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Articles 4591 - 4620 of 5699
Full-Text Articles in Engineering
Prioritization Schemes Of The Air Force Logistics Reparable Pipeline, Brigham K. Briggs
Prioritization Schemes Of The Air Force Logistics Reparable Pipeline, Brigham K. Briggs
Theses and Dissertations
This study describes the prioritization schemes utilized in the Air Force logistics reparable pipeline. The reparable pipeline is defined and illustrated with flowcharts. A literature review examines previous research conducted on the reparable pipeline, including analytic and conceptual pipeline models, and pipeline management studies. In addition, the topic of prioritization as defined in the production/operations management academic discipline is reviewed. Prioritization schemes of the reparable pipeline are reported in tabular format in addition to descriptions of the various prioritization schemes. Recommendations for pipeline improvement and further research form a basis from which pipeline operation may be improved. p9
Maximum Likelihood Estimation Of Exponentials In Unknown Colored Noise For Target Identification In Synthetic Aperture Radar Images, Matthew P. Pepin
Maximum Likelihood Estimation Of Exponentials In Unknown Colored Noise For Target Identification In Synthetic Aperture Radar Images, Matthew P. Pepin
Theses and Dissertations
This dissertation develops techniques for estimating exponential signals in unknown colored noise. The Maximum Likelihood (ML) estimators of the exponential parameters are developed. Techniques are developed for one and two dimensional exponentials, for both the deterministic and stochastic ML model. The techniques are applied to Synthetic Aperture Radar (SAR) data whose point scatterers are modeled as damped exponentials. These estimated scatterer locations (exponentials frequencies) are potential features for model-based target recognition. The estimators developed in this dissertation may be applied with any parametrically modeled noise having a zero mean and a consistent estimator of the noise covariance matrix. ML techniques …
Numerical Analysis Of Two And Three Dimensional Recessed Flame Holders For Scramjet Applications, Douglas L. Davis
Numerical Analysis Of Two And Three Dimensional Recessed Flame Holders For Scramjet Applications, Douglas L. Davis
Theses and Dissertations
This study investigated the flame holding properties of recessed cavities in supersonic flow using numerical analysis techniques. A simplified analytical model indicated that an important property for flame holding was the lower residence time. Several chemical kinetic rate models for hydrogen and hydrocarbon combustion were compared. The perfectly stirred reactor model also indicated that trace species diffusion should increase flame spreading rate, and that heat loss reduces flame holding limits. After nonreacting calibration, two-dimensional simulations confirmed the perfectly stirred reactor results for blowout limits. Also, the effect of trace species diffusion on flame spreading was shown to be negligible, and …
Development And Analysis Of A Dual-Role Fighter Deployment Footprint Logistics Planning Equation, Stanley E. Griffis, Joseph D. Martin
Development And Analysis Of A Dual-Role Fighter Deployment Footprint Logistics Planning Equation, Stanley E. Griffis, Joseph D. Martin
Theses and Dissertations
This research investigated the deployment footprint of a developing dual-role fighter, the Joint Strike Fighter (JSF). This thesis documents the creation of a point estimate linear equation and a spreadsheet model for deployment footprint analysis and provides three example applications of the spreadsheet model for the JSF. The development of the model focused towards direct application in the JSF acquisition process, however, this research also serves as a proof of concept for modeling any future weapon system's deployment footprint. The developed spreadsheet model allows the model manipulator to select a baseline weapon system then alter the various components which make …
Integration Of The Distribution And Repair In Variable Environments (Drive) Model Into Micap Policy, Bradley E. Anderson
Integration Of The Distribution And Repair In Variable Environments (Drive) Model Into Micap Policy, Bradley E. Anderson
Theses and Dissertations
