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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

The surface integral approach to radarclinometry

A radar image of the Lake Champlain West quadrangle in the Adirondack Mountains of the U.S. is synthesized and used to test the surface integral approach to radarclinometry. It is shown that the surface integral approach to radarclinometry possesses an inherent instability that can be avoided only if the radar reflectance function possesses a shallow slope over the range of operation and if terrain slopes are bounded to prevent their being either parallel or perpendicular to the Poynting vector of the radar irradiance. It is found that the noise associated with real SAR systems makes this instability worse. It is concluded that the value of the surface integral approach to radarclinometry shows little promise.

Wildey, Robert L.↗
NASA NTRS1988

Comparing hydrogen and hydrocarbon booster fuels

The present evaluation of the consequences of hydrogen and hydrocarbon fuels as the basis of launch vehicle booster rocket-stage performance notes that hydrocarbon fuels lead to lower vehicle dry mass, for low-velocity requirements, while hydrogen fuel furnishes lower dry mass. Vehicles employing both types of fuel attempt to take advantage of the low intercept and slope of hydrocarbon fuel at low velocity, and subsequently, of the slope of the hydrogen curves at higher velocities.

Martin, James A.↗
NASA NTRS1988

Oxygen electrode bifunctional electrocatalyst NiCo2O4 spinel

A significant increase in energy density may be possible if a two-unit alkaline regenerative H2-O2 fuel cell is replaced with a single-unit system that uses passive means for H2O transfer and thermal control. For this single-unit system, new electrocatalysts for the O2 electrode will be required which are not only bifunctionally active but also chemically and electrochemically stable between the voltage range of about 0.7 and 1.5 V. NiCo2O4 spinel is reported to have certain characteristics that make it useful for a study of electrode fabrication techniques. High surface area NiCo2O4 powder was fabricated into unsupported, bifunctional, PTFE-bonded, porous gas fuel cell electrodes by commercial sources using varying PTFE contents and sintering temperatures. The object of this study is to measure the bifunctional activities of these electrodes and to observe what performance differences might result from different commercial electrode fabricators. O2 evolution and O2 reduction data were obtained at 80 C (31 percent KOH). An irreversible reaction (i.e., aging) occurred during O2 evolution at potentials greater than about 1.5 V. Anodic Tafel slopes of 0.06 and 0.12 V/decade were obtained for the aged electrodes. Within the range of 15 to 25 percent, the PTFE content was not a critical parameter for optimizing the electrode for O2 evolution activity. Sintering temperatures between 300 and 340 C may be adequate but heating at 275 C may not be sufficient to properly sinter the PTFE-NiCo2O4 mixture. Electrode disintegration was observed during O2 reduction. Transport of O2 to the NiCo2O4 surface became prohibitive at greater than about -0.02 A/sq cm. Cathodic Tafel slopes of -0.6 and -0.12 V/decade were assumed for the O2 reduction process. A PTFE content of 25 percent (or greater) appears to be preferable for sintering the PTFE-NiCo2O4 mixture.

Fielder, William L.↗
NASA NTRS1988

The ridged plains as a possible landing site for the Mars sample return mission

Differences in the shape and density of crater size-frequency distribution curves have been interpreted as indicators of different impactor populations. Within the inner solar system two production populations are seen. The signature of the first is recorded in the heavily cratered regions of the Moon, Mercury, and Mars and displays a multi-sloped distribution curve which cannot be described by a power law function at all crater diameters. The signature of the second population is seen in the lightly cratered lunar and Martian plains, where the size-frequency distribution curve can be approximated by a power law function of -3 differential slope in the 8- to 70-km diameter range. Based on data obtained from the Apollo lunar samples and crater flux estimates, the first population is believed to have been emplaced during the period of heavy bombardment which, at least on the Moon, ended about 3.8 BY ago. The second population has dominated the cratering record since that time and is commonly assumed to be due to comets and asteroids.

