Dissertations / Theses on the topic 'Aerospace engineering'

Given the increasing complexity of systems and the cost associated with test and evaluation of aerospace systems, more efficient methods are sought. Randomized test designs for aviation developmental test activities and other complex systems may not enable safe test conduct and may be prohibitively costly from a financial or time point of view. This research reviews Design of Experiments (DoE) test design approaches applicable to aerospace prototype test and evaluation activities. It proposes the use of Split Plot Optimal Designs to leverage advantages of DoE while satisfying requirements for limited randomization of the test runs. Through the use of case studies, the Split Plot Optimal Design approach is demonstrated to provide a 58% cost and schedule savings versus a One Factor At a Time approach, and 53% savings from the fully randomized Central Composite Design, while maintaining relevant statistical power. Through the use of Monte Carlo data simulation, the designs are evaluated for application to linear and quadratic models, with statistically significant results measured by Chi Squared and Kolmogorov-Smirnov tests.

With the growing threat of orbital debris impacts to space structures, the development of space shielding concepts has been a critical research topic. In this study, numerical simulations of the hypervelocity impact response of stacked aluminum 6061-T6 sheets were performed to assess the effects of layering on penetration resistance. This work was initially motivated by set of experimental tests where a stack of four aluminum sheets of equal thickness was observed to have a higher hypervelocity ballistic resistance than a monolithic aluminum sheet with the same total thickness. A set of smoothed particle hydrodynamic simulations predicted a 40% increase in the ballistic limit for a 6-layer target compared to a monolithic sheet. In addition, the effect of variable sheet thickness and sheet ordering on the impact resistance was investigated, while still maintaining a constant overall thickness. A set of thin layers in front of a thick layer generally lead to a higher predicted ballistic limit than the inverse configuration. This work demonstrates an increase in the performance of advanced space shielding structures associated with multi-layering. This suggests that it may be possible to dramatically improve the performance of such structures by tailoring the material properties, interfaces, and layering concepts.

Due to millions of years of evolution, sharks have evolved to become quick and efficient ocean apex predators. Shark skin is made up of millions of microscopic scales, or denticles, that are approximately 0.2 mm in size. Scales located on the shark’s body where separation control is paramount (such as behind the gills or the trailing edge of the pectoral fin) are capable of bristling. These scales are hypothesized to act as a flow control mechanism capable of being passively actuated by reversed flow. It is believed that shark scales are strategically sized to interact with the lower 5% of a boundary layer, where reversed flow occurs at the onset of boundary layer separation. Previous research has shown shark skin to be capable of controlling separation in water. This thesis aims to investigate the same passive flow control techniques in air.

To investigate this phenomenon, several sets of microflaps were designed and manufactured with a 3D printer. The microflaps were designed in both 2D (rectangular) and 3D (mirroring shark scale geometry) variants. These microflaps were placed in a low-speed wind tunnel in the lower 5% of the boundary layer. Solid fences and a flat plate diffuser with suction were placed in the tunnel to create different separated flow regions. A hot film probe was used to measure velocity magnitude in the streamwise plane of the separated regions. The results showed that low-speed airflow is capable of bristling objects in the boundary layer. When placed in a region of reverse flow, the microflaps were passively actuated. Microflaps fluctuated between bristled and flat states in reverse flow regions located close to the reattachment zone.

A comprehensive survey of available numerical methods and models was performed on the open source computational fluid dynamics solver OpenFOAM version 2.0 for incompressible turbulent bluff body flows. Numerical methods are illuminated using source code for side-by-side comparison. For validation, the accuracy of flow predictions over a sphere in the subcritical regime and delta wing with sharp leading edge is assessed. Solutions show mostly good agreement with experimental data and data obtained from commercial software. A demonstration of the numerical implementation of a dynamic hybrid RANS/LES framework is also presented, including results from test studies.

