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"One of the general processes of interest in Pulsed Plasma Thrusters is the ablation of the solid fuel. In general, ablation occurs when a short pulse of applied energy removes a portion of the fuel surface. Although this ablation process is relatively straight-forward in simple materials that sublimate, ablation in Pulsed Plasma Thrusters is significantly more complicated. This is caused by the transient conditions and the complex behavior of Teflon that does not sublimate but rather undergoes both physical and chemical changes prior to leaving the surface. These two effects combine to make Teflon ablation a highly nonlinear function of heat flux, material property variations, changing molecular weight, and phase transformation behavior. To gain greater insight into the ablation process, a one-dimensional ablation model is developed that addresses the more detailed thermal and thermodynamic behavior of Teflon during simulated operation of a Pulsed Plasma Thruster. The mathematical model is based on the work of Clark (1971), which focused on two-phase, one-dimensional Teflon ablation in the context of thermal protection systems. The model is modified for use in simulated PPT operations and implemented numerically using an adaptive non-uniform grid, explicit finite-difference techniques, and a volume fraction method to capture the interface between the crystalline and amorphous Teflon phases. The ablation model is validated against analytical heat transfer and ablation solutions and compared with previous experimental results. The Teflon ablation model is used to analyze several general ablation scenarios in addition to specific PPT conditions to gain greater insight into long-duration thruster firing, post-pulse material ablation, variable heat flux effects, variable material property effects, and the impact of surface re-crystallization on particulate emission. These simulations are considered in the context of prior experimental investigations of Pulsed Plasma Thrusters. The results of these simulations demonstrate the success of the numerical ablation model in predicting experimental trend and suggest potential paths of moderately improving thruster efficiency and operational repeatability in the future. "

The feasibility of relocating a small (~500,000 kg) Near-Earth Asteroid (NEA) to High Earth Orbit via Solar Electric Propulsion (SEP) is evaluated with the orbital simulation software General Mission Analysis Tool (GMAT). Using prior research as a basis for the mission parameters, a retrieval mission to NEA 2008 HU4 is simulated in two parts: approach from Earth and return of the Asteroid Redirect Vehicle (ARV) with the asteroid in tow. Success of such a mission would pave the way for future missions to larger NEAs and other deep space endeavors. It is shown that for a hypothetical launch time of 24 May 2016, the ARV could arrive within 25 km of 2008 HU4 on 28 Jun 2017 with a Delta V of 0.406 km/s, begin return maneuver on 08 Dec 2017 and reach Earth altitude of 450,000 km by 23 Apr 2026 with a Delta V of 44.639 m/s.

<p dir="ltr">This work studies the kinetics of nitrous oxide decomposition over alumina-based catalysts, specifically at the high temperatures and high nitrous oxide (N2O) concentrations that would be experienced in catalyst beds for monopropellant rocket thruster applications. High- and low- order models are developed to understand the interaction between reaction kinetics and mass transfer in monolith catalyst tubes. However, nitrous oxide decomposition is not observed on monolith catalyst tubes on account of their lower geometric surface area leading to a majority of the gas not coming into contact with the catalyst surface. Pellet-bed catalysts are studied for the remainder of this work, starting from experiments with a constant-volume batch reactor. The batch experiments demonstrate N2O decomposition over catalyst pellets, and a one-dimensional, time-varying model is developed to quantify the reaction rate based on measured temperature and pressure rise from experimental data. The reaction rates predicted by the model are significantly lower than predicted in the literature for the same catalysts. The inaccuracy is attributed to the fact that the model cannot capture N2O decomposition occurring during the first few seconds of filling the batch tube. Additionally, the simplified temperature distribution applied in the model may not be accurate, and obtaining a higher fidelity temperature distribution experimentally would require more advanced diagnostics.</p><p dir="ltr">The final experiment is a conventional flow-through pellet bed reactor which uses infrared spectroscopy to measure the concentration of nitrous oxide in the decomposed gas mixture. The analysis method incorporates uncertainties from infrared measurements and other sources, and initial activity results of a cobalt oxide-on-alumina catalyst are consistent with the literature. Results from additional testing indicate that manganese oxide catalysts are more active than nickel oxide or cobalt oxide catalysts. At weight loadings of ~10%, results indicate that the Arrhenius pre-exponential constant is roughly an order or magnitude greater for manganese oxide catalysts than cobalt or nickel oxide catalysts. The results also indicate hysteresis in catalytic activity of all oxides. Surface area and x-ray diffraction measurements do not reveal any permanent change in the surface area or crystal structure of these catalysts. The findings lead to the conclusion that the temperature and surrounding environment of the catalyst (either nitrous oxide or nitrogen during system purges) cause short-lived changes to the crystal structure of the active phase, leading to the observed hysteresis.</p>