Barlow, Nadine G.↗
NASA NTRS1988

Hellas Basin ejecta: A prime stratigraphic marker and sampling target

The Hellas Basin and associated ejecta deposits dominate a large portion of the southern highlands of Mars. These deposits have been particularly well-documented at the scale 1:5,000,000 in the Sinus Sabaeus (MC-20), Iapygia (MC-21) and Mare Tyrrhenum (MC-22) quadrangles (1,2,3) where they have been mapped as ancient hilly and cratered material. In MC-22 the ejecta deposit is interpreted to thin from west to east, away from Hellas. In the eastern part of MC-22, the outlines of some large older craters, such as Hershel, can be seen through the thin ejecta cover and some of these buried craters even have enough relief to act as depositional traps for later mostly eolian sediments. Superposed on the ejecta is a population of numerous intermediate-sized to smaller craters which indicates the relatively old age of the ejecta deposit and causes the unit to have rather rough surface topography. Numerous small channels on steeply sloping crater rims and regionally sloping surface indicate erosion, transportation and redeposition of substantial amounts of ejecta materials, probably by aqueous fluvial processes. Numerous more recent volcanic constructs and flows also are superposed on the ejecta.

King, Elbert A.↗
NASA NTRS1989

Nonradiative relaxation and laser action in tunable solid state laser crystals

Room-temperature pulsed laser action was obtained in chromium-activated forsterite (Cr:Mg2SiO4) for both 532 and 1064 nm pumping. Free running laser emission in both cases is centered at 1235 nm and has a bandwidth of approximately 30 nm. Slope efficiency as high as 22 percent was measured. Using different sets of output mirrors and a single birefrigent plate as the intracavity wavelength selecting element tunability over the 1167 to 1268 nm spectral range was demonstrated. Continuous wave laser operation at room temperature was obtained for 1064 nm pumping from a CW Nd:YAG laser. The output power slope efficiency is 6.8 percent. The gain cross section is estimated to be 1.1 x 10 to the 19th sq cm. Spectroscopic studies suggest that the laser action is due to a center other than the trivalent chromium (Cr 3+), presumably the tetravalent chromium (Cr 4+) in a tetrahedrally coordinated site.

Petricevic, V.↗
NASA NTRS1989

Development of mid-infrared solid state lasers for spaceborne lidar

The laser performance of a Ho(3+):Tm(3+):Cr(3+):YAG crystal was measured under flashlamp pumping at various operating temperatures. The normal mode laser thresholds of a Ho(3+)(0.45 at. percent):Tm(3+)(2.5 at. percent):Cr(3+)(1.5 at. percent ):YAG crystal were found to range from 26 to 50 J between 120 and 200 K with slope efficiencies up to 0.36 percent with a 60 percent reflective output mirror. From the Q-switched operations a slope efficiency corresponding to 90 percent of the normal mode operation was observed. Laser wavelengths were measured for various operating conditions and fluorescence spectra were obtained at various temperatures in order to help understand the dynamic energy processes among the Ho(3+), Tm(3+), and Cr(3+) ions. A pulse forming network for a flashlamp pumped Cr:GSAG laser, which is to be used as a high power laser diode simulator in rare earth laser pumping, was completed and tested. The network provided critically damped, 1 ms FWHM, square pulses with a rise time of about 160 micrometers at an input electrical energy of 300 J.

Whitney, Donald A.↗
NASA NTRS1989

Observations of gravity waves in the mesosphere with the MU radar

Wind motions were observed at 60 to 90 km altitudes with the MU radar during daylight hours (0800 to 1600 LT) from 13 to 31 October 1986. Quasi-monochromatic gravity waves were evident on 16 of the 19 days of observations. They were characterized by typical vertical wavelength of 5 to 15 km and intrinsic periods centered at about 9 hours. The propagation direction of the gravity waves, determined by the gravity wave dispersion relation, was mostly equatorward. The vertical wave number spectra of the horizontal components of the mesoscale wind fluctuations are explained well by saturated gravity wave theory. The frequency spectrum of vertical wind component has a slope of + 1/3, while the oblique spectra have a slope of -5/3 up to 4 x 10(-3) (c/s); these agree fairly well with model gravity wave spectra. Doppler shift effects on the frequency spectra are recognized at higher frequencies. Upward flux was determined of horizontal momentum flux induced by waves with periods from 10 min to 8 hours, and westward and northward body forces of 5.1 and 4.0 m/s/day, were estimated respectively.