The United States Air Force has implemented the Distribution and Repair In Variable Environments (DRIVE) Model to a limited degree for a limited range of items, and policy makers seem unsure as to the proper level of DRIVE utilization. New asset release sequences and policies have been proposed without evidence to support those decisions. The purpose of this study is to explore different levels of DRIVE implementation relating to proposed asset release policies in order to provide some evidence on which policy could best support Air Force weapon systems. A second purpose of this study is to show how the …
Crisis Airlift Management: Effective Scheduling, David C. Penny
Crisis Airlift Management: Effective Scheduling, David C. Penny
Theses and Dissertations
This is a report on the basis, data, findings, and models developed for US Transportation Command and Headquarters Air Mobility Command. Focusing on the dynamic scheduling problem that arises during management of airlift assets into space-constrained airfields during a crisis, this research explains the conditions surrounding this event, discusses several areas of potential cross-functional applications, presents analysis of current performance, and provides several modeling possibilities for schedule efficiency. Past research in the Flow Management Problem (an airline flow control issue) is extensive, but specific applications in crisis airlift are long in coming, primarily due to the complexity of the environment. …
Numerical Analysis Of Two And Three Dimensional Recessed Flame Holders For Scramjet Applications, Douglas L. Davis
Numerical Analysis Of Two And Three Dimensional Recessed Flame Holders For Scramjet Applications, Douglas L. Davis
Theses and Dissertations
This study investigated the flame holding properties of recessed cavities in supersonic flow using numerical analysis techniques. A simplified analytical model indicated that an important property for flame holding was the lower residence time. Several chemical kinetic rate models for hydrogen and hydrocarbon combustion were compared. The perfectly stirred reactor model also indicated that trace species diffusion should increase flame spreading rate, and that heat loss reduces flame holding limits. After nonreacting calibration, two-dimensional simulations confirmed the perfectly stirred reactor results for blowout limits. Also, the effect of trace species diffusion on flame spreading was shown to be negligible, and …
Electromagnetic Scattering From Semi-Infinite Planar Arrays, Peter J. Collins
Electromagnetic Scattering From Semi-Infinite Planar Arrays, Peter J. Collins
Theses and Dissertations
A hybrid method of moments (MM) based numerical model for the electromagnetic scattering from large finite by infinite planar slot arrays is developed. The method incorporates the novel concept of a physical basis function (PBF) to dramatically reduce the number of required unknowns. The model can represent a finite number of slot columns with slots oriented along the infinite axis, surrounded by an arbitrary number of coplanar dielectric slabs. Each slot column can be loaded with a complex impedance, allowing one to tailor the edge currents to provide a desired echo width pattern. The surface equivalence theorem is used to …
An Analysis Of Air Mobility Express Requirements Operating Within A Lean Logistics Wartime Environment, Jennifer A. Bollinger, Kellie L. Davila-Martinez
An Analysis Of Air Mobility Express Requirements Operating Within A Lean Logistics Wartime Environment, Jennifer A. Bollinger, Kellie L. Davila-Martinez
Theses and Dissertations
Lean Logistics was developed in response to budget cuts, force reductions, and a new political world order. The primary objective of Lean Logistics is to minimize the total system wide costs of the Air Force organization. Currently, the Air Force is seeking to cut costs by reducing inventories, improving repair processes, and employing faster transportation where possible. The purpose of this thesis is to determine if the Air Mobility Express (AMX) current sizing plan is capable of supporting the retrograde assets generated during the sustainment portion of a war. The Dyna-METRIC version 6.4 simulation program is employed to analyze the …
An Evaluation Of U.S. Air Force Aviation Fuel Consumption Factors To Accurately Predict Aviation Fuel Costs By Aircraft Mission, Design, And Series, Jodi A. Clayton
An Evaluation Of U.S. Air Force Aviation Fuel Consumption Factors To Accurately Predict Aviation Fuel Costs By Aircraft Mission, Design, And Series, Jodi A. Clayton
Theses and Dissertations
