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At least 721 records · Page 40

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NASA NTRS1985

A vortex-lattice method for general, unsteady aerodynamics

A general method of calculating unsteady, incompressible, inviscid, three-dimensional flows around arbitrary planforms has been developed. The method is an extension of the vortex-lattice technique. It is not limited by aspect ratio, camber, or angle of attack, as long as vortex breakdown does not occur above the surface of the wing and separation occurs only along sharp edges. As the wing performs arbitrary maneuvers, the position of the wake and the distribution of circulation on the wing and in the wake are obtained as functions of time. One desirable feature of the present method is its ability to treat steady lifting flows very efficiently. Several examples of steady and unsteady flows are presented. These include rectangular wings, with and without flaps, delta, and cropped delta wings.

Konstadinopoulos, P.↗

NASA NTRS1986

Space infrared telescope facility (SIRTF) focal plane

The Infrared Array Camera (IRAC), Multiband Imaging Photometer (MIP), and the Infrared Spectrometer (IRS) of the Space Infrared Telescope Facility (SIRTF) are described. The configuration and capabilities of the SIRTF are discussed. The small silicon array module, the germanium array module, and bolometer module of MIP, which is designed to provide sensitivity limited only by natural background and telescope emissions over the 3-200 micron spectral range, and allow diffraction limited imaging, super resolution techniques, mapping, and surveying, are examined. The objectives and design of the IRAC, which is constructed for two-dimensional photometry and imaging over a wavelength region from 2-30 microns and to perform extragalactic, galactic and solar system astronomy, are studied. The use of the IRS to study the dynamical and physical phenomena of the universe in the 2.5-200 micron range is analyzed.

Ramos, R.↗

NASA NTRS1986

Plasma interactions with large spacecraft

Space is playing a rapidly expanding role in the conduct of the Air Force mission. Larger, more complex, high-power space platforms are planned and military astronauts will provide a new capability in spacecraft servicing. Interactions of operational satellites with the environment have been shown to degrade space sensors and electronics and to constrain systems operations. The environmental interaction effects grow nonlinearly with increasing size and power. Quantification of the interactions and development of mitigation techniques for systems-limiting interactions is essential to the success of future Air Force space operations.

Sagalyn, Rita C.↗

NASA NTRS1987

Experience in distributed parameter modeling of the Spacecraft Control Laboratory Experiment (SCOLE) structure

The Spacecraft Control Laboratory Experiment (SCOLE) configuration is used to compare exact and approximate solutions of the partial differential equations which define its structural dynamics. The need for a proof model for evaluating competing control laws demands that solutions be generated which not only exhibit accurate modal characteristics, but precise static deflections as well. Because precise pointing is required, the motion of the end bodies of the Shuttle-attached antenna must be known with great accuracy. Modal models are attractive because of their stable solutions but require hundreds of modes to obtain a static deflection accuracy of only one percent. Although proportional damping in bending agrees well with experimental results using the SCOLE experimental apparatus, modes which involve both torsion and bending differ significantly from proportional damping. A lumped mass model is used to generate exact static deflections, but only approximate modal characteristics. Asymptotic solutions to the distributed parameter system approximate very accurately the modal characteristics at high mode numbers. Ways are examined for refining the approximate solutions by applying a first-order variation and by employing singular perturbation techniques which are usually limited to ordinary differential equations. The most accurate solutions of the distributed parameter model of SCOLE are obtained by combining exact and asymptotic solutions.

Taylor, L. W.↗

NASA NTRS1987

Multichannel optical correlator/convolver utilizing the magnetooptic spatial light modulator

The paper describes a multichannel correlator/convolver architecture utilizing an acoustooptic light modulator for the one dimensional channel and a magnetooptic spatial light modulator (MOSLM) for the second two-dimensional parallel channel. The MOSLM allows greater parallelism to be implemented in this correlator/convolver design than was previously reported. The implementation of 24 parallel channels with a 48 x 48 device is demonstrated. Experimental data are presented and the ways of increasing the number of parallel channels using commercially available MOSLMs and other previously discussed techniques, such as frequency multiplexing, are discussed. It is shown that over 2000 parallel channels are possible at 32-bit accuracy. A technique for obtaining a limited gray scale is also discussed.

Davis, Jeffrey A.↗

NASA NTRS1989

Sufficiency and necessity in reliability modeling

Limitations of current analytic techniques in estimating the reliability of life-critical electronics systems are discussed. A new framework for specification of recovery and fault-handling submodels is suggested, and is shown through several examples to provide substantially improved modeling accuracy and flexibility. Implementation of the new technique in an X-windows based system, XHARP, is also described. The implementation allows for an automated behavioral decomposition of full system models, heretofore unavailable in such tools.