The purpose of the study is to evaluate the possibility of using gridded ion thrusters as a means of attitude control for a solar sail as a part of the sunshade project, which aims to place 10^8 solar sail sunshade spacecraft, each with an area of about 10 000 m^2, at the Sun-Earth Lagrangian point L1 in order to reduce Earth's global temperature. Two types of solar sail sunshade spacecraft were studied. The first type, referred to as the sunshade demonstrator, had an area of 100 m^2 and a mass of 10 kg, and the second type, referred to as the full-sized sunshade, had an area of 10 000 m^2 and a mass of 90 kg. To determine the significance of using ion thrusters for the attitude control system, the mass of the required fuel, as well as the total mass that had to be added to the spacecraft to implement the attitude control system, was calculated. Two types of journeys were studied for each spacecraft type: starting from Low Earth Orbit (LEO) to L1 and from Geostationary Orbit (GEO) to L1, respectively. The results showed that the duration of the journey of the full-sized spacecraft was about 570 days from LEO to L1 and 370 from GEO to L1, respectively. The required amounts of fuel for the respective journeys were 580 g and 15 g, respectively, and resulted in a total additional mass of 7.8 kg and 7.2 kg, respectively.

Testing of hardware to be used in space sometimes involves using vacuum chambers. The need to test hollow cathodes, used as neutralizers for some Electric Propulsion Thrusters, and low power Hall Effect Thrusters at OHB Sweden requires a vacuum chamber to be upgraded. The thesis aims firstly to show the process of adapting the vacuum chamber at OHB to test a heaterless hollow cathode. The requirements of the test included a maximum temperature of a 100 °C in the cathode’s bracket and a pressure inside the chamber around 10¯6 mbar during the test. To accomplish the first, the mounting structure was subjected to a thermal simulation using CREO. Once, the requirement was fulfilled the structure was manufactured and mounted. For the second requirement, a vacuum chamber characterization was done to see the pressure evolution inside the facility. Lastly, the fluid line was designed and mounted in the facility. An upgrade of the current vacuum chamber was deemed insufficient to test low power Hall Effect Thrusters. A comparison between the current vacuum chamber at OHB Sweden and other vacuum chambers designed for this purpose was made. The conclusion was drawn that a new vacuum chamber is necessary. Proposals for the design of a new vacuum facility are presented with particular focus on dimensions, the pump system, sputter protection and thermal protection. / Testandet av hårdvara som ska användas i rymden involverar ibland vakuumkammare. Behovet av att testa hollow cathodes, som används som neutraliserare för vissa jonmotorer, och Hall Effect Thrusters med låg effekt hos OHB Sweden kräver att en vakuumkammare uppgraderas. Avhandlingen syftar först till att redogöra för hur en av vakuumkammarna hos OHB Sweden kan anpassas för att testa en heaterless hollow cathode. Kraven för testet inkluderar en maximal temperatur på 100 °C grader i katodens fäste och och ett vakuum runt 10 ¯6 mbar. En jig designades genom bland annat termisk simulering i CREO. När det termiska kravet ansågs uppfyllas tillverkades och installerades denna jig. För det andra kravet karakteriserades vakuumkammaren med dess vakuumpumpar i syfte att bedöma om dessa kunde uppfylla kravet. Slutligen designades och installerades ett rörsystem som möjliggör matning av bränsle till testobjekten. En uppgradering av nuvarande vakuumkammare bedömdes otillräcklig för att testa Hall Effect Thrusters med låg effekt. En jämförelse mellan nuvarande vakuumkammare på OHB Sweden och andra vakuumkammare designade för detta ändamål gjordes och slutsatsen drogs att en ny vakuumkammare är nödvändig. Förslag på design av ny vakuumkammare presenteras med särkilt fokus på dimensioner, pumpssystemet, sputtingskydd och termiskt skydd.

