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At least 307 records · Page 17

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NASA NTRS1986

Scandium oxide antireflection coatings for superluminescent LEDs

For an employment of laser diodes as superluminescent LEDs (SLDs) or amplifiers, the facets of the diodes must be coated with antireflection films. In the work reported, scandium oxide was evaporated from an e-beam source onto Supersil II fused silica substrates. The obtained samples were used for measurements of absorption and reflectivity. Results of index measurements on e-beam evaporated films are presented. It is shown that excellent coatings with reflectivities of 0.00025 can be obtained using these films. Attention is given to the refractive indices for scandium oxide films as a function of wavelength, the power output vs current for laser before coating and after coating with Sc2O3.

Ladany, I.↗

NASA NTRS1985

The International Cometary Explorer mission to comet Giacobini-Zinner

The International Cometary Explorer (ICE), scheduled to pass the tail of Comet Giacobini-Zinner on September 11, 1985, was expected to address first-order questions bout the interaction between solar wind and comets. Since ICE was not initially intended for comet studies, it did not have sun-sensitive imaging instruments designed to image the comet nucleus. In effect, ICE was free to pass through the tail of the comet. The ICE spacecraft, a 16-faceted cylindrical drum just over five ft high with a spin axis normal to the ecliptic plane, was designed to encounter the comet at a distance of 0.47 AU from the earth. Scientific instruments on board the spacecraft include: (1) the electron plasma experiment, (2) the vector helium magnetometer, (3) the plasma wave experiment, (4) the radio waves experiment, (5) the plasma ion experiment, (6) the low-energy cosmic ray experiment, and (7) the energetic protons experiment. Targeting error was expected as a result of solar-wind induced plasma tail wagging. Comet interception was planned to be at a distance of 10,000 km from the nucleus.

Von Rosenvinge, T. T.↗

NASA NTRS1985

The effect of microwave backscatter uncertainty on satellite radar altimeter accuracy

The effect of variations in ocean surface roughness characteristics with upwind/downwind direction, reported by other investigators, is used to compute radar cross section (RCS) and to assess the errors which may arise in present and planned altimeter sensors. Based on an analysis of the rough surface impulse response, the uncertainty between attitude angle and RCS asymmetry is found to cause height errors as large as 12 cm, depending on off-nadir angles and sea state. Additionally, the previously reported data in conjunction with computed facet backscatter are found to produce RCS characteristics at large off-nadir angles which are in better agreement with experimental results than those predicted by physical optics Gaussian theory.

Miller, L. S.↗

NASA NTRS1986

Growth and characterization of epitaxial SrF2 on InP(100)

The epitaxial growth of 100-262.5-nm SrF2 films on n-type and p-type (100)InP in a conventional baked UHV system at base pressure about 200 ptorr, temperature 250-350 C, and growth rate from less than 100 to about 200 pm/s. Substrates are chemicomechanically polished, degreased, bombarded with 500-eV Ar ions for 3-4 min at 350 C, and annealed for 23-30 min at 350 C, producing a slightly In-rich (In/P = 1.02) In-island-free surface with a (4 x 1) or (1 x 1) LEED structure. Films grown at 350 C and less than 100 pm/s are found to be smooth and free of cracks in most cases, with a highly faceted (1 x 1) LEED structure. The electrical properties of the SrF2 films are found to be acceptable only when the ohmic contacts are applied prior to the substrate prior to SrF2 growth.

Sinharoy, S.↗

NASA NTRS1984

Determination of the ultrasonic properties of composites

The research described encompasses two facets of the propagation of ultrasonic waves in composite structures: imaging of delaminations and the temperature dependence of the acoustical properties of composites. Progress in each area is summarized.

Smith, B. T.↗

NASA NTRS1987

Tube coupling device

A first annular ring of a tube coupling device has a keyed opening sized to fit around the nut region of a male coupling, and a second annular ring has a keyed opening sized to fit around the nut of a female coupling. Each ring has mating ratchet teeth and these rings are biased together, thereby engaging these teeth and preventing rotation of these rings. This in turn prevents the rotation of the male nut region with respect to the female nut. For tube-to-bulkhead locking, one facet of one ring is notched, and a pin is pressed into an opening in the bulkhead. This pin is sized to fit within one of the notches in the ring, thereby preventing rotation of this ring with respect to the bulkhead.

Myers, William N.↗

NASA NTRS1987

Means for phase locking the outputs of a surface emitting laser diode array

An array of diode lasers, either a two-dimensional array of surface emitting lasers, or a linear array of stripe lasers, is phase locked by a diode laser through a hologram which focuses the output of the diode laser into a set of distinct, spatially separated beams, each one focused onto the back facet of a separate diode laser of the array. The outputs of the diode lasers thus form an emitted coherent beam out of the front of the array.