Tsuda, T.↗
NASA NTRS1988

Bidirectional reflectance properties of iron-nickel meteorites

The effects of viewing geometry and small-scale (less than 1 mm) surface roughness on the bidirectional reflectance of iron-nickel meteorites were investigated by measuring the reflectance properties of Gibeon and Canyon Diablo iron-nickel meteorites. Several cut surfaces of each of these meteorites were polished with abrasive grits to controlled surface roughness, and a series of bidirectional reflectance spectra were taken at a variety of viewing geometries using NASA's RELAB facility. The measurements show that, for optically smooth surfaces, only the specular component of reflectance exhibited red-sloped continuum characteristic of iron. Spectra of surfaces that were much rougher than the wavelength of incident light exhibited iron's characteristic red sloped continuum and could be modeled by linear combinations of facets that are oriented in specular and nonspecular geometries with respect to the observer. Spectra of this latter type exhibit good agreement with the published spectra of M-type asteroids.

Britt, D. T.↗
NASA NTRS1988

The preparation of new perfluoro ether fluids exhibiting excellent thermal-oxidative stabilities

A series of low molecular weight perfluoroalkyl ethers (PFAEs) were synthesized by direct fluorination. Viscosity-temperature properties and oxidation stabilities were determined. Viscosity-temperature correlations indicated that increases in branching and increases in the size of the branching substituent caused a deterioration in viscometric properties (i.e., an increase in ASTM slope). In addition, increasing the ratio of carbon to oxygen in these compounds also increased the ASTM slope. Preliminary oxidation stability measurements indicated that highly branched PFAE fluids (i.e., those containing quaternary carbons) may be less stable than either those containing a single trifluoromethyl pendant group or those containing no branching at all.

Jones, William R., Jr.↗
NASA NTRS1988

Nd:LNA laser optical pumping of He-4 - Application to space magnetometers

Results obtained from laser pumping in a helium magnetometer sensor, using a tunable Nd:LNA laser pumped with a high-power diode laser, are reported. It is shown that it was possible to observe both the Hanle signals and the n = 0, p = 1 parametric resonance by monitoring the pumping radiation passing through the cell. As the diode laser-pumped Nd:LNA laser was tuned through the D0, D1, and D2 transitions, three distinct resonance signals were produced. A comparison of the slope of lamp-pumped signals and laser-pumped D1 signals showed that, under otherwise identical conditions, the slope of the D1 laser signal was 45 times greater than the lamp-pumped signal.

Slocum, R. E.↗
NASA NTRS1988

Experimental study of thin-walled composite beams

Thin-walled rectangular section composite beams are built out of graphite-epoxy prepreg tape using an autoclave molding technique. Cantilevered beams are tested under static loads using a specially-built apparatus and bending slope and elastic twist at a station are measured using a laser system. Measured bending slope and bending-induced twist distributions are correlated with predicted results obtained using a simple beam analysis as well as a refined finite element analysis. Natural vibration characteristics at different rotational speeds are determined experimentally by testing beams in a specially built 10 ft diameter vacuum chamber. The rotor is excited by piezo-electric crystals and response is determined from strain gages mounted on the surface. For composite beams with bending-twisting couplings (symmetric layup), effects of section warping and transverse shear on structural response are found to be substantial. These results were substantiated by the analyses.

