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At least 271 records · Page 15

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NASA NTRS1977

Lunar regolith dynamics based on analysis of the cosmogenic radionuclides Na-22, Al-26, and Mn-53

Depth profiles of Na-22 and Al-26 in the upper portions of five lunar cores are analyzed. From the analyses, it is concluded that the natural gardening processes on the lunar surface result in mixing of the regolith to a depth of 2-3 cm over a time period which is short compared with the half-life of Al-26 (0.73 m.y.). It is also concluded that the rotary drill processes which were used to obtain the deep drill samples generally resulted in loss and/or mixing of the upper portions of the cores. In contrast, the near-surface regions of the drive tube cores appear to have a well-preserved stratigraphy. Analysis of Mn-53 in samples of six lunar rocks helps substantiate the accuracy of age date estimates by other means, and provides definite information that the total lunar surface exposure of two of these rocks has occurred during a single surface event which continued to their collection.

Fruchter, J. S.↗

NASA NTRS1993

Automated Precision Orbit Determination for TOPEX/Poseidon with GPS

A highly automated GPS data processing system for the orbit determination of TOPEX/Poseidon is described. The orbit is recovered to an estimated accuracy of better than 4 cm in altitude, 6 cm crosstrack, and 11 cm down track. The RMS postfit residuals on the ionospherically calibrated carrier phase observable are less than 5 mm. The RMS difference over a 4.5-hour overlap period between two 30-hour data arcs is 1 cm in altitude, 5 cm cross track, and 4 cm down track. These results can be obtained within two days of onboard GPS data collection. Most of the data processing for a 30-hour arc of GPS data can be performed on a single workstation in less than 6 hours of CPU time. The estimation scenarios are explained, the automated data processing steps are described, and means to assess solution quality are discussed.

automation GPS data processing orbit determination↗

NASA NTRS1994

Observations of cloud liquid water path over oceans: Optical and microwave remote sensing methods

Published estimates of cloud liquid water path (LWP) from satellite-measured microwave radiation show little agreement, even about the relative magnitudes of LWP in the tropics and midlatitudes. To understand these differences and to obtain more reliable estimate, optical and microwave LWP retrieval methods are compared using the International Satellite Cloud Climatology Project (ISCCP) and special sensor microwave/imager (SSM/I) data. Errors in microwave LWP retrieval associated with uncertainties in surface, atmosphere, and cloud properties are assessed. Sea surface temperature may not produce great LWP errors, if accurate contemporaneous measurements are used in the retrieval. An uncertainty of estimated near-surface wind speed as high as 2 m/s produces uncertainty in LWP of about 5 mg/sq cm. Cloud liquid water temperature has only a small effect on LWP retrievals (rms errors less than 2 mg/sq cm), if errors in the temperature are less than 5 C; however, such errors can produce spurious variations of LWP with latitude and season. Errors in atmospheric column water vapor (CWV) are strongly coupled with errors in LWP (for some retrieval methods) causing errors as large as 30 mg/sq cm. Because microwave radiation is much less sensitive to clouds with small LWP (less than 7 mg/sq cm) than visible wavelength radiation, the microwave results are very sensitive to the process used to separate clear and cloudy conditions. Different cloud detection sensitivities in different microwave retrieval methods bias estimated LWP values. Comparing ISCCP and SSM/I LWPs, we find that the two estimated values are consistent in global, zonal, and regional means for warm, nonprecipitating clouds, which have average LWP values of about 5 mg/sq cm and occur much more frequently than precipitating clouds. Ice water path (IWP) can be roughly estimated from the differences between ISCCP total water path and SSM/I LWP for cold, nonprecipitating clouds. IWP in the winter hemisphere is about 3 times the LWP but only half the LWP in the summer hemisphere. Precipitating clouds contribute significantly to monthly, zonal mean LWP values determined from microwave, especially in the intertropical convergence zone (ITCZ), because they have almost 10 times the liquid water (cloud plus precipitation) of nonprecipitating clouds on average. There are significant differences among microwave LWP estimates associated with the treatment of precipitating clouds.