The purpose of this thesis was to undertake a systematic, scientific study of the accuracy of Air Force published fuel factors to estimate fuel costs at MAJCOM level by mission, design, and series (MDS) since the implementation of the Fuels Automated Management System (FAMS) under the current environment of decentralized aviation fuel (AVFUEL) funding. The research found that, at MAJCOM level, the use of USAF published AVFUEL factors in estimating out-year costs would have overstated costs in both fiscal year (FY) 1994 and FY 1995 by $2.5 M (FY 1994), by $.25 M (FY 1995), and may potentially understate costs …
An Analysis Of The Relationship Between Risk Alignment And Cost Performance, Christopher M. Wegner
An Analysis Of The Relationship Between Risk Alignment And Cost Performance, Christopher M. Wegner
Theses and Dissertations
This thesis investigates the relationship between risk alignment and cost performance in DoD procurement. Both the government and the contractor conduct independent assessments of the perceived risk in a contract. The results of inaccurate risk assessments are undesirable consequences, one of which is the inability to achieve cost targets. If both the contractor's and the government's perceptions of risk are in alignment with the actual risk involved in a contract, then we would expect minimal impact. If, however, either or both parties fail to accurately assess the actual risk, then we would expect consequences, such as cost overrun/underrun. The level …
Information Requirements Analysis: A Qualitative Characterization Of The Flightline Expediter For The Integrated Maintenance Information System, John C. Gorla Jr.
Information Requirements Analysis: A Qualitative Characterization Of The Flightline Expediter For The Integrated Maintenance Information System, John C. Gorla Jr.
Theses and Dissertations
The Integrated Maintenance Information System (IMIS) concept design includes the addition of management tools to access IMIS databases and provide communication capabilities between flightline technicians and supervisors. Armstrong Laboratory has developed a portable maintenance aid for technicians, and sponsored this research to investigate the requirements for a computer based tool for the expediter, The basic hardware and software requirements document for IMIS, the System/Segment Specification (SSS), contains task information that closely corresponds to the expediter job description as defined in Air Combat Command Instruction 21-166. This research compiled a list of information requirements for the expediter from the IMIS SSS …
The Value Of The Civil Reserve Air Fleet: How Much Could The Dod Spend On Incentives?, Pamela S. Donovan
The Value Of The Civil Reserve Air Fleet: How Much Could The Dod Spend On Incentives?, Pamela S. Donovan
Theses and Dissertations
This study evaluated the value of the Civil Reserve Air Fleet (CRAF) program to the DoD and explored the amount that could be spent to remove potential obstacles to participation with aviation insurance and lost market share. In comparing the value of the CRAF and the cost of current incentives, it was determined that up to $1.4 million could be spent on additional incentives, annually. For multiple aircraft losses and liability claims, the Air Force would need to tap into the Defense Business Operating Fund. Therefore, a sensitivity analysis was conducted and found that for low valued aircraft, such as …
Parameter Estimation For Superimposed Weighted Exponentials, Edwards A. Ingham
Parameter Estimation For Superimposed Weighted Exponentials, Edwards A. Ingham
Theses and Dissertations
The approach of modeling measured signals as superimposed exponentials in white Gaussian noise is popular and effective. However, estimating the parameters of the assumed model is challenging, especially when the data record length is short, the signal strength is low, or the parameters are closely spaced. In this dissertation, we first review the most effective parameter estimation scheme for the superimposed exponential model: maximum likelihood. We then provide a historical review of the linear prediction approach to parameter estimation for the same model. After identifying the improvements made to linear prediction and demonstrating their weaknesses, we introduce a completely tractable …
Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup
Low-Velocity Impact On Composite Sandwich Plates, Eric J. Herup
Theses and Dissertations