Colloid thrusters are under consideration for NASA missions such as the Laser Interferometer Space Antenna (LISA), which requires the continuous cancellation of external disturbances (approximately 25 microNewtons over a 3-10 year mission). Emissive probes are one diagnostic for the measurement of plasma potential, which can provide valuable information on the level of space-charge neutralization in a thruster plume. Understanding how to achieve effective space-charge neutralization of the positive-droplet thruster plume is important for efficient operation and to minimize the risk of contamination. In this Thesis we describe a laboratory electrospray (colloid) source and accompanying power processing electronics developed for testing of diagnostics in colloid thruster plumes. We present results of an initial series of emissive probe measurements using floating probe and swept bias probe techniques. These measurements were carried out using a single needle emitter operating on a mixture of EMI-IM (an ionic liquid) and tributyl phosphate. For a spray operating at a discharge voltage and current of 2.0kV and 200nA respectively, a potential of 5.0V was measured using the floating probe technique with the probe located at a distance of 2.7cm from the electrospray source. The interpretation of this floating potential as the plasma potential is discussed. In a separate set of tests, we used the swept bias emissive probe technique at the same distance and measured a plasma potential of 2.0V at a discharge voltage of 2.0kV. The discharge current in this latter test was somewhat unstable and varied from approximately 250 nA to over 1000nA. Numerical integration of the Poisson equation was performed to better understand space charge limitations of a probe emitting into a low density plasma. These results are presented and some implications for the measurements discussed. While the electrospray droplet number density was not measured, calculations to estimate this number density are also presented. Based on these estimates and our numerical calculations, the“knee" in the current voltage characteristic measured using the swept probe technique is estimated to be within 1.3 V of the actual plasma potential.

This work focuses on improving the thrust-to-power ratio of Hall effect thrusters using in-channel electrodes to reduce ion-wall neutralization and focus the ion beam. A higher thrust-to-power ratio would give Hall thrusters increased thrust with the limited power available on spacecraft. A T-220HT Hall thruster is modified in this work to include a pair of ring electrodes within inside the discharge channel. The electrodes are biased above anode potential to repel ions from the walls and toward the channel centerline. Theoretical analysis of ion loss factors indicate that ion-wall neutralizations remove almost 13% of the total ions produced. Reduced wall losses could significantly improve the thruster performance without increased discharge power or propellant consumption. The thruster performance, plume ion characteristics, and internal plasma contours are experimentally measured. The plume and internal plasma measurements are important to determine the cause of the performance changes. The thruster is tested in three conditions: no electrode bias, low bias (10 V), and high bias (30 V). The performance measurements show the electrodes do indeed improve the thrust and thrust-to-power ratio, the latter only at the low bias level. Adding bias increases the ion density and decreases the plume angle compared to the no bias case. The plume measurements indicate that the performance improvements at low bias are due to increased ion number density as opposed to increased ion energy. The increased ion density is attributed to reduced wall losses, not increased ionization. The in-channel measurements support this due to little change in the acceleration potential or the electron temperature. At the high bias level, a drop in thrust-to-power ratio is seen, even though a larger increase in thrust is observed. This is due to increased power draw by the electrodes. Plume measurements reveal the increased thrust is due to ion acceleration. The internal measurements show increased acceleration potential and electron energy which can lead to increased ionization. At the high bias condition, the electrodes become the dominant positive terminal in the thruster circuit. This causes the increased ion acceleration and the creation of domed potential contours that conform to the near-wall cusp-magnetic fields. The domed contours produce focused electric fields, which cause the decreased wall losses and plume angle.

Dans un propulseur à courant de Hall, la création des ions et leur accélération sont régis par le même phénomène physique. L'idée du propulseur de Hall double étage (DSHT) est de découpler l'ionisation du gaz (poussée) et l'accélération des ions (ISP), de sorte à rendre le système davantage versatile. Les travaux menés durant cette thèse visent à démontrer, grâce à des essais expérimentaux, la pertinence et la faisabilité d'un tel concept. Dans un premier temps, un prototype de DSHT, baptisé ID-HALL, a été conçu et assemblé. Il est constitué d'une source inductive magnétisée insérée dans un tube en céramique et d'un étage d'accélération identique à une barrière magnétique de propulseur simple étage. La source inductive a été optimisée de sorte à réduire le couplage capacitif et à maximiser l'efficacité du transfert de puissance par ajout de pièces en ferrite et diminution de la fréquence RF d'excitation. Dans un deuxième temps, la source inductive du propulseur a été caractérisée indépendamment du propulseur en argon et xénon pour différentes pressions. Le dispositif expérimental a permis notamment de tracer une cartographie 2D de la densité et de la température. Enfin, le propulseur a été monté dans son caisson et des mesures préliminaires (caractéristiques courant-tension, mesures par sonde RPA) ont été menées. En parallèle, des simulations utilisant un modèle hybride 2D ont été effectuées en mode simple et double étage. Elles mettent en évidence un fonctionnement versatile du moteur pour des tensions inférieures à 150 V. A terme, on visera à démontrer que la densité de courant et l'énergie des ions peuvent être, dans certaines conditions, significativement découplées. / In Hall thrusters, the same physical phenomenon is used both to generate the plasma and to accelerate ions. Furthermore, only a single operating point is experimentally observed. The double stage Hall thruster (DSHT) design could allow a separate control of ionization (thrust) and ions acceleration (ISP) to make the system more versatile. The work carried out during this PhD aims to experimentally demonstrate the relevance and the feasibility of this concept. Firstly, a new design of DSHT, called ID-HALL, was proposed and a new prototype was built. It combines the concentric cylinder configuration of a single stage Hall thruster with a magnetized inductively coupled RF plasma source (ICP) whose coil is placed inside the inner cylinder. The ICP source was improved in terms of power coupling efficiency by adding ferrite parts and by decreasing the heating RF frequency. The ICP source used in the ID-HALL thruster was then characterized independently of the thruster using argon and xenon and varying pressure. The experimental setup has allowed to measure the spatial variations of the electron density and temperature. Finally, the thruster was mounted in its vacuum chamber and preliminary measures (voltage-current characteristics, RPA measurements) were led. At the same time, simulations using a two-dimensional hybrid model were performed in single and double stage. A versatile operation for voltages lower than 150 V was highlighted. An emphasis will be given to demonstrate that the current density (given by the ion flux probe) and the ions energy (given by the RPA) might be significantly decoupled.