Lin, Bing↗

NASA NTRS2003

Isotopic Composition of Silicon Carbide in the CO3 Chondrite Colony

Presolar grains have been identified in primitive members of all chondrite classes. The isotopic compositions of presolar grains provide probes of galactic evolution and nucleosynthesis in stars, while the abundances and characteristics of presolar grains contain a record of thermal processing in the solar system. Most of the detailed isotopic work has been done on SiC from Murchison and Orgueil, supplemented by a few studies of ordinary and enstatite chondrites. This work investigates SiC in the Colony CO3.O meteorite. SiC is present in Colony at a matrix-normalized abundance of approximately 3.7 ppm, much less than in CI chondrites and the matrixes of CM and primitive ordinary and enstatite chondrites. The abundances of SiC and other presolar grains in Colony seem to correlate with the chemical processing that produced CO3 chondrites. This implies that the known presolar grains experienced the same processing as the bulk CO3 material and assumes that the parent material was the same as that for other chondrites, including CI. That parent material is most plausibly the average material in the sun s parent molecular cloud. One test of this idea is to look for primary differences between SiC in Colony (and other meteorites) and that in CI and CM chondrites. True differences not related to thermal processing would falsify the assumption that all chondrite classes originated from the same reservoir of presolar dust.

Smith, J. B.↗

NASA NTRS1979

Development and fabrication of improved power transistor switches

A new class of high-voltage power transistors was achieved by adapting present interdigitated thyristor processing techniques to the fabrication of npn Si transistors. Present devices are 2.3 cm in diameter and have V sub CEO (sus) in the range of 400 to 600V. V sub CEO (sus) = 450V devices were made with an (h sub FE)(I sub C) product of 900A at V sub CE = 2.5V. The electrical performance obtained was consistent with the predictions of an optimum design theory specifically developed for power switching transistors. The device design, wafer processing, and assembly techniques are described. Experimental measurements of the dc characteristics, forward SOA, and switching times are included. A new method of characterizing the switching performance of power transistors is proposed.

Hower, P. L.↗

NASA NTRS1973

Investigations of an urban area and its locale using ERTS-1 data supported by U-photography

An urban area in central Pennsylvania and the surrounding locality were investigated separately at first by photointerpretation of ERTS-1 imagery and by computer processing of MSS tapes. Next the photointerpretation and processing were coordinated. The results of the cooperative effort of photointerpreters and computer processing analysts were much improved over independent efforts. It was found that single frames of U-2 photography could be projected onto printer output maps with little recognizable distortion in areas 10 to 25 cm square. In this way targets could be identified for use as training areas for computer processed signature identification. In addition, at any stage of category mapping, the level of success in correct classification could be assessed by this method. The results of the classification of the study area are discussed.

Weeden, H. A.↗

NASA NTRS2000

Continuing Measurements of CO2 Crystals with a Hand-Held 35 GHz Radiometer

In order to increase our knowledge of the Martian polar caps, an improved understanding of the behavior of both frozen H2O and CO2 in different parts of the electromagnetic spectrum is needed. The thermal microwave part of the spectrum has received relatively little attention compared to the visible and infrared wavelengths. A simple experiment to measure the brightness temperature of frozen CO2 was first performed in the winter of 1998 using a 35 GHz radiometer. in experiments performed during the winter of 1999 and 2000, passive microwave radiation emanating from within layers of manufactured CO2 (dry ice) crystals was again measured with a 35 GHz handheld radiometer. Both large (0.8 cm) and small (0.3 cm) cylindrical-shaped dry ice pellets, at a temperature of 197 K (-76 C), were measured. A 1 sq m plate of aluminum sheet metal was positioned beneath the dry ice so that microwave emissions from the underlying soil layers would be minimized. Non-absorbing foam was positioned around the sides of the plate in order to keep the dry ice in place and to assure that the incremental deposits were level. Thirty-five GHz measurements of this plate were made through the dry ice deposits in the following way. Layers of dry ice were built up and measurements were repeated for the increasing CO2 pack. First, 7 cm of large CO2 pellets were poured onto the sheet metal plate, then an additional 7 cm were added, and finally, 12 cm were added on top of the 14 cm base. Hand-held 35 GHz measurements were made each time the thickness of the deposit was increased. The same process was repeated for the smaller grain pellets. Furthermore, during the past winter, 35 GHz measurements were taken of a 25 kg (27 cm x 27 cm x 27 cm) solid cube Of CO2, which was cut in half and then re-measured. Additional information is contained in the original extended abstract.

Foster, J.↗

NASA NTRS1979

Study of Automated Module Fabrication for Lightweight Solar Blanket Utilization

Cost-effective automated techniques for accomplishing the titled purpose; based on existing in-house capability are described. As a measure of the considered automation, the production of a 50 kilowatt solar array blanket, exclusive of support and deployment structure, within an eight-month fabrication period was used. Solar cells considered for this blanket were 2 x 4 x .02 cm wrap-around cells, 2 x 2 x .005 cm and 3 x 3 x .005 cm standard bar contact thin cells, all welded contacts. Existing fabrication processes are described, the rationale for each process is discussed, and the capability for further automation is discussed.

