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At least 163 records · Page 9

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NASA NTRS1991

Failure prediction of thin beryllium sheets used in spacecraft structures

The primary objective of this study is to develop a method for prediction of failure of thin beryllium sheets that undergo complex states of stress. Major components of the research include experimental evaluation of strength parameters for cross-rolled beryllium sheet, application of the Tsai-Wu failure criterion to plate bending problems, development of a high order failure criterion, application of the new criterion to a variety of structures, and incorporation of both failure criteria into a finite element code. A Tsai-Wu failure model for SR-200 sheet material is developed from available tensile data, experiments carried out by NASA on two circular plates, and compression and off-axis experiments performed in this study. The failure surface obtained from the resulting criterion forms an ellipsoid. By supplementing experimental data used in the the two-dimensional criterion and modifying previously suggested failure criteria, a multi-dimensional failure surface is proposed for thin beryllium structures. The new criterion for orthotropic material is represented by a failure surface in six-dimensional stress space. In order to determine coefficients of the governing equation, a number of uniaxial, biaxial, and triaxial experiments are required. Details of these experiments and a complementary ultrasonic investigation are described in detail. Finally, validity of the criterion and newly determined mechanical properties is established through experiments on structures composed of SR200 sheet material. These experiments include a plate-plug arrangement under a complex state of stress and a series of plates with an out-of-plane central point load. Both criteria have been incorporated into a general purpose finite element analysis code. Numerical simulation incrementally applied loads to a structural component that is being designed and checks each nodal point in the model for exceedance of a failure criterion. If stresses at all locations do not exceed the failure criterion, the load is increased and the process is repeated. Failure results for the plate-plug and clamped plate tests are accurate to within 2 percent.

Roschke, Paul N.↗

NASA NTRS2018

The Harmonized Landsat and Sentinel-2 Surface Reflectance Data Set

The Harmonized Landsat and Sentinel-2 (HLS) project is a NASA initiative aiming to produce a VirtualConstellation (VC) of surface reflectance (SR) data acquired by the Operational Land Imager (OLI) and MultiSpectral Instrument (MSI) aboard Landsat 8 and Sentinel-2 remote sensing satellites, respectively. The HLS products are based on a set of algorithms to obtain seamless products from both sensors (OLI and MSI): atmospheric correction, cloud and cloud-shadow masking, spatial co-registration and common gridding, bidirectional reflectance distribution function normalization and spectral bandpass adjustment. Three products are derivedfrom the HLS processing chain: (i) S10: full resolution MSI SR at 10 m, 20 m and 60 m spatial resolutions; (ii)S30: a 30 m MSI Nadir BRDF (Bidirectional Reflectance Distribution Function)-Adjusted Reflectance (NBAR);(iii) L30: a 30 m OLI NBAR. All three products are processed for every Level-1 input products from Landsat 8/OLI (L1T) and Sentinel-2/MSI (L1C). As of version 1.3, the HLS data set covers 10.35 million km2 and spans from first Landsat 8 data (2013); Sentinel-2 data spans from October 2015. The L30 and S30 show a good consistency with coarse spatial resolution products, in particular MODIS Collection 6 MCD09CMG products (overall deviations do not exceed 11%) that are used as a reference for quality assurance. The spatial co-registration of the HLS is improved compared to original Landsat 8 L1T and Sentinel 2A L1C products, for which misregistration issues between multi-temporal data are known. In particular, the resulting computed circular errors at 90% for the HLS product are 6.2 m and 18.8 m, for S10 and L30 products, respectively. The main known issue of the current data set remains the Sentinel-2 cloud mask with many cloud detection omissions. The cross-comparison with MODIS was used to flag products with most evident non-detected clouds. A time series outlier filtering approach is suggested to detect remaining clouds. Finally, several time series are presented to highlight the high potential of the HLS data set for crop monitoring.

Landsat Sentinel-2↗

NASA NTRS1982

Analysis of broadcasting satellite service feeder link power control and polarization

Statistical analyses of carrier to interference power ratios (C/Is) were performed in assessing 17.5 GHz feeder links using (1) fixed power and power control, and (2) orthogonal linear and orthogonal circular polarizations. The analysis methods and attenuation/depolarization data base were based on CCIR findings to the greatest possible extent. Feeder links using adaptive power control were found to neither cause or suffer significant C/I degradation relative to that for fixed power feeder links having similar or less stringent availability objectives. The C/Is for sharing between orthogonal linearly polarized feeder links were found to be significantly higher than those for circular polarization only in links to nominally colocated satellites from nominally colocated Earth stations in high attenuation environments.