This thesis studies the feasibility of using neural networks to ''learn" the vortex panel method. This study is motivated by the desire for the rapid and accurate prediction of fluid flows during the preliminary design of engineering systems, where traditional computational fluid dynamics (CFD) are too computationally costly. The results show that a two-layer neural network can estimate the pressure coefficient and elements in the vortex-panel influence-coefficient matrix. However, when the neural-network-predicted influence-coefficient matrix is used to estimate the pressure coefficients, the results are in poor agreement with the baseline prediction, although general trends are captured.

Within the field of Guidance, Navigation, and Control, the navigation process refers to accurately determining one's position in space. The quest of accurate navigation has shaped human history. Early navigation techniques involved dead reckoning with infrequent measurement updates from line-of-sights to stars or landmarks on the shore. The first practical inertial navigation system (INS) attributed to the German V-2 missile in 1942. In the early 1960's the Kalman filter was developed to aid in the merging of mathematical models and measurement updating. Throughout the space age and continuing into today's remote systems the hardware has made vast improvements; however, the navigation filtering theory has remained mostly stagnant with the multiplicative Extended Kalman Filter (MEKF) still being the workhorse of most modern INS applications.

In most INS applications, the state vector usually consists of the attitude, position, velocity, and Inertial Measurement Unit (IMU) calibration parameters such as biases and scale factors. Because position-type measurements are usually only given, such as pseudoranges to GPS satellites, the observability of the attitude and gyroscope calibration parameters is weak. Since the early days of employing the MEKF for INS applications, and even modern-day applications, the state errors are defined as a simple algebraic difference between the truth and the estimate.

It has been argued that a new state-error definition is required in which state-error quantities are defined using elements expressed in a common frame. This provides a realistic framework to describe the actual errors. In previous work, the errors were put into a common frame using the estimated attitude error, which led to the ``geometric EKF'' (GEKF). The GEKF provides extra transport terms, due to error-attitude coupling with the states, in the filter. This previous work focused strictly on attitude estimation, which incorporated only ``body-frame" errors. In this work the GEKF is extended to the INS formulation. Here, errors are considered for both the body frame and the filter's navigation frame.

In this work, it is shown how these body-frame and navigation-frame errors are related through a similarity transformation. The body-frame error, and the navigation-frame error through this similarity transformation, are first examined in a Planar Inertial Navigation (PIN) problem. For the PIN problem, the MEKF and GEKF algorithms are derived using the same kinematic and measurement models. These algorithms are then studied for a single simulation test case; these results were then verified via Monte Carlo analysis. For this example, it was shown that the body-frame errors had a faster convergence for the GEKF; however, the navigation-frame states showed slower convergence. It is argued here that the direct comparison of these results is inconclusive since both filters employ different error definitions; therefore, the errors being examined utilize a different error metric. It can be stated that the errors realized by the GEKF are more representative of the real errors experienced by the system.

The body-frame and navigation-frame errors are also used to derive the GEKF for three navigation filters. Specifically, this work examines the absolute Earth-Centered-Earth-Fixed (ECEF) navigation, relative ECEF navigation, and North-East-Down (NED) navigation filters. The counterpart MEKF filters are also derived in this work to illustrate the differences seen in the state matrices due to the additional coupling terms. These filters are also studied via simulation studies. However, now the body-framed vectors do not show faster convergence for the GEKF. This is because the measurement update for this specific example is unaffected by the new error definition. The measurements are not affected by the new error definition because these filters only utilize pseudo-GPS position measurements, and it is shown that the position error still utilizes the old error definition due to its kinematic relation to the velocity error.

Finally, this work conducts a linearize analysis of a simplified INS where the GEKF in the NED frame is considered. It is shown via a stationary analysis that the fundamental frequencies of the GEKF are the same as those of the MEKF. This is because during the stationary analysis all of the additional coupling terms seen in the state error matrix are neglected due to assumptions made about the vehicle's motion.