Lesh, James R.↗

NASA NTRS1986

The cometary nucleus: Current concepts

Ideas concerning the nature of cometary nuclei as modified by observations of recent years, particularly by those of Halley's comet during the 1986 apparition are reviewed. The major trend is to increase the postulated dimensions of the nuclei and reduce their albedos. The Vega and Giotto missions establish an invaluable checkpoint with regard to comet nuclei. These and other observations are affecting opinions concerning many facets of cometary nature, origin, evolution, and decay.

Whipple, Fred L.↗

NASA NTRS1986

The high energy transmission grating spectrometer for AXAF

Attention is given to a high energy transmission grating spectrometer that operates over the range 0.4-8 keV, giving resolving powers of 100-1000 and effective areas of 10-200 sq cm. The instrument, which is part of the MIT high resolution X-ray spectroscopy investigation, consists of a single array of grating facets of two types: medium energy gratings of 0.6-micron period, 0.5-micron thick silver mounted behind the outer three AXAF mirrors, and high energy gratings of 0.2-micron period, 1.0-micron thick gold mounted behind the inner three mirrors. The gratings are oriented so as to correct for coma and so that the medium and high energy spectra form a shallow 'X' at the AXAF focal plane. Likely targets include normal stars, binary X-ray sources, active galactic nuclei and quasars. The HETGS can also be used to give moderate resolution spectra of slightly extended sources and monochromatic images of sources with strong lines, such as supernova remnants in nearby galaxies.

Canizares, C. R.↗

NASA NTRS1986

Further observations of SCC in alpha-beta brass Considerations regarding the appearance of crack arrest markings during SCC

A series of constant displacement and constant extension rate stress corrosion cracking (SCC) tests was performed on an alpha-beta brass alloy in 1 N Na2SO4 solutions. The chosen mechanical and electrochemical conditions resulted in predominantly transgranular, cleavage-like failure at high (about 8 to 50 microns/s) average crack propagation rates. Crack arrest markings were observed on selected transgranular facets, which almost exclusively bordered regions of ductile overload failure. It is proposed that the observed crack velocities and the specific nature of the arrest mark appearance are most consistent with a cracking mechanism involving adsorption or some other interaction with a damaging environmental species.

Hintz, M. B.↗

NASA NTRS1987

Aeroelastic tailoring - Creative uses of unusual materials

This paper discusses the fundamental aspects of the use of aeroelastic tailoring to enhance the performance of flexible fixed wing aircraft. Tailoring with advanced composites is seen as part of a natural evolutionary process in the everchanging field of design. Among topics discussed are: static divergence; lift effectiveness; drag reduction; control effectiveness; and, vibration and flutter. In addition, terminology is reviewed, together with descriptive formulas that characterize stiffness coupled structures. Finally, a summary of design features favorable to different facets of aeroelastic tailoring is given to illustrate conflicts and compromises that must be considered.

Weisshaar, Terrence A.↗

NASA NTRS1987

Inferring spectral reflectances of plant elements by simple inversion of bidirectional reflectance measurements

Inverting previously developed explicit expressions for a vertical architecture, bidirectional reflectances measured over corn viewing from the solar quadrant at azimuths near the principal plane are used to determine the spectral reflectances of plant elements. The leaf reflectance values extracted in three visible bands at viewing zenith angles of 70 deg, 60 deg, and 45 deg agree closely with laboratory-measured reflectances of corn leaves. At viewing zenith angle of 30 deg, the inversion breaks down, inasmuch as the inferred plant element reflectances are too high. Satisfactory results are also achieved when the same approach is applied to bidirectional reflectances measured over potted balsam firs, but when applied to soybeans reflectances, the procedure yields unreasonably high leaf reflectances. The failure in this case is attributed to the nonvertical architecture of the soybean canopy; however, for this canopy, inversion based on horizontal architecture is possible. The bidirectional reflectances measured from the solar quadrant, at viewing angles appreciably far from 'hot spot' viewing, approximately equal in magnitude half of the leaf reflectance. The 0.5 ratio is predicted by a previous analysis of opaque horizontal Lambertian facets, as the asymptotic value for a dense canopy at any viewing angle. For soybeans, this ratio applies very closely at 15 deg viewing zenith angle. The results suggest that inversion based on simple architecture, applying explicit expressions, might be of value, either in itself or as a preliminary step before inversion applying complex models.

Otterman, J.↗

NASA NTRS1987

Polarization of light scattered by clover

This study was undertaken in order to better understand the factors that govern the polarization of light scattered from vegetation and soils. This phenomenon is not well understood but is potentially of interest for remote sensing of the earth. The intensity and polarization of light scattered by clover in vivo and soil were measured at a number of different angles of incidence and reflectance. Both individual leaves and natural patches of vegetation were measured. The Umov effect, or inverse relation between polarization and reflectance noted by many earlier workers, was observed here and is shown to be a very general property of diffusely scattering surfaces. The light transmitted through the leaves was found to be negatively polarized. The polarization of light scattered from aggregations of leaves is affected by this negatively polarized, transmitted light. The light scattered from the upper leaf surfaces was found to be positively polarized in a manner which could be accounted for quantitatively by specular Fresnel reflection from small, randomly oriented facets on the surfaces of the leaves.

