SEARCH · Engineering Papers

Results for “Surface structure”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33

All sources NASA NTRS DOE OSTI

NASA NTRS1983

Critique of 'Elastic thickness of the Venus lithosphere estimated from topography and gravity' by A. Cazenave and K. Dominh

A critique of a model of the near-surface structure of Venus by Cazenave and Dominh (1981) based on Pioneer Venus data on gravity and topography is presented. Two objections are raised, the first an assumption of Airy compensation at a depth of 6 km, and the second that a gravity-topography relation is sufficient to study the subsurface characteristics. It is shown that determination of the gravity field along the line of sight, and not along the vertical, is necessary for a valid geophysical model with the available data. It is concluded that the spacecraft data are sufficient to define constraints on models of the subsurface characteristics, but are not sufficient for actually describing the substrate.

Reasenberg, R. D.↗

NASA NTRS1982

Nitridation of Si(111) by nitrogen atoms. II

The reaction between Si(111) and nitrogen atoms has been investigated by LEED, Auger, and electron energy loss (ELS) techniques. The early stage of the reaction at 850-1050 C involves the formation of a chemically reacted monolayer which grows in islands. The sticking coefficient for the submonolayer is estimated to be of the order of unity. Two electronically different surface structures can be formed during these stages, yielding the '8 x 8' and the quadruplet LEED patterns; the quadruplet unit cell is the one with the highest nitrogen density. A pure quadruplet structure is obtained from samples with a carbon contamination of the Si monolayer of about 5 pct. During the multilayer stages, the influence of the substrate is not as important, and the reactions at 850 C and above produce quadruplet-like local structures.

Schrott, A. G.↗

NASA NTRS1983

Solar related waves in the Venusian atmosphere from the cloud tops to 100 km

A quasi-linear diagnostic model using observed solar-related temperatures and a specified solar mean circulation and surface structure to find the solar-related circulation above the clouds of Venus is presented. Despite the greater dependence of model-derived, solar-related circulation on the mean flow than is the case for terrestrial tides, as well as the uncertainty concerning this mean flow, significant conclusions are drawn for the solar-related circulation and thermal structure of Venus. An anomalously large response is found in the polar regions, due to the model's requirement of a process such as dissipation which will act as a major sink for momentum. Dissipation is specified in the model as Rayleigh friction with an unknown free parameter coefficient. In view of this, dissipation is either very efficient by terrestrial standards and accompanied by small solar-related circulation, or similar to that of earth and possessed of a circulation large enough to have an impact on the mean circulation.

Elson, L. S.↗

NASA NTRS1983

The Influence of Sensor and Flight Parameters on Texture in Radar Images

Texture is known to be important in the analysis of radar images for geologic applications. It was previously shown that texture features derived from the grey-level co-occurrence matrix (GLCM) can be used to separate large scale texture in radar images. The influence of sensor parameters, specifically the spatial and radiometric resolution and flight parameters, i.e., the orientation of the surface structure relative to the sensor, on the ability to classify texture based on the GLCM features is investigated. It was found that changing these sensor and flight parameters greatly affects the usefulness of the GLCM for classifying texture on radar images.

Frost, V. S.↗

NASA NTRS1984

Advanced leading edge thermal-structure concept. Direct bond reusable surface insulation to a composite structure

An advanced leading-edge concept was analyzed using the space shuttle leading edge system as a reference model. The comparison indicates that a direct-bond system utilizing a high temperature (2700 F) fibrous refractory composite insulation tile bonded to a high temperature (PI/graphite) composite structure can result in a weight savings of up to 800 lb. The concern that tile damage or loss during ascent would result in adverse entry aerodynamics if a leading edge tile system were used is addressed. It was found from experiment that missing tiles (as many as 22) on the leading edge would not significantly affect the basic force-and-moment aerodynamic coefficients. Additionally, this concept affords a degree of redundancy to a thermal protection system in that the base structure (being a composite material) ablates and neither melts nor burns through when subjected to entry heating in the event tiles are actually lost or damaged during ascent.

Riccitiello, S. R.↗

NASA NTRS1984

The influence of sensor and flight parameters on texture in radar images

Texture is known to be important in the analysis of radar images for geologic applications. It has previously been shown that texture features derived from the grey level co-occurrence matrix (GLCM) can be used to separate large scale texture in radar images. Here the influence of sensor parameters, specifically the spatial and radiometric resolution and flight parameters, i.e., the orientation of the surface structure relative to the sensor, on the ability to classify texture based on the GLCM features is investigated. It was found that changing these sensor and flight parameters greatly affects the usefulness of the GLCM for classifying texture on radar images.