La question de la propulsion spatiale a été un enjeu politique au coeur de la guerre froide et reste un enjeu stratégique de nos jours. La technologie chimique déjà en place sur les moteurs fusées s'avère être limitée par la vitesse d'éjection et la durée de vie des appareils. La propulsion électrique et plus particulièrement le moteur à effet Hall apparait ainsi comme la technologie la plus performante et la plus utilisée pour diriger un satellite dans l'espace. Cependant, la physique à l'intérieur d'un propulseur étant complexe, de par les champs électromagnétiques ou les processus de collisions importants, toutes les particularités de fonctionnement du moteur ne sont pas parfaitement expliquées. Au bout de centaines d'heures d'essais, certains prototypes voient leur paroi s'éroder de façon anormale et des instabilités électromagnétiques se développent au sein de la chambre d'ionisation. La mobilité des électrons mesurée est en contradiction avec les modèles analytiques et soulèvent des problématiques sur la physique du plasma à l'intérieur de ces moteurs. Par conséquent, le code AVIP a été développé afin de proposer un code 3D massivement parallèle et non-structuré à Safran Aircraft Engines modélisant le plasma instationnaire à l'intérieur du propulseur. Des méthodes lagrangiennes et eulériennes sont utilisées et intégrées dans le code et mon travail s'est concentré sur le développement d'un modèle fluide, étant plus rapide et donc mieux adapté à la conception et au design industriel. Le modèle fluide est basé sur un modèle aux moments avec une expression rigoureuse des termes de collisions et une description précise des conditions limites pour les gaines. Ce modèle a été implémenté numériquement dans un formalisme non structuré et optimisé de façon à être performant sur les nouvelles architectures de calcul. La modélisation retenue et les efforts d'optimisation ont permis de réaliser un calcul réel de moteur à effet Hall afin de retrouver les propriétés globales de fonctionnement telles que l'accélération des ions ou encore la localisation de la zone d'ionisation. Un second cas d'application a finalement reproduit avec succès les instabilités azimutales dans le propulseur avec un modèle fluide et a justifié le rôle de ces instabilités dans le transport anormal des électrons et l'érosion des parois / The space propulsion has been a political issue in the midst of the Cold War and remains nowadays a strategic and industrial issue. The chemical propulsion on rocket engines is limited by its ejection velocity and its lifetime. Electric propulsion and more particularly Hall effect thrusters appear then as the most powerful and used technology for space satellite operation. The physic inside a thruster is complex because of the electromagnetic fields and important collision processes. Therefore, all specificities of the engine operation are not perfectly understood. After hundreds of hours of tests, thruster walls are curiously eroded and electromagnetic instabilities are developping within the ionization chamber. The measured electron mobility is in contradiction with the analytical models and raises issues on the plasma behavior inside the discharg chamber. As a result, the AVIP code was developed to provide a massively parallel and unstructured 3D code to Safran Aircraft Engines modeling unsteady plasma inside the thruster. Lagrangian and Eulerian methods are used and integrated in the solver and my work has focused on the development of a fluid model which is faster and therefore better suited to industrial conception. The model is based on a set of equations for neutrals, ions and electrons without drift-diffusion hypothesis, combined with a Poisson equation to describe the electric potential. A rigorous expression of collision terms and a precise description of the boundary conditions for sheaths have been established. This model has been implemented numerically in an unstructured formalism and optimized to obtain good performances on new computing architectures. The model and the numerical implementation allow us to perform a real Hall effect thruster simulation. Overall operating properties such as the acceleration of the ions or the location of the ionization zone are captured. Finally, a second application has successfully reproduced azimuthal instabilities in the Hall thruster with the fluid model and justified the role of these instabilities in the anomalous electron transport and in theerosion of the walls