The objective of the research is to analytically model the response of composite sandwich plates to low velocity impact. A displacement based, plane stress, finite element code is modified for this purpose. Major new algorithms include 5th order Hermitian interpolation, three dimensional equilibrium integration for transverse stress calculations, sandwich core modeling as an elastic plastic foundation, loading by simulated contact with a spherical indentor, adaptive mesh, damage prediction, damage progression via stiffness reduction, and local-global analysis for displacement. An experimental effort is also included in which composite sandwich plates with graphite epoxy facesheets and Nomex honeycomb core are subjected to …
Reaction Based Grasp Force Assignment, Mark W. Hunter
Reaction Based Grasp Force Assignment, Mark W. Hunter
Theses and Dissertations
An iterative method is developed by which the contact forces required to apply an arbitrary wrench (six elements of force and moment) to a stably grasped object may be calculated quickly. The assignment of contact forces, given a required object wrench, is accomplished with the use of fuzzy logic. This concept is referred to as the fuzzy logic reactive system (FLRS). The solution is versatile with respect to goals inherent in the rulebase and the input parameters, and is also applicable for an arbitrary number of contacts. The goal presented in this research, to illustrate the concept of the FLRS, …
Structures And Techniques For Implementing And Packaging Complex, Large Scale Microelectromechanical Systems Using Foundry Fabrication Processes, John H. Comtois
Structures And Techniques For Implementing And Packaging Complex, Large Scale Microelectromechanical Systems Using Foundry Fabrication Processes, John H. Comtois
Theses and Dissertations
Microelectromechanical Systems, or 'MEMS' is a broad new field of research into devices that range in size from a few microns to a few millimeters. Much of the technology supporting MEMS research is borrowed from the microelectronics industry; so MEMS holds out the promise of batch fabrication of microminiaturized machines that can be easily integrated with electronics. This dissertation research investigated structures and methods for implementing and packaging complex, large scale microelectromechanical devices and systems using commercially available foundry fabrication processes. It specifies methods for creating and packaging large, complex MEM systems, allowing the exploration of new MEMS architectures at …
Simultaneous Composite Tailoring And Bending Control Optimization For Damping The Torsional Vibration Of A Plate, Douglas W. Dehart
Simultaneous Composite Tailoring And Bending Control Optimization For Damping The Torsional Vibration Of A Plate, Douglas W. Dehart
Theses and Dissertations
A method for the simultaneous structural and control optimization for torsional vibration of a composite plate which simulates a spacecraft solar array structure was developed in this study. Included in the optimization of the plate are the location of the piezoceramic actuators and sensors that provide bending/torsion actuation, the control gains, and the orientation of the graphite fibers in the composite plies. This research included the effects of using composite tailoring to promote the coupling of the twisting-bending mode to enhance the damping system. The plate was modeled by classical lamination plate theory using a linear elastic strain-displacement theory. This …
Residual Strength After Fatigue Of Unidirectional And Cross-Ply Metal Matrix Composites At Elevated Temperature, Sen-Tzer Chiou
Residual Strength After Fatigue Of Unidirectional And Cross-Ply Metal Matrix Composites At Elevated Temperature, Sen-Tzer Chiou
Theses and Dissertations
This study investigated the residual strength of the unidirectional and cross-ply laminates of SCS-6 / Ti-15-3, metal matrix composite at elevated temperature 427°C (800°F) after under tension-tension load controlled mode. For this purpose, several specimens were fatigued to various fractions of the fatigue life and then loaded monotonically to failure. The purpose of this study was to determine the effects of different levels of fatigue damage on the composite's strength. The unidirectional specimens were cycled at a 900 MPa maximum stress at a frequency of 10 Hz, while, the cross-ply specimens were tested at both 300 MPa and 450 MPa …
Effect Of Transient Body Force On The Performance Of An Axial Grooved Heat Pipe, Wen-Lung Wang
Effect Of Transient Body Force On The Performance Of An Axial Grooved Heat Pipe, Wen-Lung Wang
Theses and Dissertations