Chandra, Ramesh↗
NASA NTRS1989

The hard X-ray to low-energy gamma-ray spectrum of the Crab Nebula

Data from the UCSD/MIT Hard X-ray and Low-Energy Gamma-Ray Experiment on the first High Energy Astronomy Observatory (HEAO 1) were used to determine the spectrum of the Crab Nebula in the 10 keV-5 MeV energy range. The spectral change in the 100 keV-1 MeV region is characterized. When the combined spectrum is fitted to a broken power-law model, the data require a description with the following distinct spectral indices: a power-law slope of 2.08 in the 1-100 keV region and a slope of about 2.5 above 150 keV.

Jung, G. V.↗
NASA NTRS1989

Crater formation and modification on the icy satellites of Uranus and Saturn - Depth/diameter and central peak occurrence

Fresh crater shapes, in particular crater depth/diameter ratios d/D, have been measured for craters on nine of the icy satellites. Crater depths were determined from photoclinometry plus some shallow measurements. Interior morphology, including rimwall slope, was also examined from photoclinometry profiles. Central peak statistics and the characteristic break in slope of d/D curves were determined in order to estimate the transition diameter from simple to complex craters. Crater morphology and transition diameters on the icy satellites were then compared with crater morphology on the terrestrial rocky planets in order to separate or evaluate the relative importance and similarity of surface gravity and target characteristics in controlling crater formation and modification in icy and rocky targets. Estimates of effective rheologies of the target materials were also obtained based on morphologic changes.

Schenk, Paul M.↗
NASA NTRS1989

2.07-micron CW diode-laser-pumped Tm,Ho:YLiF4 room-temperature

Continuous-wave action is obtained at 2.07 microns from a 2-mm-long Tm-sensitized Ho:YLiF4 crystal at room temperature when longitudinally pumped by a pair of diode-laser arrays. Laser output power at 300 K is 26 mW, with a 30-percent slope efficiency and a lasing threshold of 108 mW. A maximum output power of 187 mW is obtained from a 4-mm-long crystal at 77 K, with a 67 percent slope efficiency. A preliminary demonstration of cavity Q switching produced 165 microJ of pulse energy at a repetition rate of 100 Hz.

Hemmati, Hamid↗
NASA NTRS1989

A reevaluation of the infrared-radio correlation for spiral galaxies

The infrared radio correlation has been reexamined for a sample of 237 optically bright spiral galaxies which range from 10 to the 8th to 10 to the 11th solar luminosities in far-infrared luminosity. The slope of the correlation is not unity. A simple model in which dust heating by both star formation and the interstellar radiation field contribute to the far-infrared luminosity can account for the nonunity slope. The model differs from previous two component models, however, in that the relative contribution of the two components is independent of far-infrared color temperature, but is dependent on the far-infrared luminosity.

Devereux, Nicholas A.↗
NASA NTRS1988

Observations of the properties of the water surface roughness structure under the action of wind and waves

The statistical properties of a water surface roughness structure subjected to wind and waves are analyzed in a laboratory wind wave channel. The surface slope is derived using elevation measurements and the pitot tube is employed to measure wind speed. The transient responses of the surface slope to a calm condition and low, medium, and high wind conditions are studied. Two methods for determining a critical wind speed range are described.

Long, Steven R.↗
NASA NTRS1989

Excited state absorption of pump radiation as a loss mechanism in solid-state lasers

The characteristics of optical pumping dynamics in laser-pumped, rare-earth-doped, solid-state laser materials are investigated by using a tunable alexandrite laser to pump Y3Al5O12:Nd(3+) in an optical cavity. It is found that the slope efficiency of the Nd laser operation depends strongly on the wavelength of the pump laser. For pump wavelengths resulting in low slope efficiencies, intense fluorescence emission is observed from the sample in the blue-green spectral region. This is attributed to the excited-state absorption of pump photons which occurs during radiationless relaxation from the pump band to the metastable state. This type of process is an important loss mechanism for monochromatic pumping of laser systems at specific pump wavelengths.

Kliewer, Michael L.↗