A tethered satellite system consists of two or more orbiting satellites linked together by a tether (or cable). Although much theoretical and experimental work has contributed to a good understanding of the short-term dynamics of tethered systems, their long-term behaviour remains unexplored. Hence, a detailed mathematical model and a software have been developed to analyse the long-term effect of the low Earth orbit environment on tethered systems. The software predicts the trajectory and the attitude of the system, as well as the temperature and the longitudinal vibrations of the tether. The program accounts for the effects of atmospheric lift and drag, asphericity of the Earth (zonal and sectorial harmonics), solar and Earth radiation, electromagnetic forces, lunisolar attraction, and material damping. / The thesis reviews previous research work in the field and extends it using more detailed models of external perturbations. Particular attention is given to the three major external forces influencing the dynamics of tethered systems: atmospheric forces, Earth oblateness effects, and electromagnetic forces. Furthermore, analytical solutions are provided for the problem of atmospheric drag induced shift of the equilibrium angle. / It was noted that the present formulation can predict the long-term motion of non-conductive librating tethered systems (such as TiPS) with greater accuracy than previous models. The simulation software is also used to study the behaviour of spinning and conductive systems. The results show that bare conductive tethers can decay the orbit of spent rocket stages or dysfunctional satellites over 100 kg at a lower "weight cost" than traditional rocket systems and much faster than atmospheric drag.

<p> A dynamic hybrid Reynolds-averaged Navier-Stokes (RANS)-Large Eddy Simulation (LES) modeling framework has been investigated and further developed to improve the Computational Fluid Dynamics (CFD) prediction of turbulent flow features along with laminar-to-turbulent transitional phenomena. In recent years, the use of hybrid RANS/LES (HRL) models has become more common in CFD simulations, since HRL models offer more accuracy than RANS in regions of flow separation at a reduced cost relative to LES in attached boundary layers. The first part of this research includes evaluation and validation of a dynamic HRL (DHRL) model that aims to address issues regarding the RANS-to-LES zonal transition and explicit grid dependence, both of which are inherent to most current HRL models. Simulations of two test cases&mdash;flow over a backward facing step and flow over a wing with leading-edge ice accretion&mdash;were performed to assess the potential of the DHRL model for predicting turbulent features involved in mainly unsteady separated flow. The DHRL simulation results are compared with experimental data, along with the computational results for other HRL and RANS models. In summary, these comparisons demonstrate that the DHRL framework does address many of the weaknesses inherent in most current HRL models. </p><p> Although HRL models are widely used in turbulent flow simulations, they have limitations for transitional flow predictions. Most HRL models include a fully turbulent RANS component for attached boundary layer regions. The small number of HRL models that do include transition-sensitive RANS models have issues related to the RANS model itself and to the zonal transition between RANS and LES. In order to address those issues, a new transition-sensitive HRL modeling methodology has been developed that includes the DHRL methodology and a physics-based transition-sensitive RANS model. The feasibility of the transition-sensitive dynamic HRL (TDHRL) model has been investigated by performing numerical simulations of the flows over a circular cylinder and a PAK-B airfoil. Comparisons with experimental data along with computational results from other HRL and RANS models illustrate the potential of TDHRL model for accurately capturing the physics of complex transitional flow phenomena.</p>