An experimental investigation was performed to determine the effect of transient body forces on the performance of an ammonia/aluminum axial groove heat pipe. The effects of increased body forces on the dryout and rewet performance were simulated by tilting the heat pipe to different inclination angles. Theoretical calculations predicted the dryout length varied with different final inclination angles. The steady state experimental work was performed by tilting the heat pipe from 0 degrees to different final inclination angles for a long period of time until totally dryout occurred. Then, from the plot of the changing temperature at each location along …
Design Of Gradient Index Optical Thin Films, Jeffrey J. Druessel
Design Of Gradient Index Optical Thin Films, Jeffrey J. Druessel
Theses and Dissertations
Gradient index thin films provide greater flexibility for the design of optical coatings than the more conventional 'layer' films. In addition, gradient index films have higher damage thresholds and better adhesion properties. This dissertation presents an enhancement to the existing inverse Fourier transform gradient index design method, and develops a new optimal design method for gradient index films using a generalized Fourier series approach. The inverse Fourier transform method is modified to include use of the phase of the index profile as a variable in rugate filter design. Use of an optimal phase function in Fourier-based filter designs reduces the …
An Investigation Of The Characteristics Of Regenerative Heat Exchangers, Timothy J. Murphy
An Investigation Of The Characteristics Of Regenerative Heat Exchangers, Timothy J. Murphy
Theses and Dissertations
The objective of the current research was to investigate the effects of reducing screen thickness on the volume and compactness factor of stacked, wire-screen regenerators. An improved transient step-change method was devised which integrated experimental data with a numerical model of the flow to determine the heat transfer coefficient and friction factor. The improvements to the approach are: (1) the measured inlet temperature trace is used, (2) the heat transfer coefficient is based on a parameter called the sponge effect delay time, and (3) the important effect of the tube surrounding the matrix is included in the numerical model. The …
Optimal Continuous-Thrust Orbit Transfers, James D. Thorne
Optimal Continuous-Thrust Orbit Transfers, James D. Thorne
Theses and Dissertations
The minimum time orbital transfer problem for spacecraft with steerable, continuous thrust of constant magnitude may be solved using Euler-Lagrange theory, which leads to the optimal control law in terms of Lagrange multipliers. However, the initial values of the Lagrange multipliers are not known from the orbital boundary conditions. Using analytical and empirical results, the optimal initial costates are modeled as functions of the problem parameters which are the initial thrust acceleration, A, and the final orbit radius, H, in canonical units. For circle to circle, coplanar orbit transfers, these approximate initial costate models lead to convergence in the shooting …
Direct Measurement Of Skin Friction In High Temperature And Impulsively Started Supersonic Flowfields, David M. Hazelton
Direct Measurement Of Skin Friction In High Temperature And Impulsively Started Supersonic Flowfields, David M. Hazelton
Theses and Dissertations
This study further developed and improved a miniature plastic cantilever floating element gauge for direct measurement of skin friction in shock tunnel tests. Gauge durability and usability were improved, and potential sources of error were quantified. A high frequency 30 kHz gauge (compared to the baseline 10 kHz gauge) was constructed by using an I-beam cantilever rather than a round beam. A directional gauge was developed and demonstrated by measuring the flowfield about a fin in the AFIT shock tunnel. The 10 kHz and 30 kHz gauges were used in two scramjet test sequences conducted at Mach 14 enthalpy (stagnation …
Gain-Scheduled Aircraft Control Using Linear Parameter-Varying Feedback, Martin R. Breton
Gain-Scheduled Aircraft Control Using Linear Parameter-Varying Feedback, Martin R. Breton
Theses and Dissertations
Systems which vary significantly over an operating envelope, such as fighter aircraft, generally cannot be controlled by a single linear time-invariant controller. As a result, gain-scheduling methods are employed to design control laws which can provide the desired performance. This thesis examines a relatively new approach to gain-scheduling, in which the varying controller is designed from the outset to guarantee robust performance, thereby avoiding the disadvantages of point designs. Specifically, the parameter-varying (LPV) aircraft model is linearized using linear fractional transformations (LFT's), and the resulting control problem is characterized as the solution to a set of four linear matrix inequalities …
