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At least 613 records · Page 34

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NASA NTRS1975

Soil moisture and evapotranspiration predictions using Skylab data

The author has identified the following significant results. Multispectral reflectance and emittance data from the Skylab workshop were evaluated for prediction of evapotranspiration and soil moisture for an irrigated region of southern Texas. Wavelengths greater than 2.1 microns were required to spectrally distinguish between wet and dry fallow surfaces. Thermal data provided a better estimate of soil moisture than did data from the reflective bands. Thermal data were dependent on soil moisture but not on the type of agricultural land use. The emittance map, when used in conjunction with existing models, did provide an estimate of evapotranspiration rates. Surveys of areas of high soil moisture can be accomplished with space altitude thermal data. Thermal data will provide a reliable input into irrigation scheduling.

Myers, V. I.↗

NASA NTRS1976

Space shuttle electromagnetic environment experiment. Phase A: Definition study

Methods for carrying out measurements of earth electromagnetic environment using the space shuttle as a measurement system platform are herein reported. The goal is to provide means for mapping intentional and nonintentional emitters on earth in the frequency range 0.4 to 40 GHz. A survey was made of known emitters using available data from national and international regulatory agencies, and from industry sources. The spatial distribution of sources, power levels, frequencies, degree of frequency re-use, etc., found in the survey, are here presented. A concept is developed for scanning the earth using a directive antenna whose beam is made to rotate at a fixed angle relative to the nadir; the illuminated area swept by the beam is of the form of cycloidal annulus over a sphere. During the beam's sojourn over a point, the receiver sweeps in frequency over ranges in the order of octave width using sweeping filter bandwidths sufficient to give stable readings.

Haber, F.↗

NASA NTRS1976

Fundamental studies of black chrome for solar collector use

The thicknesses of black chrome plated for various times have been measured from electron photomicrographs and correlated with the solar spectrum absorptance and infrared emittance as calculated from spectral reflectance measurements. The maximum absorptance is reached at an average thickness of 0.5 micrometer. The emittance increases only slightly up to 1.0 micrometer but increases rapidly at thickness above 1.0 micrometer.

Mcdonald, G.↗

NASA NTRS1975

Spectral reflectance properties of electroplated and converted zinc for use as a solar selective coating

The spectral reflectance properties of electroplated and chemically converted zinc were measured for both chromate and chloride conversion coatings. The reflectance properties were measured for various times of conversion and for conversion at various chromate concentrations. The values of absorptance, alpha, integrated over the solar spectrum, and of infrared emittance, epsilon, integrated over black body radiation at 250 F were then calculated from the measured reflectance values. The interdependent variations of alpha and epsilon were plotted. The results indicate that the optimum combination of the highest absorptance in the solar spectrum and the lowest emittance in the infrared of the converted electroplated zinc is produced by chromate conversion at 1/2 concentration of the standard NEOSTAR chromate black solution for 0.50 minute or by chloride conversion for 0.50 minute.

Mcdonald, G. E.↗

NASA NTRS1975

Time dependences of the 0.51 and 2.2 MeV lines in solar flares

The time dependence of the 0.51 MeV line in solar flares is determined by the following factors: variation with time of the rate of nuclear reactions which produce the positron emitters; mean lives of the positron emitters; slowing-down of the positrons which depends on the ambient density and magnetic field; and annihilation or positronium formation times of the positrons with free or bound electrons. The discussion is limited to the observed intensity-time profile of a 0.35-8 MeV gamma-ray emission and of a 0.51-MeV line calculated for three different ambient densities in the annihilation region. Major conclusions are that (1) MeV electrons and protons of at least 30 MeV/nuc have similar time dependences and hence a common origin in the flare region, which support a two-phase acceleration model in solar flares; and (2) the delayed character of the 0.51-MeV emission is clearly defined, where at a time when the rate of nuclear reactions is zero, there still is considerable 0.51-MeV emission.

Ramaty, R.↗

NASA NTRS1967

Optimized Selective Coatings for Solar Collectors

The spectral reflectance properties of black nickel electroplated over stainless steel and of black copper produced by oxidation of copper sheet were measured for various plating times of black nickel and for various lengths of time of oxidation of the copper sheet, and compared to black chrome over nickel and to converted zinc. It was determined that there was an optimum time for both plating of black nickel and for the oxidation of copper black. At this time the solar selective properties show high absorptance in the solar spectrum and low emittance in the infrared. The conditions are compared for production of optimum optical properties for black nickel, black copper, black chrome, and two black zinc conversions which at the same conditions had absorptances of 0.84, 0.90, 0.95, 0.84, and 0.92, respectively, and emittances of 0.18, 0.08, 0.09, 0.10, and 0.08, respectively.