Sullivan, T. M.↗

NASA NTRS1973

Stress intensity factors for long, deep surface flaws in plates under extensional fields

Using a singular solution for a part circular crack, a Taylor Series Correction Method (TSCM) was verified for extracting stress intensity factors from photoelastic data. Photoelastic experiments were then conducted on plates with part circular and flat bottomed cracks for flaw depth to thickness ratios of 0.25, 0.50 and 0.75 and for equivalent flaw depth to equivalent ellipse length values ranging from 0.066 to 0.319. Experimental results agreed well with the Smith theory but indicated that the use of the ''equivalent'' semi-elliptical flaw results was not valid for a/2c less than 0.20. Best overall agreement for the moderate (a/t approximately 0.5) to deep flaws (a/t approximatelly 0.75) and a/2c greater than 0.15 was found with a semi-empirical theory, when compared on the basis of equivalent flaw depth and area.

Harms, A. E.↗

NASA NTRS1995

Storing Data In Circulating Electrons

Proposed memory device contains electrons traveling in circular orbit in vacuum. Orientations of electron spins (up or down) represents binary ones or zeros, respectively. Assuming length of each memory element 100 times classical fictitious electron radius, device theoretically capable of storing 446 gigabytes of data per meter of circumference of orbit.

Eastman, Ted R.↗

NASA NTRS1987

Ion mass spectrometer experiment for ISIS-2 spacecraft

The International Satellite for Ionospheric Studies (ISIS) program of NASA was the longest duration program in NASA history. A number of satellites were flown under this program, the last being called ISIS-2, which was launched on April 1, 1971 and operated successfully for over 13 years. An experiment called the Ion Mass Spectrometer (IMS) was flown on the ISIS-2 spacecraft. It operated for 10 years providing a large data base of positive ion composition and ion flow velocities along the orbit of the satellite, the latter being circular at 1400 km with a 90 degree inclination. The data were processed and reside in the National Space Sciences Data Center.

Hoffman, John H.↗

NASA NTRS2012

Improving the Inventory of Large Lunar Basins: Using LOLA Data to Test Previous Candidates and Search for New Ones

Topography and crustal thickness data from LOLA altimetry were used to test the validity of 98 candidate large lunar basins derived from photogeologic and earlier topographic and crustal thickness data, and to search for possible new candidates. We eliminate 23 previous candidates but find good evidence for 20 new candidates. The number of basins greater than 300 km diameter on the Moon is almost certainly a factor 2 (maybe 3?) larger than the number of named features having basin-like topography. Unified Lunar Control Net 2005 data [1] and model crustal thickness data [2] were previously used to search for possible previously unrecognized large lunar impact basins [3,4]. An inventory of 98 candidate topographic basins greater than 300 km in diameter was found [5]. This includes 33 named features (only those having basin-like topography) out of the 45 listed by Wilhelms [6], 38 additional Quasi-Circular Depressions (QCDs) found in the ULCN2005 topography, and 27 Circular Thin Areas (CTAs) found in model crustal thickness data [2]. Most named features and additional QCDs have strong CTA signatures, but there may be a class of CTAs that are not easily recognized in the old and low resolution ULCN2005 topography. Lunar Orbiter Laser Altimeter (LOLA) data have recently become publically available. We used these data to (a) refine the center and ring diameters of known basins, (b) test the viability of the candidate basins previously found (as described above), and (c) search for additional candidate basins not revealed by the earlier lower resolution data. We used the LOLA topography directly but also a recent new model crustal thickness data that includes Kaguya gravity data [7]. We repeated a Topographic Expression (TE) and a Crustal Thickness Expression (CTE) scoring exercise originally done with the basins found in ULCN and earlier model crustal thickness data [5]. Each candidate was scored on a scale from 0 (no topographic basin or circular thin area signature) to 5 (strong circular low or strong circular thin area signature). These were combined into a total score used to rank the probability for each candidate basin. We used the same GRIDVIEW software to stretch, contour and profile the LOLA and new crustal thickness data as was done with the ULCN2005 and older model crustal thickness data.

Frey, H. V.↗

NASA NTRS1974

Variable beamwidth monopulse feed for Tracking and Data Relay Satellite (TDRS)

The Tracking and Data Relay Satellite with a set of circularly-polarized, amplitude-sensing monopulse patterns suitable for acquiring and tracking user spacecraft at Ku-band (15.0 GHz) is discussed. The possibility of increasing the less than 0.4-degree half-power beamwidth of the data beam to almost 1.0 degree during the acquisition phase is predicated on the use of feeds situated in the first bright-ring of the Airy diffraction structure. A complex-vector simulation equivalent to the Kirchhoff-Kottler or Franz formulations is used to compute transmitted and received field information for a dual-reflector (Cassegrain) antenna configuration in a three-dimensional space.