Geist, Robert↗

NASA NTRS1989

An Entropy-Based Approach to Nonlinear Stability

Many numerical methods used in computational fluid dynamics (CFD) incorporate an artificial dissipation term to suppress spurious oscillations and control nonlinear instabilities. The same effect can be accomplished by using upwind techniques, sometimes augmented with limiters to form Total Variation Diminishing (TVD) schemes. An analysis based on numerical satisfaction of the second law of thermodynamics allows many such methods to be compared and improved upon. A nonlinear stability proof is given for discrete scalar equations arising from a conservation law. Solutions to such equations are bounded in the L sub 2 norm if the second law of thermodynamics is satisfied in a global sense over a periodic domain. It is conjectured that an analogous statement is true for discrete equations arising from systems of conservation laws. Analysis and numerical experiments suggest that a more restrictive condition, a positive entropy production rate in each cell, is sufficient to exclude unphysical phenomena such as oscillations and expansion shocks. Construction of schemes which satisfy this condition is demonstrated for linear and nonlinear wave equations and for the one-dimensional Euler equations.

Merriam, Marshal L.↗

NASA NTRS1990

Low-resistance noble metal contacts to high-temperature superconductors

Low-resistance contacts were made to both YBa2Cu3O(7-x) and Bi2BaSr2Cu2O8, and to related superconducting compounds by melting gold or silver pads onto the samples before the final oxygen treatment. Scanning electron microscope studies show that both gold and silver do not diffuse far from the contact area. The surface contact resistivity of the best contacts made by the melting technique has an upper limit value in the 10 to the -8th ohm sq cm range at 77 K. This contact resistivity shows no significant change in its value over a period of 17 months. Furthermore, an electron radiation dose of 5.7 x 10 to the 17th electron/sq cm only doubled the contact resistivity.

Selim, R.↗

NASA NTRS1991

Radiographic Detection Of Voids In SiC And SiN Ceramics

Two reports describe experimental studies of efficacy of radiographic detection of voids in "green" (unfired) and sintered SiC and SiN ceramics. Projection radiography provides greater sensitivity of detection. Studies motivated by need for nondestructive technique to detect flaws limiting strengths of sintered structural ceramic components.

Baaklini, George Y.↗

NASA NTRS1991

A novel boundary layer sensor utilizing domain switching in ferroelectric liquid crystals

This paper describes the design and the principles of operation of a novel sensor for the optical detection of a shear stress field induced by air or gas flow on a rigid surface. The detection relies on the effects of shear-induced optical switching in ferroelectric liquid crystals. It is shown that the method overcomes many of the limitations of similar measuring techniques including those using cholesteric liquid crystals. The present method offers a preferred alternative for flow visualization and skin friction measurements in wind-tunnel experiments on laminar boundary layer transition investigations. A theoretical model for the optical response to shear stress is presented together with a schematic diagram of the experimental setup.

Parmar, D. S.↗

NASA NTRS1991

Extended behavioral decomposition for estimating ultrahigh reliability

Limitations of some analytic techniques in approximating the reliability of life-critical electronic systems are discussed, and a framework for the specification of recovery and fault-handling submodels is suggested. The framework makes full use of the instantaneous jump theorem by viewing the collection of interfering, premature exits from any fault handling and recovery submodel as defining a new, competing process submodel. This approach allows a greater flexibility in submodel representation, since submodels may contain arbitrary entrance arcs, exit arcs, and competing, interfering transitions with arbitrary destinations. Since the effects of near-coincident faults need not be represented as system failure events, the reliability estimates produced need not be unduly conservative. Comparisons on small models, where exact results can be computed, show substantial improvement in accuracy over earlier techniques. Implementation of the technique in an X-windows-based system, XHARP, is described.

Geist, Robert↗

NASA NTRS1992

A portable lidar using a diode-pumped YAG laser

A Mie lidar system is technically established and is used for monitoring air pollution, stratospheric and boundary layer aerosol distribution, plume dispersion, visibility, and the study of atmospheric structure and cloud physics. However, a lidar system is not widely used because of its cumbersome handling and unwieldy portability. Although the author developed a laser diode lidar system based on RM-CW technique, it has a limit of measurement distance. Here we report the development of an all solid Mie lidar system using a diode-pumped Nd:YAG laser and a Si-APD detector. This was constructed as a prototype of a handy lidar system.

Takeuchi, N.↗

NASA NTRS1991

Channel coding for digital HDTV terrestrial broadcasting

The Federal Communications Commission of the United States has ruled that high-definition television (HDTV) will occupy no more than 6 MHz of the VHF and UHF bands now used for conventional TV. In order to transmit the HDTV signal in 6 MHz, the four United States digital HDTV proponents, the DigiCipher, DSC-HDTV, ADTV, and ATVA-P systems, are reducing the video data rate of HDTV to 15-17 Mb/s, a compression ratio of approximately 60-70 times. The high compression dictates that channel coding be used to avoid block errors and multiframe error propagation. High efficiency in channel utilization required by the 6-MHz limitation means that the channel must be properly equalized and that the multipath and interfering signals must be severely limited. The channel coding techniques used for error reduction include data interleaving, error detection and replacement, and error correction at different levels of protection for bits and blocks of unequal importance.