<p> Recently, manned space capsules have been recognized as beneficial and reasonable human space vehicles again. The Dragon capsule already achieved several significant successes. The Orion capsule is going to be sent to a high-apogee orbit without crews for experimental purposes in September 2014. For such human-rated space capsules, the study of acceleration impacts against the human body during splashdown is essential to ensure the safety of crews. Moreover, it is also known that wearing a full pressure rescue suit significantly increases safety of a crew, compared to wearing a partial pressure suit. This is mainly because it enables the use of a personal life support system independently in addition to that which installed in the space vehicle. However, it is unclear how the inflation of the full pressure suit due to pressurization affects the crew safety during splashdown, especially in the case of the new generation manned space vehicles. </p><p> Therefore, the purpose of this work is to investigate the effect of the suit inflation on crew safety against acceleration impact during splashdown. For this objective, the displacements of the safety harness in relation with the suit, a human surrogate, and the crew seats during pressurizing the suit in order to determine if the safety and survivability of a crew can be improved by wearing a full pressure suit. </p><p> For these tests, the DL/H-1 full pressure IVA suit, developed by Pablo de Leon and Gary L. Harris, will be used. These tests use image analysis techniques to determine the displacements. It is expected, as a result of these tests, that wearing a full pressure suit will help to mitigate the impacts and will increase the safety and survivability of a crew during landing since it works as a buffer to mitigate impact forces during splashdown. </p><p> This work also proposes a future plan for sled test experiments using a sled facility such as the one in use by the Civil Aerospace Medical Institute (CAMI) for experimental validation of the work presented as part of this thesis. </p>

<p> A method is introduced to consider flush air data system (FADS) pressures using a technique based on inertial navigation to reconstruct the trajectory of an atmospheric entry vehicle. The approach augments the recently-developed Inertial Navigation Statistical Trajectory and Atmosphere Reconstruction (INSTAR), which is an extension of inertial navigation that provides statistical uncertainties by utilizing Monte Carlo dispersion techniques and is an alternative to traditional statistical approaches to entry, descent, and landing trajectory and atmosphere reconstruction.</p><p> The method is demonstrated using flight data from the Mars Science Laboratory (MSL) entry vehicle, which contained an inertial measurement unit and a flush air data system called the Mars Entry Atmospheric Data System (MEADS). An MSL trajectory and atmosphere solution that was updated using landing site location in INSTAR is first presented. This solution and corresponding uncertainties, which were obtained from Monte Carlo dispersions, are then used in a minimum variance algorithm to obtain aerodynamic estimates and uncertainties from the MEADS observations. MEADS-derived axial force coefficient and freestream density estimates and uncertainties are also derived from the minimum variance solutions independent of the axial force coefficients derived from computation fluid dynamics (CFD), which have relatively high <i>a priori</i> uncertainty. Results from probabilistic analyses of the solutions are also presented.</p><p> This dissertation also introduces a method to consider correlated CFD uncertainties in INSTAR. From <i>a priori</i> CFD uncertainties, CFD force and pressure coefficients are dispersed in a Monte Carlo sense and carried over into the reconstructions. An analysis of the subsequent effects on the trajectory, atmosphere, and aerodynamic estimates and statistics is presented.</p><p> Trajectory, atmospheric, and aerodynamic estimates compare favorably to extended Kalman filter solutions obtained by the MSL reconstruction team at NASA Langley Research Center. The uncertainties obtained through the methods from this work are generally smaller in magnitude because of assumptions made regarding sources of error in the MEADS pressure transducer uncertainties. Using data-derived uncertainties in the pressure measurement noise covariance results in aerodynamic parameter estimate uncertainties that are in better agreement with the uncertainties derived from the Monte Carlo dispersions. CFD database errors dominate the uncertainties of parameters derived from aerodatabase axial force coefficients.</p>