Schmidt, R. F.↗

NASA NTRS2012

Preliminary Crater Retention Ages for an Expanded Inventory of Large Lunar Basins

Based on LOLA topography and a new crustal thickness model, the number of candidate lunar basins greater than 300 km in diameter is at least a factor 2 larger than the traditional number based on photogeology alone, and may be as high as 95. Preliminary N(50) crater retention ages for this population of candidate basins shows two distinct peaks. Frey [1] suggested, based on Clementine-era topography (ULCN2005) and a crustal thickness model based on Lunar Prospector data [2], that there could be as many as 98 lunar basins greater than 300 km diameter. Many of the weaker cases have not stood up to recent testing [3,4,5] using LOLA data and a newer crustal thickness model based on Kaguya gravity data and LOLA topography data [6]. As described in companion abstracts [4,5], we have deleted from the earlier inventory 1 more named feature (Sikorsky- Rittenhouse; LOLA data show that its diameter is actually less than 300 km), 11 Quasi-Circular Depressions (QCDs) identified in the ULCN topography, and 11 Circular Thin Areas (CTAs) found in the earlier crustal thickness model [2]. We did this by repeating the scoring exercise originally done in [1] but with the new data [4,5]. Topographic Expression (TE) and Crustal Thickness Expression (CTE) scores were determined for each candidate on a scale of 0 to 5 (5 being a strong, circular signature, 0 for those with no discernible circular topographic or crustal thickness signature). These scores are added together to produce a Summary Score which has a range of 0 to 10. We eliminated all candidates with a Summary Score less than 3, as well as other cases where, for example, the TE went to zero because what looked like a single large circular QCD in the lower resolution ULCN data was in fact a cluster of smaller deep impacts readily apparent in the newer higher resolution LOLA data. This process reduced the original inventory from 98 to 75 candidates.

Frey, H. V.↗

NASA NTRS1982

Pressure losses and heat transfer in non-circular channels with hydraulically smooth walls

The influence of channel geometry on pressure losses and heat transfer in noncircular channels with hydraulically smooth walls was studied. As a basic assumption for the description of this influence, integral geometrical criteria, selected according to experimental experience, were introduced. Using these geometrical criteria, a large set of experimental data for pressure losses and heat transfer in circular and annular channels with longitudinal fins was evaluated. In this way it as empirically proved that the criteria described channel geometry fairly well.

Malak, J.↗

NASA NTRS1994

Spectral polarization analysis of the interplanetary magnetic field fluctuations

A new computational method and algorithm, based on complex Fourier analysis, is used to derive the spectral density of plane and circularly polarized fluctuation components of the interplanetary magnetic field. Applications of the method have been made using HEOS 2 (1 AU), Pioneer 10 (5 AU), Pioneer 11 (20 AU), and International Cometary Explorer (ICE) (Giocabini-Zinner's comet) data sets. The results show the existence of circularly polarized magnetohydrodynamic (MHD) waves in all cases.

Polygiannakis, J. M.↗

NASA NTRS1976

Stress intensity factors for long, deep surface flaws in plates under extensional fields

Using a singular solution for a part circular crack by Smith, a Taylor Series Correction Method (TSCM) was verified for extracting stress intensity factors (SIFs) from photoelastic data. Photoelastic experiments were then conducted on plates with part circular and flat bottomed cracks for flaw depth to thickness ratios of 0.25, 0.50, and 0.75 and for equivalent flaw depth to equivalent ellipse length values ranging from 0.066 to 0.319. Experimental results agreed well with the Smith theory but indicated that the use of the 'equivalent' semi-elliptical flaw for correlating the part circular flaw results with semi-elliptical flaw results was not valid for a/2c much less than 0.20. Best overall agreement for the moderate (a/t about 0.5) to deep flaws (a/t about 0.75) and a/2c greater than 0.15 was found with a semi-empirical theory due to Newman when compared on the basis of equivalent flaw depth and area. The Smith theory, when correlated on the basis of flaw depth and area, appears to yield reasonable estimates (within 10%) of the SIF for flat bottomed flaws for the geometries studied here.

Harms, A. E.↗

NASA NTRS1983

Measurement of ultrasonic fields in transparent media using a scanning differential interferometer

An experimental system for the detection of three dimensional acoustic fields in optically transparent media using a dual beam differential interferometer is described. In this system, two coherent, parallel, focused laser beams are passed through the specimen and the interference fringe pattern which results when these beams are combined shifts linearly by an amount which is related to the optical pathlength difference between the two beams. It is shown that for small signals, the detector output is directly proportional to the amplitude of the acoustic field integrated along the optical beam path through the specimen. A water tank and motorized optical platform were constructed to allow these dual beams to be scanned through an ultrasonic field generated by a piezoelectric transducer at various distances from the transducer. Scan data for the near, Fresnel, and far zones of a uniform, circular transducer are presented and an algorithm for constructing the radial field profile from this integrated optical data, assuming cylindrical symmetry, is described.