Beakley, Guy W.↗

NASA NTRS1992

The role of the National Launch System in support of Space Station Freedom

A study was performed to determine the most appropriate potential use of the National Launch System (NLS) for Space Station Freedom (SSF) logistics resupply and growth assembly needs. Objectives were to estimate earth-to-SSF cargo requirements, identify NLS sizing trades, and assess operational constraints of a shuttle and NLS transportation infrastructure. Detailed NLS and Shuttle flight manifests were developed to model varying levels of NLS support. NLS delivery of SSF propellant, and in some cases, cryoenic fluids, yield significant shuttle flight savings with minimum impact to the baseline SSF design. Additional cargo can be delivered by the NLS if SSF trash disposal techniques are employed to limit return cargo requirements. A common vehicle performance level can be used for both logistics resupply and growth hardware delivery.

Green, J. L.↗

NASA NTRS1991

Methods development for total organic carbon accountability

This report describes the efforts completed during the contract period beginning November 1, 1990 and ending April 30, 1991. Samples of product hygiene and potable water from WRT 3A were supplied by NASA/MSFC prior to contract award on July 24, 1990. Humidity condensate samples were supplied on August 3, 1990. During the course of this contract chemical analyses were performed on these samples to qualitatively determine specific components comprising, the measured organic carbon concentration. In addition, these samples and known standard solutions were used to identify and develop methodology useful to future comprehensive characterization of similar samples. Standard analyses including pH, conductivity, and total organic carbon (TOC) were conducted. Colorimetric and enzyme linked assays for total protein, bile acid, B-hydroxybutyric acid, methylene blue active substances (MBAS), urea nitrogen, ammonia, and glucose were also performed. Gas chromatographic procedures for non-volatile fatty acids and EPA priority pollutants were also performed. Liquid chromatography was used to screen for non-volatile, water soluble compounds not amenable to GC techniques. Methods development efforts were initiated to separate and quantitate certain chemical classes not classically analyzed in water and wastewater samples. These included carbohydrates, organic acids, and amino acids. Finally, efforts were initiated to identify useful concentration techniques to enhance detection limits and recovery of non-volatile, water soluble compounds.

Benson, Brian L.↗

NASA NTRS1993

Intercomparisons between passive and active microwave remote sensing, and hydrological modeling for soil moisture

Soil moisture estimations from a distributed hydrological model and two microwave sensors were compared with ground measurements collected during the MAC-HYDRO'90 experiment. The comparison was done with the purpose of evaluating the performance of the hydrological model and examining the limitations of remote sensing techniques used in soil moisture estimation. An image integration technique was used to integrate and analyze rainfall, soil properties, land cover, topography, and remote sensing imagery. Results indicate that the hydrological model and microwave sensors successfully picked up temporal variations of soil moisture and that the spatial soil moisture pattern may be remotely sensed with reasonable accuracy using existing algorithms.

Wood, E. F.↗

NASA NTRS1957

Theoretical and Experimental Investigation of Random Gust Loads Part I : Aerodynamic Transfer Function of a Simple Wing Configuration in Incompressible Flow

Sinusoidally oscillating downwash and lift produced on a simple rigid airfoil were measured and compared with calculated values. Statistically stationary random downwash and the corresponding lift on a simple rigid airfoil were also measured and the transfer functions between their power spectra determined. The random experimental values are compared with theoretically approximated values. Limitations of the experimental technique and the need for more extensive experimental data are discussed.

RESEARCH TECHNIQUE, AERODYNAMIC↗

NASA NTRS1993

Applications of SLR

Satellite Laser Ranging (SLR) has a rich history of development which began in the 1960s with 10 meter-level first generation systems. These systems evolved with order of magnitude improvements to the systems that now produce several millimeter single shot range precisions. What began, in part, as an interesting application of the new laser technology has become an essential component of modern, precision space geodesy, which in turn enables contributions to a variety of science areas. Modern space geodesy is the beneficiary of technological developments which have enabled precision geodetic measurements. Aside from SLR and its closely related technique, Lunar Laser Ranging (LLR), Very Long Baseline Interferometry (VLBI) has made prominent science contributions also. In recent years, the Global Positioning System (GPS) has demonstrated a rapidly growing popularity as the result of demonstrated low cost with high precision instrumentation. Other modern techniques such as DORIS have demonstrated the ability to make significant science contributions; furthermore, PRARE can be expected to contribute in its own right. An appropriate question is 'why should several techniques be financially supported'? While there are several answers, I offer the opinion that, in consideration of the broad science areas that are the benefactors of space geodesy, no single technique can meet all the requirements and/or expectations of the science areas in which space geodesy contributes or has the potential for contributing. The more well-known science areas include plate tectonics, earthquake processes, Earth rotation/orientation, gravity (static and temporal), ocean circulation, land, and ice topography, to name a few applications. It is unfortunate that the modern space geodesy techniques are often viewed as competitive, but this view is usually encouraged by funding competition, especially in an era of growing needs but diminishing budgets. The techniques are, for the most part, complementary and the ability to reduce the data to geodetic parameters from several techniques promotes confidence in the geophysical interpretations. In the following sections, the current SLR applications are reviewed in the context of the other techniques. The strengths and limitations of SLR are reviewed and speculation about the future prospects are offered.

Schutz, Bob E.↗