimentale et numérique sont combinées. Nous développons d’abord une approche théorique minimale afin de déterminer, à volume immergé et puissance donnés, les rapports d’aspect optimaux des coques de bateau, qui sont discutés et comparés aux rapports d’aspect de bateaux réels. L’effet de l’asymétrie avant-arrière des coques est ensuite discuté. Dans une deuxième partie, nous étudions la propulsion en aviron et dans la nage avec palme. Dans le cas de l’aviron, nous réexaminons la question de la synchronisation des rameurs sur le bateau à l’aide d’un modèle réduit de bateau robotisé et cherchons quelle est la synchronisation qui permet à l’équipage d’aller le plus vite. Enfin, nous analysons l’effet de la géométrie des palmes pour trouver les stratégies de nage optimales. / We consider a few optimisation problems — in water and at its surface — ranging from drag minimisation on ship hulls to propulsion efficiency in rowing and swimming with fins. We use theoretical, experimental and numerical methods. In a first part, we focus on the question of optimal hull shapes. We develop a minimal theoretical approach to determine, at given load and propulsive power, the optimal aspect ratios of ship hulls, which are discussed and confronted to empirical data. Then, the effect of the fore-aft hull asymmetry is addressed. In a second part, we study the question of propulsion in rowing and swimming with fins. In particular, the long-standing question of whether rowers should be synchronised or not is brought up to date with a scaled rowing robot. Finally, we analyse the optimal shape of fins in order to find optimal swimming strategies.

Le xénon est utilisé par la plupart des systèmes de propulsion électrique à grilles. Cependant sa rareté, son coût de production important ainsi que son usage dans de nombreuses applications industrielles font apparaître la nécessité de trouver une alternative à ce propergol. Il est apparu que l'iode était un candidat potentiel pour cela, étant beaucoup moins cher à produire et beaucoup moins rare. Il se présente sous forme de cristaux violacés dans les conditions standards de pression et de température et possède une pression de vaporisation peu élevée ainsi qu'un potentiel d'ionisation plus bas que celui du xénon. Un modèle global d'un plasma d'iode dans un propulseur électrique à grilles a donc été développé afin d'étudier le comportement et les performances d'un tel dispositif. Ces résultats de l'iode sont comparés à ceux du même dispositif obtenus par l'utilisation du xénon, les conditions d'opération étant bien évidemment similaires. Le modèle prédit une efficacité globale du propulseur 15% plus grande pour l'iode. Les résultats du modèle global en iode sont également comparés avec des résultats expérimentaux obtenus dans un propulseur électrique à grilles, sous des conditions d'opération et paramètres d'entrée similaires. Un band d'essai expérimental entièrement dédié à l'étude de l'iode comme nouveau propergol pour la propulsion à grilles a en effet été assemblé avec toutes les précautions nécessaires, l'iode étant un élément corrosif et chimiquement actif avec certains matériaux. Le banc d'essai en iode fut également utilisé pour effectuer la preuve de concept en iode du propulseur PEGASES. / Most state-of-the-art electric space propulsion systems such as gridded and Hall thrusters use xenon as the propellant gas. However, xenon is rare, expensive to produce and used in a number of competing industrial applications. Alternatives to xenon are currently being investigated, and iodine has emerged as a potential candidate. Its lower cost, larger availability, its solid state at standard temperature and pressure, its low vapour pressure and its low ionization potential makes it an attractive option. A global model of iodine plasma inside an ion gridded thruster has therefore been developed to study behaviour and performances of this propellant. We compared the iodine results with ones obtained in xenon under otherwise similar conditions. The model predicts a thruster efficiency 15% higher for iodine compared to xenon. Results of the iodine global model were compared with experimental data obtained under similar operating conditions and input parameters in a gridded ion thruster. An experimental test bench dedicated to iodine plasma study, inside a classic ion gridded thruster and PEGASES thruster, has been assembled with all precautions needed. Iodine is a corrosive gas and chemically active with certain metals and the right choice of materials is therefore important. The positive ion and electron densities obtained by the model and in experiments appeared to show close values, indicating that the iodine chemistry and reaction set used in the global model seem relevant to a first order approximation.