Gibson, C. E.↗

NASA NTRS1980

Characterization of deliberately nickel-doped silicon wafers and solar cells

Microstructural and electrical evaluation tests were performed on nickel-doped p-type silicon wafers before and after solar cell fabrication. The concentration levels of nickel in silicon were 5 x 10 to the 14th power, 4 x 10 to the 15th power, and 8 x 10 to the 15th power atoms/cu cm. It was found that nickel precipitated out during the growth process in all three ingots. Clumps of precipitates, some of which exhibited star shape, were present at different depths. If the clumps are distributed at depths approximately 20 micron apart and if they are larger than 10 micron in diameter, degradation occurs in solar cell electrical properties and cell conversion efficiency. The larger the size of the precipitate clump, the greater the degradation in solar cell efficiency. A large grain boundary around the cell effective area acted as a gettering center for the precipitates and impurities and caused improvement in solar cell efficiency. Details of the evaluation test results are given.

Salama, A. M.↗

NASA NTRS1983

Upgrading and performance of the SAO laser ranging system in Matera

The performance of the SAO lasers was improved considerably in terms of accuracy, range noise, data yield, and reliability. With the narrower laser pulse (2.5-3.0 nsec) and a new analog pulse processing system, the systematic range errors were reduced to 3-5 cm and range noise has been reduced to 5-15 cm on low satellites and 10-18 cm on Lageos. Pulse repetition rate was increased to 30 ppm and considerable improvement has been made in signal-to-noise ratio by using a 3 Angstrom interference filter and by reducing the range gate window down to 200-400 nsec. The first upgraded system was installed in Arequipa, Peru in the spring of 1982. The second upgraded system is now in operation in Matera, Italy. The third system is expected to be installed in Israel during 1984.

Maddox, J.↗

NASA NTRS1984

Zirconium fluoride glass - Surface crystals formed by reaction with water

The hydrated surfaces of a zirconium barium fluoride glass, which has potential for application in optical fibers and other optical elements, were observed by scanning electron microscopy. Crystalline zirconium fluoride was identified by analysis of X-ray diffraction patterns of the surface crystals and found to be the main constituent of the surface material. It was also found that hydrated zirconium fluorides form only in highly acidic fluoride solutions. It is possible that the zirconium fluoride crystals form directly on the glass surface as a result of its depletion of other ions. The solubility of zirconium fluoride is suggested to be probably much lower than that of barium fluoride (0.16 g/100 cu cm at 18 C). Dissolution was determined to be the predominant process in the initial stages of the reaction of the glass with water. Penetration of water into the glass has little effect.

Doremus, R. H.↗

NASA NTRS1985

Stress concentration around a small circular hole in the HiMAT composite plate

Anisotropic plate theory is used to calculate the anisotropic stress concentration factors for a composite plate (AS/3501-5 graphite/epoxy composite, single ply or laminated) containing a circular hole. This composite material is used on the highly maneuverable aircraft technology (HiMAT) vehicle. It is found that the anisotropic stress concentration factor could be greater or less than 3 (the stress concentration factor for isotropic materials), and that the locations of the maximum tangential stress points could shift with the change of fiber orientation with respect to the loading axis. The effect of hole size on the stress concentration factor is examined using the Point Stress Criterion and the Averaged Stress Criterion. The predicted stress concentration factors based on the two theories compared fairly well with the measured values for the hole size 0.3175 cm (1/8 in). It is also found that through the lamination process, the stress concentration factor could be reduced drastically, indicating an improvement in structural performance.

Ko, W. L.↗

NASA NTRS1988

Temperature dependence of the abundances of elements in solar He-3 rich events

The element abundances in solar He-3-rich events are observed to vary with the soft X-ray temperature of the parent flare. The observations are consistent with thermal collisional ionization of the elements to the equilibrium-charge states expected within the flare-heated plasma at (10-15) x 10 to the 6th K. Proposed photoionization mechanisms appear to be excluded. Rapid ionization of the ions requires that the heating occur sufficiently low in the corona that the electron density exceed about (1-5) x 10 to the 8th/cu cm. Heavy-element enhancements might be a signature of material processed in the flash phase of events in contrast to unheated ambient material accelerated by a coronal shock.

Reames, D. V.↗

NASA NTRS1989

Plasma flow processes within magnetic nozzle configurations

An attempt is made to separate the interactive processes occurring in electromagnetic devices and to examine the acceleration of ionized gas within magnetic fields which can guide it through an expansion process. The plasma was generated in a single turn coil 50 cm long with a radius of 5.25 cm which surrounded a quartz tube with an internal radius of 3.81 cm and a length of 100 cm. Choking was found to occur at both sonic and cusp velocities which were coincident in the present experiment.

