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At least 289 records · Page 16

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NASA NTRS1978

Ion beam sputtering of fluoropolymers

Etching and deposition of fluoropolymers are of considerable industrial interest for applications dealing with adhesion, chemical inertness, hydrophobicity, and dielectric properties. This paper describes ion beam sputter processing rates as well as pertinent characteristics of etched targets and films. An argon ion beam source was used to sputter etch and deposit the fluoropolymers PTFE, FEP, and CTFE. Ion beam energy, current density, and target temperature were varied to examine effects on etch and deposition rates. The ion etched fluoropolymers yield cone or spire-like surface structures which vary depending upon the type of polymer, ion beam power density, etch time, and target temperature. Also presented are sputter target and film characteristics which were documented by spectral transmittance measurements, X-ray diffraction, ESCA, and SEM photomicrographs.

Sovey, J. S.↗

NASA NTRS1979

Analysis of Lunar Seismic Signals: Determination of Instrumental Parameters and Seismic Velocity Distributions

Inverse filters were designed to correct the effect of instrumental response, coupling of the seismometer to the ground, and near surface structures. The least squares technique was used to determine the instrumental constants and the transfer functions of the long period lunar seismographs. The influence of noise and the results of these calculations are discussed.

Horvath, P.↗

NASA NTRS1981

Rotation and precession of cometary nuclei

The given review is concerned with a recent breakthrough in the investigation of polar-axis orientations and spin rates, and with the implications for the surface structure of cometary nuclei. The rotation data for twelve comets are summarized in a table. The progress achieved in the past few years has demonstrated that seemingly complex relations among observed physical and dynamical properties of comets can consistently be interpreted in terms of Whipple's (1950, 1951) model of a rotating icy-conglomerate nucleus. The nucleus, probably a few kilometers in diameter for an average comet, is believed to lose mass more or less continuously. Attention is given to the outgassing asymmetry, a formulation of the geometry of the directed ejection, and the role of rotation in cometary outbursts and splitting.

Sekanina, Z.↗

NASA NTRS1981

Design for active and passive flutter suppression and gust alleviation

Analytical design techniques for active and passive control of aeroelastic systems are based on a rational approximation of the unsteady aerodynamic loads in the entire Laplace domain, which yields matrix equations of motion with constant coefficients. Some existing schemes are reviewed, the matrix Pade approximant is modified, and a technique which yields a minimal number of augmented states for a desired accuracy is presented. The state-space aeroelastic model is used to design an active control system for simultaneous flutter suppression and gust alleviation. The design target is for a continuous controller which transfers some measurements taken on the vehicle to a control command applied to a control surface. Structural modifications are formulated in a way which enables the treatment of passive flutter suppression system with the same procedures by which active control systems are designed.

Karpel, M.↗

NASA NTRS1982

Volcanism and igneous processes in small icy satellites

Evidence for the production of endogenic processes of the small Saturnian satellites by igneous activity of low melting point NH3-H2O magma heated radiogenically is presented. An initial state of the inner satellites is modeled as a homogeneous mixture of particulate silicates and ices. Conductive transport is assumed to have either combined with heat capacity in a mixture of crystalline phases, or to have been reduced by the presence of dust, defects, vitreous or amorphous phases, and clathrates. The extent to which a eutectic melt could form is calculated in terms of the volume percentage of melt, with all heating above 175 K going toward overcoming the latent heat of the eutectic mix. Mimas and Enceladus were treated as free from radiogenic heat sources, which were significant for Tethys, Dione, Rhea, and Iapetus. The migrations of inner materials to form surface structures and seal off the interiors of the moons are described.

Stevenson, D. J.↗

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 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 NTRS1981

Exploration for fractured petroleum reservoirs using radar/Landsat merge combinations

Since fractures are commonly propagated upward and reflected at the earth's surface as subtle linears, detection of these surface features is extremely important in many phases of petroleum exploration and development. To document the usefulness of microwave analysis for petroleum exploration, the Arkansas part of the Arkoma basin is selected as a prime test site. The research plan involves comparing the aircraft microwave imagery and Landsat imagery in an area where significant subsurface borehole geophysical data are available. In the northern Arkoma basin, a positive correlation between the number of linears in a given area and production from cherty carbonate strata is found. In the southern part of the basin, little relationship is discernible between surface structure and gas production, and no correlation is found between gas productivity and linear proximity or linear density as determined from remote sensor data.

Macdonald, H.↗

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

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↗