Investigation Of Radio Wave Propagation In The Martian Ionosphere Utilizing Hf Sounding Techniques, Robert J. Yowell
Investigation Of Radio Wave Propagation In The Martian Ionosphere Utilizing Hf Sounding Techniques, Robert J. Yowell
Theses and Dissertations
This thesis presents a preliminary design of an ionospheric sounder to be carried aboard one or more of NASA's Mars Surveyor landers. Past Russian and American probes have indicated the existence of an ionosphere, but none of these missions remotely sensed this atmospheric layer from the surface. The rationale for utilizing a surface-based Martian ionospheric sounder is discussed. Based on NASA's choice of launch vehicle and power source, a low-weight, low-powered Chirp sounder using a horizontally-polarized dipole antenna is recommended for the sounder experiment. The sounder experiment should be conducted for at least one Martian year, in order to investigate …
Nonlinear Analysis Of Airfoil Flutter At Transonic Speeds, Scott A. Morton
Nonlinear Analysis Of Airfoil Flutter At Transonic Speeds, Scott A. Morton
Theses and Dissertations
Hopf-bifurcation analysis is used to determine flutter boundaries of a pitch and plunge airfoil (PAPA) at transonic Mach number conditions. The PAPA model is a coupling of the Euler equations and a two-degree-of-freedom structural model composed of linear and torsional springs. The Euler equations are discretized using an upwind total variation diminishing scheme (TVD) of Harten and Yee. Equilibrium solutions of the PAPA model are computed using Newton's method and dynamic solutions are explicitly integrated in time with first-order accuracy. The Hopf-bifurcation point, which models the flutter condition, is computed directly by solving an extended system of equilibrium equations following …
An Investigation Of Learning Curves And Their Use In Simulation, Jennie H. Lommel
An Investigation Of Learning Curves And Their Use In Simulation, Jennie H. Lommel
Theses and Dissertations
In 1995, the C-17 Factory Simulation Model (FSM) was developed to enable analysts to address 'what-if' questions about the resources required to build future aircraft, and is based on learning curve models that are used to both portray and simulate future aircraft production. In this thesis, we examine and develop alternate learning curve models that also utilize a small amount of initial production data to portray the relationship between the number of aircraft built and the resources required to build them. The goal is to identify a model which not only provides a good fit and forecast based on a …
Generalized Hidden Filter Markov Models Applied To Speaker Recognition, John M. Colombi
Generalized Hidden Filter Markov Models Applied To Speaker Recognition, John M. Colombi
Theses and Dissertations
Classification of time series has wide Air Force, DoD and commercial interest, from automatic target recognition systems on munitions to recognition of speakers in diverse environments. The ability to effectively model the temporal information contained in a sequence is of paramount importance. Toward this goal, this research develops theoretical extensions to a class of stochastic models and demonstrates their effectiveness on the problem of text-independent (language constrained) speaker recognition. Specifically within the hidden Markov model architecture, additional constraints are implemented which better incorporate observation correlations and context, where standard approaches fail. Two methods of modeling correlations are developed, and their …
An Investigation Into The Effects Of Lateral Aerodynamic Asymmetries, Lateral Weight Asymmetries, And Differential Stabilator Bias On The F-15 Directional Flight Characteristics At High Angles Of Attack, David R. Evans
Theses and Dissertations
The F-15 is a stable aircraft throughout most of its flight envelope. However, it still exhibits an uncommanded yawing and rolling tendency at high angles-of-attack. Identified influencing factors of this uncommanded motion are lateral aerodynamic asymmetries, lateral weight asymmetries, and differential stabilator bias. Previous research into the effects of these influencing factors has been qualitative. This thesis quantifies the effects and then identifies a symmetric F-15 configuration. The quantifying metric presented is net yaw acceleration. This thesis used both computer simulation and experimental flight test to quantify these effects. A discussion of each influencing factors effects on the F-15B high …