Mcdonald, G.↗

NASA NTRS1976

High efficiency thermionic converter studies

The objective is to improve thermionic converter performance by means of reduced interelectrode losses, greater emitter capabilities, and lower collector work functions until the converter performance level is suitable for out-of-core space reactors and radioisotope generators. Electrode screening experiments have identified several promising collector materials. Back emission work function measurements of a ZnO collector in a thermionic diode have given values less than 1.3 eV. Diode tests were conducted over the range of temperatures of interest for space power applications. Enhanced mode converter experiments have included triodes operated in both the surface ionization and plasmatron modes. Pulsed triodes were studied as a function of pulse length, pulse potential, inert gas fill pressure, cesium pressure, spacing, emitter temperature and collector temperature. Current amplifications (i.e., mean output current/mean grid current) of several hundred were observed up to output current densities of one amp/sq cm. These data correspond to an equivalent arc drop less than 0.1 eV.

Huffman, F. N.↗

NASA NTRS1976

Inert gas thrusters

Inert gases are of interest as possible alternatives to the usual electric thruster propellants of mercury and cesium. The multipole discharge chamber investigated was shown capable of low discharge chamber losses and flat ion beam profiles with a minimum of optimization. Minimum discharge losses were 200 to 250 eV/ion for xenon and 300 to 350 eV/ion for argon, while flatness parameters in the plane of the accelerator grid were 0.85 to 0.95. The design used employs low magnetic field strengths, which permits the use of sheet-metal parts. The corner problem of the discharge chamber was resolved with recessed corner anodes, which approximately equalized both the magnetic field above the anodes and the electron currents to these anodes. Argon hollow cathodes were investigated at currents up to about 5 amperes using internal thermionic emitters. Cathode chamber diameter optimized in the 1.0 to 2.5 cm range, while orifices diameter optimized in the 0.5 to 5 mm range. The use of a bias voltage for the internal emitter extended the operating range and facilitated starting. The masses of 15 and 30 cm flight type thrusters were estimated at about 4.2 and 10.8 kg.

Kaufman, H. R.↗

NASA NTRS1977

Techniques of Global analysis applied to gravitation theories: A cosmological black hole?

An elementary model of freely falling observers and emitters within a black hole's radius is examined to determine the redshift spectrum reaching a typical observer. The model is independent of scale, the fundamental unit being the radius (mass) of the black hole. The observers/emitters all follow the same kinds of trajectories: radially inward and starting from rest at spatial infinity. The test-particle role is assumed throughout; i.e., the observers/emitters do not themselves contribute to the gravitational field of the system. By means of redshift formulas and luminosity distance to the emitters, a picture of actual redshifts and blueshifts, with their intensities, emerges for an observer within the black hole's radius. No luminosity distances greater than approximately one-half the radius are considered in this particular study; nevertheless, redshifts and blueshifts up to approximately 0.6 are seen in portions of the observer's celestial sphere. An exotic application can be made, as a curiosity, to a black hole the size of the universe, resulting in a particular anisotropic "cosmology."

Debney, G.↗

NASA NTRS1963

Spectral emissivity of metals after damage by particle impact

The reflectance, R, solar absorption, alpha, thermal emittance epsilon, and the alpha/epsilon ratio were determined for several metals before and after bombardment by high velocity microprojectiles. Observed decreases of 1 to 26 percent in the reflectance of various metals were apparently dependent on the amount of surface area damaged as well as on the velocity of the impinging projectiles. The thermal emittance was observed to increase more than the solar absorptance, which resulted in a net decrease in the alpha/epsilon ratio in all metals investigated except stainless steel 304. In the case of stainless steel 304, alpha and epsilon were observed to increase equally, resulting in an alpha/epsilon ratio apparently insensitive to microprojectile bombardment. Recommendations for the continuation of these investigations are outlined.

Laszlo, T. S.↗

NASA NTRS1976

NASA thermionic-conversion program

The NASA applied research and technology (ART) program for thermionic energy conversion (TEC) is progressing effectively. Current out-of-core emphases allow converter material and design freedoms previously prohibited by in-core nucleonic and geometric restrictions. As a result, potential improvements indicate possibilities for severalfold increases in efficiencies. The new TEC-ART program concentrated initially on low-work-function collectors and interelectrode-loss reduction and revealed much in a short time. For example, arc-drop studies verified the necessity of stable emitters that operate well with little or no adsorbed cesium. This new emission capability coupled with improved collectors that maintain performance with emitter-vapor deposit accumulations are requisites for efficient, enduring thermionic converters. The accomplishments and contributors in these areas are discussed.