Dockery, G. D.↗

NASA NTRS1988

Numerical analysis of three-dimensional viscous internal flows

A 3-D Navier-Stokes code has been developed for analysis of turbomachinery blade rows and other internal flows. The Navier-Stokes equations are written in a Cartesian coordinate system rotating about the x-axis, and then mapped to a general body-fitted coordinate system. Streamwise viscous terms are neglected using the thin-layer assumption, and turbulence effects are modeled using the Baldwin-Lomax turbulence model. The equations are discretized using finite differences on stacked C-type grids and are solved using a multistage Runge-Kutta algorithm with a spatially-varying time step and implicit residual smoothing. Calculations have been made of a horseshoe vortex formed in front of a flat plate with a round leading edge standing in a turbulent endwall boundary layer. Comparisons are made with experimental data taken by Eckerle and Langston for a circular cylinder under similar conditions. Computer and measured results are compared in terms of endwall flow visualization pictures and total pressure loss contours and vector plots on the symmetry plane. Calculated details of the primary vortex show excellent agreement with the experimental data. The calculations also show a small secondary vortex not seen experimentally.

Chima, Rodrick V.↗

NASA NTRS1988

Numerical analysis of three-dimensional viscous internal flows

Chima, Rodrick V.↗

NASA NTRS1984

Radar and photoelectric observations of asteroid 2100 Ra-Shalom

Asteroid Ra-Shalom was subjected to 13-cm wavelength radar and V-filter photoelectric observations during its August-September 1981 apparition; the radar data yield echoes in the same sense of circular polarization as transmitted (SC) as well as in the opposite sense (OC). SC-to-OC echo power ratio estimates indicate that most of the backscattering is due to single reflections from surface elements that are relatively smooth at decimeter scales. The photoelectric data suggest a value of 19.79 hours for the synodic rotation period.

Ostro, S. J.↗

NASA NTRS1982

Comparison between computations and experimental data in unsteady three-dimensional transonic aerodynamics, including aeroelastic applications

Comparisons were made of computed and experimental data in three-dimensional unsteady transonic aerodynamics, including aeroelastic applications. The computer code LTRAN3, which is based on small-disturbance aerodynamic theory, was used to obtain the aerodynamic data. A procedure based on the U-g method was developed to compute flutter boundaries by using the unsteady aerodynamic coefficients obtained from LTRAN3. The experimental data were obtained from available NASA publications. All the studies were conducted for thin, unswept, rectangular wings with circular-arc cross sections. Numerical and experimental steady and unsteady aerodynamic data were compared for a wing with an aspect ratio of 3 and a thickness ratio of 5% at Mach numbers of 0.7 and 0.9. Flutter data were compared for a wing with an aspect ratio of 5. Two thickness ratios, 6% at Mach numbers of 0.715, 0.851, and 0.913, and 4% at Mach number of 0.904, were considered. Based on the unsteady aerodynamic data obtained from LTRAN3, flutter boundaries were computed; they were compared with those obtained from experiments and the code NASTRAN, which uses linear aerodynamics.

Guruswamy, P.↗

NASA NTRS1992

Atmosphere and climate studies of Mars using the Mars Observer pressure modulator infrared radiometer

Studies of the climate and atmosphere of Mars are limited at present by a lack of meteorological data having systematic global coverage with good horizontal and vertical resolution. The Mars Observer spacecraft in a low, nearly circular, polar orbit will provide an excellent platform for acquiring the data needed to advance significantly our understanding of the Martian atmosphere and its remarkable variability. The Mars Observer pressure modulator infrared radiometer (PMIRR) is a nine-channel limb and nadir scanning atmospheric sounder which will observe the atmosphere of Mars globally from 0 to 80 km for a full Martian year. PMIRR employs narrow-band radiometric channels and two pressure modulation cells to measure atmospheric and surface emission in the thermal infrared. PMIRR infrared and visible measurements will be combined to determine the radiative balance of the polar regions, where a sizeable fraction of the global atmospheric mass annually condenses onto and sublimes from the surface. Derived meteorological fields, including diabatic heating and cooling and the vertical variation of horizontal winds, are computed from the globally mapped fields retrieved from PMIRR data.

Mccleese, D. J.↗