<p> A modern approach to satellite based experimentation has evolved from large, multi-instrumented satellites, to cheaper, smaller, almost disposable yet still reliable small spacecrafts. These small satellites are either sent to the International Space Station (ISS) to be dropped out into low earth orbit (LEO), or dropped off as a secondary payload into various orbits. While it is cheap to have small spacecraft accomplishing these missions, the lifetime expectancy is very short. Currently there are no commercialized propulsion systems that exist to keep them flying for prolonged periods of time. Recently researched at the Micro Propulsion and Nanotechnology Lab (MPNL), at the George Washington University (GWU), have been developments of a variety of Vacuum Arc Thrusters (VAT's) dubbed Micro-Cathode Vacuum Arc Thrusters (&mu;CATs). &mu;CAT's provide an inert electric means of propulsion for small spacecraft. The issue with these &mu;CATs has been their efficiency levels and low amounts of thrust that they provide. The &mu;CATs can provide &mu;N levels of thrust per pulse. While being proficient for small spacecrafts, an increase in thrust is highly sought for, but the improvements must retain a small footprint and low power consumption. </p><p> The topic of this thesis is the development and characterization of a new type of &mu;CAT. The interest in this new design has been conceptualized based on experiments for plasma coating techniques. By utilizing the physics of evaporation, which has been used to decrease macroparticles (MP's) for thin film deposition, it has been theorized to also be applied to VAT technology. The concept is to increase levels of thrust with the &mu;CAT, and provide higher levels of efficiency. This effect can be created without many additional components nor multiple additional loads to the thruster subsystem. </p><p> Development of this new mechanic for thruster technology has been investigated through a variety of tests for fundamental proofs of concept. Running in two operations modes, the Heated-Anode Cathode Arc Thruster (HA-CAT), has undergone current efficiency tests, mass measurements, and cross examination through the use of a Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). This research hopes to explore an old territory of plasma engineering for future developments with &mu;CAT and VAT technology.</p>

Carbon fibre (CF) reinforced polymers have become the most widely used composites in theaerospace industry. However, ensuring the integrity of composite structures remains one of themain challenges. By measuring the change in electrical resistance of these materials, it is feasibleto monitor strains and damage initiation and accumulation in-situ and in real-time. The objectiveof this work is to investigate the potential of adding carbon nanotubes (CNTs) to existing CFpolymersto improve strain self-sensing. First, the DC and AC conductivities of epoxy containingdifferent CNT weight concentrations are measured in order to characterize the percolationthreshold. Second, the variation in electrical resistance as a function of electrode distance isinvestigated for CF-epoxy and CF-CNT-epoxy composites. The results show that the addition ofCNTs increases through-thickness conductivity by primarily reducing CF-CF contact resistancerather than increasing the number of CF-CF contacts. In addition, the presence of CNTsengenders a more homogeneous CF-CF contact resistance distribution. Third, theelectromechanical behaviours of CF-epoxy and CF-CNT-epoxy composites are compared viatension, compression, and flexure experiments while simultaneously monitoring electricresponses. The addition of CNTs results in: 1) improved sensitivity under compression due to thecreation of more CF-CF contacts; 2) better linearity under tension due to a more gradual changein CF-CF contact resistance; 3) better electric response reproducibility and repeatability due tomore homogeneous distribution of CF-CF contact resistance. Finally, an existing analyticalmodel is modified to estimate the change in surface resistance on both the tension andcompression sides of flexural specimens using the results of tension- and compression-only testsas inputs. The modelling data is then compared with the measured change in surface resistanceunder flexural load and shows good agreement. The model advances CF-CF contact as thedominating mechanism in the change of surface resistance under mechanical loading—especiallyin compression. / Les polymères à fibre de carbone (FC) sont de nos jours les composites les plus largementutilisés dans le domaine aéronautique. Cependant, assurer l'intégrité de ces structures compositesreste l'un des principaux défis. En mesurant la variation de la résistance électrique descomposites polymère-FC, il est possible de surveiller les déformations et les dommages in-situen temps réel. L'objectif de ce travail est d'étudier l'influence de l'addition de nanotubes decarbone (NTCs) dans des polymère-FC sur la capacité d'auto-détection des déformations et desendommagements structurels. Premièrement, les conductivités continues et alternatives derésines époxy contenant différentes concentrations en poids de NTC sont mesurées afin decaractériser le seuil de percolation. Deuxièmement, la variation de la résistance électrique enfonction de la distance de l'électrode est étudiée pour les composites époxy-FC et époxy-CFNTC. Les résultats démontrent que l'ajout de NTC augmente la conductivité à travers l'épaisseur,en réduisant principalement la résistance de contact FC-FC plutôt que d'augmenter le nombre decontacts FC-FC. De plus, l'ajout de NTC permet une répartition plus homogène de la résistancedes contacts FC-FC. Troisièmement, les comportements électromécaniques peuvent êtrecomparés en mesurant les réponses électriques d'échantillons en temps réel soumis à des tests detraction, compression, et flexion. L'ajout de NTC conduit à: 1) une amélioration de la sensibilitésous contraintes de compression due à davantage de contacts FC-FC créés, 2) une meilleurelinéarité de la résistance électrique sous contraintes de traction due à un changement plusprogressif de la résistance de contact FC-FC, 3) une meilleure reproductibilité et répétabilité desréponses électriques grâce à une répartition plus homogène de la résistance de contact FC-FC.Enfin, un modèle analytique est modifié pour estimer la variation de la résistance de surface enflexion en utilisant les résultats des essais de traction et de compression comme donnéesd'entrée. Les données obtenues par le modèle sont ensuite comparées avec la variation de larésistance de surface mesurée lors des essais en flexion, et présentent une bonne corrélation. Cemodèle démontre en plus que la variation des contacts FC-FC domine la variation de résistancede surface sous chargement mécanique, en particulier la compression.