Morris, J. F.↗

NASA NTRS1977

The NASA thermionic-conversion /TEC-ART/ program

The NASA program for applied research and technology in thermionic energy conversion is described, and the approaches used to obtain the goal of reduced interelectrode losses, improved emitters, and improved collectors are indicated. The current emphasis on out-of-core thermionics allows materials and designs previously prohibited by in-core nucleonics and geometrics. Since high-temperature material effects are important for several components, tungsten, 25%-rhenium technology is a subject of research. Emitter-vaporization, collector-deposition effects were studied, and improvements in metallic-fluid heat pipes are considered. Some required characteristics of electrodes are examined.

Morris, J. F.↗

NASA NTRS1976

Fundamental studies of black chrome for solar collector use

Mcdonald, G.↗

NASA NTRS1977

Active control of potential of the geosynchronous satellites ATS-5 and ATS-6

The ATS-6 geosynchronous satellite carries two cesium ion thrusters as part of the technology experiment package. Their effectiveness in controlling the spacecraft potential was shown during normal operation of the thrusters. Auroral particle detectors were used to estimate spacecraft potential, amount of cesium propellant backflow to the spacecraft, and electron signature of the thruster. Operation of the thruster clamps the spacecraft to about -10 v in the presence of a wide range of ambient particle flux. Some positive ions from the thruster are returned to the spacecraft during this period. The plasma bridge neutralizer of the thruster is also effective in controlling the spacecraft potential. Laboratory simulation tests of the ATS-5 ion thruster thermionic emitter neutralizer showed that negative differential charging on the spacecraft surface probably prevents the emitter from completely discharging the spacecraft in flight.

Goldstein, R.↗

NASA NTRS1977

Measurement of material parameters that limit the open-circuit voltage in P-N-junction silicon solar cells

The greatest gains in solar energy conversion efficiency of p-n-junction silicon solar cells come from increasing the open-circuit voltage V sub OC; it is important to understand and characterize the material parameters that limit the V sub OC. Strong experimental evidence exists to support the assertion that either an anomalously large minority carrier charge storage or an anomalously small minority carrier lifetime in the quasi-neutral emitter region limits the open circuit voltage. A method is presented for measuring charge storage and effective lifetime. Static and transient measurements are analyzed using physical models of the solar cell characteristics. This analysis yields the emitter charge storage and life-time, which then can be related to the various physical mechanisms, such as energy band gap shrinkage, that have been proposed earlier as responsible for limiting V sub OC.

Lindholm, F. A.↗

NASA NTRS1977

Optical and electrical properties of ion beam textured Kapton and Teflon

An electron bombardment argon ion source was used to ion etch polyimide (Kapton) and fluorinated ethylene, FEP (Teflon). Samples of polyimide and FEP were exposed to (0.5-1.0) keV Ar ions at ion current densities of (1.0-1/8) mA/sq cm for various exposure times. Changes in the optical and electrical properties of the samples were used to characterize the exposure. Spectral reflectance and transmittance measurements were made between 0.33 and 2.16 micron m using an integrating sphere after each exposure. From these measurements, values of solar absorptance were obtained. Total emittance measurements were also recorded for some samples. Surface resistivity was used to determine changes in the electrical conductivity of the etched samples. A scanning electron microscope recorded surface structure after exposure. Spectral optical data, resistivity measurements, calculated absorptance and emittance measurements are presented along with photomicrographs of the surface structure for the various exposures to Ar ions.

Mirtich, M. J.↗

NASA NTRS1977

Black chrome on commercially electroplated tin as a solar selecting coating

The reflectance properties of black chrome electroplated on commercially electroplated tin were measured for various black chrome plating times for both the solar and infrared spectrum. The values of absorptance and emittance were calculated from the measured reflectance values. The results indicate that the optimum combination of the highest absorptance in the solar region and the lowest emittance in the infrared of the black chrome plated on commercially electroplated tin is obtained for a black chrome plating time of between one and two minutes.

Mcdonald, G. E.↗

NASA NTRS1978

Infrared spectra of lunar soils

Lunar sample measurements were made both under ambient nitrogen pressure and under vacuum conditions. Average emittance values were analyzed for a number of samples which were determined to include silicate minerals, pyroxenes, and fine particle feldspar. Information is also provided on modifications made to emittance apparatus in order to work with smaller sample sizes and to prevent accidental loss or contamination of the lunar sample material.

Aronson, J. R.↗