Woessner, Paul↗

NASA NTRS1987

Failure Control Techniques for the SSME

Since ground testing of the Space Shuttle Main Engine (SSME) began in 1975, the detection of engine anomalies and the prevention of major damage have been achieved by a multi-faceted detection/shutdown system. The system continues the monitoring task today and consists of the following: sensors, automatic redline and other limit logic, redundant sensors and controller voting logic, conditional decision logic, and human monitoring. Typically, on the order of 300 to 500 measurements are sensed and recorded for each test, while on the order of 100 are used for control and monitoring. Despite extensive monitoring by the current detection system, twenty-seven (27) major incidents have occurred. This number would appear insignificant compared with over 1200 hot-fire tests which have taken place since 1976. However, the number suggests the requirement for and future benefits of a more advanced failure detection system.

Taniguchi, M. H.↗

NASA NTRS1987

Impact of solids on composite materials

The failure modes of composite materials as a result of low velocity impact were investigated by simulating the impact with a finite element analysis. An important facet of the project is the modeling of the impact of a solid onto cylindrical shells composed of composite materials. The model under development will simulate the delamination sustained when a composite material encounters impact from another rigid body. The computer equipment was installed, the computer network tested, and a finite element method model was developed to compare results with known experimental data. The model simulated the impact of a steel rod onto a rotating shaft. Pre-processing programs (GMESH and TANVEL) were developed to generate node and element data for the input into the three dimensional, dynamic finite element analysis code (DYNA3D). The finite element mesh was configured with a fine mesh near the impact zone and a coarser mesh for the impacting rod and the regions surrounding the impacting zone. For the computer simulation, five impacting loads were used to determine the time history of the stresses, the scribed surface areas, and the amount of ridging. The processing time of the computer codes amounted from 1 to 4 days. The calculated surface area were within 6-12 percent, relative error when compated to the actual scratch area.

Bronson, Arturo↗

NASA NTRS1986

Biogeochemical cycling in the ocean. Part 1: Introduction to the effects of upwelling along the west coast of North America

Coastal upwelling is examined as it relates to the cycling of chemical species in coastal waters along the west coast of North America. The temporal and spatial features of upwelling phenomena in the Eastern boundary regions of the North Pacific Ocean are presented and discussed in terms of upwelling episodes. Climate conditions affecting upwelling include: thermal effects, wind-induced shear stress which moves surface layers, and the curl of the wind stress vector which is thought to affect the extent and nature of upwelling and the formation of offshore convergent downwelling fronts. These effects and the interaction of sunlight and upwelled nutrients which result in a biological bloom in surface waters is modeled analytically. The roles of biological and chemical species, including the effects of predation, are discussed in that context, and relevant remote sensing and in situ observations are presented. Climatological, oceanographic, biological, physical, chemical events, and processes that pertain to biogeochemical cycling are presented and described by a set of partial differential equations. Simple preliminary results are obtained and are compared with data. Thus a fairly general framework has been laid where the many facets of biogeochemical cycling in coastal upwelled waters can be examined in their relationship to one another, and to the whole, to whatever level of detail or approximation is warranted or desired.

Howe, John T.↗

NASA NTRS1988

Space station solar concentrator materials research

The Space Station will represent the first time that a solar dynamic power system will be used to generate electrical power in space. In a system such as this, sunlight is collected and focused by a solar concentrator onto the receiver of a heat engine which converts the energy into electricity. The concentrator must be capable of collecting and focusing as much of the incident sunlight as possible, and it must also withstand the atomic oxygen bombardment which occurs in low Earth orbit (LEO). This has led to the development of a system of thin film coatings applied to the concentrator facet surface in a chamber designed especially for this purpose. The system of thin film coatings employed gives both the necessary degree of reflectance and the required protection from the LEO atomic oxygen environment.

Gulino, Daniel A.↗

NASA NTRS1988

Design and demonstration of a system for the deposition of atomic-oxygen durable coatings for reflective solar dynamic power system concentrators

A system for the vacuum deposition of atomic-oxygen durable coatings for reflective solar dynamic power systems (SDPS) concentrators was designed and demonstrated. The design issues pertinent to SDPS were developed by the Government Aerospace Systems Division of the Harris Corporation and are described in NASA-CR-179489. Both design and demonstration phases have been completed. At the time of this report the deposition system was ready for coating of facets for SDPS concentrators. The materials issue relevant to the coating work were not entirely resolved. These issues can only be resolved when substrates which are comparable to those which will be used in flight hardware are available. The substrates available during the contract period were deficient in the areas of surface roughness and contamination. These issues are discussed more thoroughly in the body of the report.

Mcclure, Donald J.↗