Frost, V. S.↗

NASA NTRS1985

Laboratory studies of Kapton degradation in an oxygen ion beam

Results are presented from a preliminary laboratory investigation of the degradation of the widely used polyimide Kapton under oxygen ion bombardment. Recent space shuttle flights have shown that Kapton and some other materials exposed to the apparent ram flow of residual atmosphere (at orbital velocity in low Earth orbit) lose mass and change their optical properties. It was hypothesized that these changes are caused by chemical interaction with atomic oxygen, aided by the 5-eV impact energy of atmospheric oxygen atoms in the ram. The reaction rate under O(+) bombardment seemed to be independent of incident energy over a wide range of energies. Although the flux of thermal ions in this experiment was much greater than the accelerated flux, the observed Kapton degradation was limited to the beam area and ram flow direction. This is consistent with an activation energy above the thermal energies but well below the beam energies. The results reproduce well the material loss, optical changes, SEM surface structure, and ram directionality of the samples returned by the shuttle. These factors, along with the lack of degradation under argon ion bombardment, are convincing evidence for ram flow oxidation as the mechanism of degradation.

Ferguson, D. C.↗

NASA NTRS1985

Chemolithotrophy and physiology of bacterial nutrient limitation

An overview of the physiology of chemolithotrophic bacteria, particularly the thiobacilli, was presented. In these bacteria unique physiological traits are expressed during nutrient limited growth. Different physiological types of chemolithotrophs, pathways of sulfur oxidation, and electron transport in the thiobacilli, problems encountered by chemolithotrophs in the generation of reducing power, and some explanations of the phenomenon of obligate chemolithotrophy were considered. Mixotrophy in the thiobacilli has been studied extensively both under nutrient excess and limitation. In nature, bacteria usually grow under nutrient limitation. Yet the bulk of our knowledge of microbial metabolic function is derived from bacteria grown in laboratory batch cultures containing a great abundance of nutrients. Microbial behavior in these two types of environments can be very different, indicating the need for basing an understanding of microbial ecology on studies that rely on cultivation of microorganisms under nutrient limitation. Nutrient limited bacteria differ in several ways from those growing in large quantities of nutrients. They have different surface structures and make a much fuller use of their metabolic potential, especially by the synthesis of unique pathways of catabolic enzymes.

Matin, A.↗

NASA NTRS1986

Large space reflector technology on the Space Station

This paper discusses the role of the Space Station in the evolutionary development of large space reflector technology and the accommodation of mission systems which will apply large space reflectors during the late 1990s and the early part of the next century. Reflectors which range from 10 to 100 meters in size and which span the electromagnetic spectrum for applications that include earth communications, earth observations, astrophysics and solar physics, and deep space communications are discussed. The role of the Space Station in large space reflector technology development and system performance demonstration is found to be critical; that role involves the accommodation of a wide variety of technology demonstrations and operational activities on the Station, including reflector deployment and/or assembly, mechanical performance verification and configuration refinement, systematic diagnostics of reflector surfaces, structural dynamics and controls research, overall system performance characterization and modification (including both radio frequency field pattern measurements and required end-to-end system demonstrations), and reflector-to-spacecraft integration and staging. A unique facility for Space Station-based, large space reflector research and development is proposed. A preliminary concept for such a Space Station-based Large Space Reflector Facility (LSRF) is described.

Mankins, J. C.↗

NASA NTRS1987

The relationship of active comets, 'extinct' comets, and dark asteroids

The possibility of distinguishing between outer solar system asteroids and dark cometary objects is discussed. Planetesimals that could be comets in various evolutionary phases have been detected at distances greater than about 3 AU. Comparisons are made among observed planetesimal and cometary albedos and spectra. It is noted that carbonaceous material, assumed to originate from comets, has been captured in the stratosphere. The data indicate that extinct or dormant comets have loose aggregate surface structures consisting of reddish organic compounds. A protoplanetary nebular condensation scenario is discussed which would have produced the dark cometary bodies.

Hartmann, William K.↗

NASA NTRS1987

Spaceborne imaging radar on EOS

A multiparameter synthetic aperture imaging radar is planned as a facility instrument for the Earth Orbiting System (EOS). This sensor will operate at L, C, and X and at all possible polarizations (HH, VV, HV, VH), thus allowing the acquisition of detailed information about the surface physical and electrical properties. When combined with the visible and IR imaging spectrometry data and the surface topography, a full description of the surface structure, composition, thermal properties and physical properties could then be extracted.

Elachi, Charles↗

NASA NTRS1987

A review: Quantitative models for lava flows on Mars

The purpose of this abstract is to review and assess the application of quantitative models (Gratz numerical correlation model, radiative loss model, yield stress model, surface structure model, and kinematic wave model) of lava flows on Mars. These theoretical models were applied to Martian flow data to aid in establishing the composition of the lava or to determine other eruption conditions such as eruption rate or duration.

Baloga, S. M.↗

NASA NTRS1987

Knox-Thompson images of 4 Vesta

Reconstructed images of the asteroid 4 Vesta reveal dark and bright patterns which can be followed across the disk as the asteroid rotates. Reasonable agreement is found with its visible light curve by assigning relative albedo 0 to the three darkest features, and relative albedo 2 to the three brightest features. From power-spectrum analysis of sets of images at two different epochs, Vesta's triaxial principal diameters, the orientation of its pole, and its rotational rate and sense are obtained. The images confirm the dominant role of surface structure, particularly the dark spots, in determining Vesta's light-curve signature.