York, Thomas M.↗

NASA NTRS1990

Getting lunar ilmenite - From soils or rocks?

Lunar soils or rocks can be mined as sources of ilmenite for producing oxygen. However, separable crystals of loose ilmenite in lunar soils are rare (less than 2 percent) and small (less than 200 microns); most ilmenite in the regolith is locked together with silicate minerals as rock fragments. If fragmentation of rock sources must be attempted to win appreciable amounts of ilmenite (approximately 10 percent or more), selective collection of high-Ti basalt fragments larger than 1 cm for fragmentation and ilmenite beneficiation may be advantageous over extensive processing of fine lunar soil. Many alternative processing schemes for fragmenting rocks on the Moon have been proposed; one process which was tested early in the Apollo program successfully disaggregated lunar and terrestrial basalts by passive exposure to low-pressure alkali (K) vapor. This process is worthy of reinvestigation.

Vaniman, D. T.↗

NASA NTRS1998

Study of Atmospheric Nitric Oxide

We investigated the contribution of energetic nitrogen atoms to the production of nitric oxide in the thermosphere and their influence on the infrared emission spectrum. The nitric oxide molecules are important contributors to the cooling of the atmosphere. We first pointed out that in determining the energy distribution of the nitrogen atoms, it is important to take into account the thermal motion of the atmospheric gases. It had been ignored in all earlier studies. The source spectra are broadened considerably by the center of mass motion of the reactants. We worked out the consequences for the production of nitric oxide at night, using as sources of energetic N atoms, NO(+) + e yield N + O, N(D-2) + O yield N + O. The high energy tail is enhanced by orders of magnitude. We had earlier suggested (Sharma et al. 1993) that the reaction of energetic nitrogen atoms with O2 was responsible for the rotationally enhanced NO identified in the infrared spectrum. Our calculations provided quantitative confirmation of the suggestion. We proceeded to explore the validity of another approximation used in earlier analyses, the hard sphere approximation for the energy loss in elastic collisions. We carried out precise quantum mechanical calculations of the elastic 2 differential scattering of nitrogen atoms in collisions with oxygen atoms and showed that although the hard sphere approximation was nowhere of high precision, reasonable results could be obtained with an effective cross section of 6 x 10(exp 15)sq cm. We also initiated a program to include inelastic energy loss processes in the determination of the energy distribution function. We began a calculation of the rotation and vibrational excitation cross sections of molecular nitrogen and nitrogen atoms and developed a method for including inelastic energy loss as a function of scattering angle in the Boltzmann equation. A procedure for obtaining the solution of the Boltzman equation was worked out.

Dalgarno, A.↗

NASA NTRS1998

Credible Software and Simulation Uncertainty

The utility of software primarily depends on its reliability and performance; whereas, its significance depends solely on its credibility for intended use. The credibility of simulations confirms the credibility of software. The level of veracity and the level of validity of simulations determine the degree of credibility of simulations. The process of assessing this credibility in fields such as computational mechanics (CM) differs from that followed by the Defense Modeling and Simulation Office in operations research. Verification and validation (V&V) of CM simulations is not the same as V&V of CM software. Uncertainty is the measure of simulation credibility. Designers who use software are concerned with management of simulation uncertainty. Terminology and concepts are presented with a few examples from computational fluid dynamics.

Mehta, Unmeel B.↗

NASA NTRS2013

Testing and Analytical Modeling for Purging Process of a Cryogenic Line

The purging operations for cryogenic main propulsion systems of upper stage are usually carried out for the following cases: 1) Purging of the Fill/Drain line after completion of propellant loading. This operation allows the removal of residual propellant mass; and 2) Purging of the Feed/Drain line if the mission is scrubbed. The lines would be purged by connections to a ground high-pressure gas storage source. The flowrate of purge gas should be regulated such that the pressure in the line will not exceed the required maximum allowable value. Exceeding the maximum allowable pressure may lead to structural damage in the line. To gain confidence in analytical models of the purge process, a test series was conducted. The test article, a 20-cm incline line, was filled with liquid hydrogen and then purged with gaseous helium (GHe). The influences of GHe flowrates and initial temperatures were evaluated. The Generalized Fluid System Simulation Program, an in-house general-purpose computer program for flow network analysis, was utilized to model and simulate the testing. The test procedures, modeling descriptions, and the results will be presented in the final paper.

Hedayat, A.↗