The variable pitch quadrotor is not a new concept but has been largely ignored in small unmanned aircraft, unlike the fixed pitch quadcopter which is controlled only by changing the RPM of the motors and only has about 30 minutes of total flight time. The variable pitch quadrotor can be controlled either by the change of the motor RPM or rotor blade pitch angle or by the combination of both. This gives the variable pitch quadrotor potential advantages in payload, maneuverability and long endurance flight. This research is focused on the design methodology for a variable pitch quadrotor using a single motor with potential applications for a long endurance flight. This variable pitch quadcopter uses a single power plant to power all four rotors through a power transmission system. All four rotors have the same rpm but vary the blade pitch angle to control its attitude in the air. A proof of concept variable pitch quadcopter is developed for testing the drivetrain mechanism on the vehicle and evaluating performance of the vehicle through numbers of testing.

In today’s space industry, satellite formation flying has become a cost-efficient alternative solution for science, on-orbit repair and military time-critical missions. While in orbit, the satellites are exposed to the space environment and unpredictable spacecraft on-board disturbances that negatively affect the attitude control system’s ability to reduce relative position and velocity error. Satellites utilizing a PID or adaptive controller are typically tune to reduce the error induced by space environment disturbances. However, in the case of an unforeseen spacecraft disturbance, such as a fault in an IMU, the PID based attitude control system effectiveness will deteriorate and will not be able to reduce the error to an acceptable magnitude.

In order to address the shortcomings a PID-Extended Memory RNN (EMRNN) adaptive controller is proposed. A PID-EMRNN with a short memory of multiple time steps is capable of producing a control input that improves the translational position and velocity error transient response compared to a PID. The results demonstrate the PID-EMRNN controller ability to generate a faster settling and rise time for control signal curves. The PID-EMRNN also produced similar results for an altitude range of 400 km to 1000 km and inclination range of 40 to 65 degrees angles of inclination. The proposed PID-EMRNN adaptive controller has demonstrated the capability of yielding a faster position error and control signal transient response in satellite formation flying scenario.

Carbon-Fiber Reinforced Polymers (CFRPs) provides superior mechanical properties and low weight, enabling their extensive use in the aerospace industry. Susceptibility to internal damage due to out-of-plane loads and poor electrical properties are some of their major challenges that require to be addressed in order to increase the utilization of composites in further aerospace structures. Lightning strikes can lead to catastrophic damage, inflicting high repair and certification costs. Lightning Strike Protection (LSP) solutions such as integration of metallic meshes or foils into the composite structures, even though effective, impose extra costs and hinders the aircraft performance due to the increased weight of the aircraft.

This research aims at the development of a different LSP solution, by enhancing the electrical conductivity of composite, while maintaining a sufficient degree of mechanical properties. The use of non-woven conductive interlayers was proposed for manufacturing of conductive composites. Highly-conductive, low-aerial-weight carbon veil was utilized to manufacture prepreg-based CF/Epoxy laminates, which are generally toughened, in order to improve their conductivity using vacuum bag only (VBO) and heat-pressing techniques. Further, a bi-functional interlayer of graphene coated Polyamide (PA) was developed using interfacial trapping method. This conductive thermoplastic interlayer was then utilized for manufacturing Benzoxazine (BZ) infused carbon fabric laminate with Vacuum-assisted resin transfer molding (VARTM) method, which acted as a conductive toughener and improves the Inter-laminar Fracture Toughness (ILFT) as well as to increase the electrical conductivity.

The effects of the incorporation of non-woven interlayers on the electrical conductivity, thermal behavior of composites, and mechanical properties such as shear strength, compressive strength, and the ILFT (Mode-I and Mode-II) were investigated in this study. In both types of composites, an increase in electrical properties, as well as mechanical properties, were observed. The only exception was in the Mode-I ILFT of the CF/Epoxy prepregs, which decreased with the increase of the areal weight of the interleaved carbon veils. The mechanical properties increased in the range of 9%–138% with the only decrement observed in Mode-I ILFT of CF/Epoxy with carbon veils of 25%. The volume resistivity of the CF/Epoxy samples decreased significantly by approximately 50% due to the incorporation of the conductive interlayer. This added feature was used to develop a structural health monitoring (SHM) procedure. The conductive composite showed an increased sensitivity in detecting the pre-identified damage location in the composites.