Drummond, J. D.↗

NASA NTRS1988

Optical Backplane Interconnect Technology (OBIT)

We describe and analyze a novel approach to implementing an Optical Backplane Interconnect Technology (OBIT) that is capable of optically connecting any row of a 32x32 backplane array to any row of a second 32x32 array. Each backplane array is formed monolithically on a wafer. The technology is based on the use of Grating Surface Emitting (GSE) waveguides formed on a wafer containing quantum-well and separate confinement waveguide layers. These layers are used for transverse guiding, gain, modulation, detection, and for the formation of wavelength tunable distributed-Bragg reflector lasers. The required surface structures are formed photolithographically. The GSE waveguides act as efficient antennae that radiate light at angles selected by tuning the wavelength of the lasers. The same waveguides may be used as the receiving antennae when the array is used in the receiving mode. Thus, wavelength tuning is used to direct each row of the transmitting array to the desired row of the receiving array. In summary: The optical backplane array will have the following characteristics: Any row of a 32x32 GSE array may be optically connected to any row of a second 32x32 array. Only one switch decision is required to switch 32 parallel connections to any one of 32 positions. Each monolithic array can be used as both transmitter and receiver by switching the bias on the quantum-well switch-detectors. Separate transmitting and receiving structures could be provided for duplex operation. For a bit error rate of 10 sup 9 at 100-MHz data rate, a required laser power of 12 mW is calculated based on an estimated total optical loss of 40 dB.

Hammer, J. M.↗

NASA NTRS1989

A new structure for comparing surface passivation materials of GaAs solar cells

The surface recombination velocity (S sub rec) for bare GaAs is typically as high as 10 to the 6th power to 10 to the 7th power cm/sec, which dramatically lowers the efficiency of GaAs solar cells. Early attempts to circumvent this problem by making an ultra thin junction (xj less than .1 micron) proved unsuccessful when compared to lowering S sub rec by surface passivation. Present day GaAs solar cells use an GaAlAs window layer to passivate the top surface. The advantages of GaAlAs in surface passivation are its high bandgap energy and lattice matching to GaAs. Although GaAlAs is successful in reducing the surface recombination velocity, it has other inherent problems of chemical instability (Al readily oxidizes) and ohmic contact formation. The search for new, more stable window layer materials requires a means to compare their surface passivation ability. Therefore, a device structure is needed to easily test the performance of different passivating candidates. Such a test device is described.

Desalvo, Gregory C.↗

NASA NTRS1988

ECLS systems for a lunar base - A baseline and some alternate concepts

A baseline ECLS system for a lunar base manned intermittently by four crewmembers and later permanently occupied by eight crewmembers has been designed. A summary of the physical characteristics for the intermittently manned and the continuously manned bases is given. Since Space Station inheritance is a key assumption in the mission models, the ECLS system components are distributed within Space Station modules and nodes. A 'core assembly' concept is then developed to meet the objectives of both phases of the ECLS system. A supplementary study is discussed which assessed tankage requirements, penalties incurred by adding subsystem redundancy and by pressurizing large surface structures, and difficulties imposed by intermittent occupancy. Alternate concepts using lunar-derived oxygen, the gravitational field as a design aid, and a city utility-type ECLS system are also discussed.

Hypes, Warren D.↗

NASA NTRS1987

Lockheed laminar-flow control systems development and applications

Progress is summarized from 1974 to the present in the practical application of laminar-flow control (LFC) to subsonic transport aircraft. Those efforts included preliminary design system studies of commercial and military transports and experimental investigations leading to the development of the leading-edge flight test article installed on the NASA JetStar flight test aircraft. The benefits of LFC on drag, fuel efficiency, lift-to-drag ratio, and operating costs are compared with those for turbulent flow aircraft. The current activities in the NASA Industry Laminar-Flow Enabling Technologies Development contract include summaries of activities in the Task 1 development of a slotted-surface structural concept using advanced aluminum materials and the Task 2 preliminary conceptual design study of global-range military hybrid laminar flow control (HLFC) to obtain data at high Reynolds numbers and at Mach numbers representative of long-range subsonic transport aircraft operation.

Lange, Roy H.↗

NASA NTRS1989

Local parallel models for integration of stereo matching constraints and intrinsic image combination

Parallel relaxation computations such as those of connectionist networks offer a useful model for constraint integration and intrinsic image combination in developing a general-purpose stereo matching algorithm. This paper describes such a stereo algorithm that incorporates hierarchical, surface-structure, and edge-appearance constraints that are redefined and integrated at the level of individual candidate matches. The algorithm produces a high percentage of correct decisions on a wide variety of stereo pairs. Its few errors arise when the correlation measures defined by the constraints are either weakened or ambiguous, as in the case of periodic patterns in the images. Two additional mechanisms are discussed for overcoming the remaining errors.

Stewart, Charles V.↗