<p> Launch Vehicles are governed by a complex set of nonlinear and highly coupled differential equations. In general, these equations are linearized about an equilibrium point and a linear controller is designed based upon these linearized dynamics. The linear controller is then applied to the actual system which is, of course, the original nonlinear system. This, in turn, leads to tracking errors and poor performance when the vehicle experiences significant deviation from the equilibrium condition. Also, the linear controller has difficulty handling unknown variations in the system parameters which also give rise to poor performance when applied to the actual vehicle.</p><p> A new type of neural network known as a Variable Memory Recurrent Neural Network (VMRNN) has been designed that, when used in conjunction with a linear PID controller, offers improved transient response characteristics when encountering uncertainty in the dynamic model and external disturbances.</p>

<p> Micro air vehicles have a maximum dimension of 15 cm or less, which makes them ideal in confined spaces such as indoors, urban canyons, and caves. Flapping wing micro air vehicles have an additional advantage over fixed wing or rotary wing micro air vehicles in that the flapping motion mimics birds and insects, thus concealing their appearance while also providing benefits of unsteady aerodynamics. Considerable research has been invested in the areas of unsteady and low Reynolds number aerodynamics, as well as techniques to fabricate small scale prototypes. Control of these vehicles has been less studied, and most control techniques proposed have only been implemented within simulations without concern for power requirements, sensors and observers, or actual hardware demonstrations. In this work, power requirements while using a piezo-driven, resonant flapping wing control scheme, Bi-harmonic Amplitude and Bias Modulation, were studied. In addition, the power efficiency versus flapping frequency was studied and shown to be maximized while flapping at the piezo-driven system's resonance. Then prototype hardware of varying designs were used to capture the impact of a specific component of the flapping wing micro air vehicle, the passive rotation joint. The passive rotation joint was optimized through a range of different angle of attack stops and rotation joint stiffness to maximize lift and thrust force development. Optical tracking software was then developed to provide feedback information for use in closed-loop control experiments. Finally, closed-loop control of different constrained configurations were demonstrated using the resonant flapping Bi-harmonic Amplitude and Bias Modulation scheme with the optimized hardware. This work is important in the development and understanding of eventual free-flight capable flapping wing micro air vehicles.</p>

<p> Aircraft are frequently involved in ground damage incidents, and repair costs are often accepted as part of doing business. The Flight Safety Foundation (FSF) estimates ground damage to cost operators $5-10 billion annually. Incident reports, documents from manufacturers or regulatory agencies, and other resources were examined to better understand the problem of ground damage in aviation. Major contributing factors were explained, and two versions of a computer-based model were developed to project costs and show what is possible. One objective was to determine if the models could match the FSF's estimate. Another objective was to better understand cost savings that could be realized by efforts to further mitigate the occurrence of ground incidents. Model effectiveness was limited by access to official data, and assumptions were used if data was not available. However, the models were determined to sufficiently estimate the costs of ground incidents.</p>

No description available.