La physique de nage d’une nageoire caudale flexible est étudiée expérimentalement grâce à une plaque flexible immergée dans un écoulement uniforme. Le bord d’attaque est forcé par un mouvement harmonique, tandis que le bord de fuite répond passivement au forçage. Une résonance en amplitude dans la réponse est mise en évidence et révèle une forte interaction entre les fréquences naturelles de la plaque et celles du forçage. Les résultats expérimentaux suggèrent un rôle non trivial de l’amplitude de forçage, qui souligne le rôle des non linéarités dans ce problème. Pour avoir une meilleure compréhension de l'origine de ces non linéarités, un modèle analytique faiblement non linéaire a été développé. Nous supposons une plaque d'épaisseur nulle immergée dans un écoulement potentiel, sujette à une force de traînée résistive. La déflection de la plaque a ensuite été décomposée en un mode rigide, mimant le forçage en pilonnement et en modes propres de flexion d’une poutre dans le vide. Les prédictions de la réponse en amplitude et en fréquence du système forcé sont alors calculées. Les fréquences de résonances, ainsi que l’enveloppe de la déflection, sont en bon accord avec les résultats expérimentaux. Les performances du système, mesurées à travers la poussée générée, est également correctement prédite par la modélisation. L’accord entre les expériences et le modèle est étendu à une étude trouvée dans la littérature. Une optimisation analytique a été conduite et étendue à l’application de la bio-robotique. / The physics of the swimming with a flexible caudal fin is studied experimentally by the means of an elastic plate immersed in a uniform water flow. The leading edge of the plate is forced into a harmonic motion, while its trailing edge responds passively to this actuation. A resonance response in amplitude is demonstrated, revealing a strong interaction between the natural frequencies of the plate and the forcing frequencies. Experimental results suggest a non-trivial role of the forcing amplitude, emphasizing the role of non linearities in this problem. To gain better insight into the origin of these non linearities, a weakly non linear model is developed. We model a quasi two-dimensional plate of zero thickness immersed in a potential flow and subject to a resistive drag-like force. The plate deflection is then decomposed into a forcing heaving mode and natural flexural modes. Predictions of the response in amplitude and frequency to a heave forcing system are then calculated. The frequencies of the resonances, as well as the shapes of the deflection, match the experimental results. The performance of the system measured through the generated thrust, is well predicted by the modelling. The experimental and modelling results presented here show (very) good agreement with the literature. Finally, an analytical optimization is undertaken and potential applications to bio-robotic are suggested.

High Efficiency Multistage Plasma Thrusters (HEMPTs) are a relatively new form of electric propulsion that show promise for use on a variety of missions and have several advantages over their older EP competitors. One such advantage is their long predicted lifetime and minimal wall erosion due to a unique periodic permanent magnet system. A laboratory HEMPT was built and donated by JPL for testing at Cal Poly. Previous work was done to characterize the performance of this thruster and it was found to exhibit a large plume divergence, resulting in decreased thrust and specific impulse. This thesis explores the design and application of a magnetic shield to modify the thruster’s magnetic field to force more ion current towards the centerline. A previous Cal Poly thesis explored the same concept, and that work is continued and furthered here. The previous thesis tested a shield which increased centerline current but decreased performance. A new shield design which should avoid this performance decrease is studied here. Magnetic modelling of the thruster was performed using COMSOL. This model was verified using guassmeters to measure the field strength at many discrete points within and near the HEMPT, with a focus on the ionization channel and exit plane. A shield design which should significantly reduce the radial field strength at the exit plane without affecting the ionization channel field was modelled and implemented. The HEMPT was tested in a vacuum chamber with and without the shield to characterize any change to performance characteristics. Data were collected using a nude Faraday probe and retarding potential analyzer. The data show a significant increase in centerline current with the application of the shield, but due to RPA malfunction and thruster failure the actual change in performance could not be concluded. The unshielded HEMPT was characterized, however, and was found to produce 12.1 +/- 1.3 mN of thrust with a specific impulse of 1361 +/- 147s. The thruster operated with a total efficiency of 10.63 +/- 3.66%, an efficiency much lower than expected. A large contributor to this low efficiency is likely the use of argon in place of xenon. Its lower mass and higher ionization energy make it a less efficient propellant choice. Further, the thruster is prone to overheating, indicating that significant thermal losses are present in this design.