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At least 109 records · Page 6

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NASA NTRS1971

Integrated operations/payloads/fleet analysis. Volume 3: System costs. Appendix A: Program direct costs

Individualized program direct costs for each satellite program are presented. This breakdown provides the activity level dependent costs for each satellite program. The activity level dependent costs, or, more simply, program direct costs, are comprised of the total payload costs (as these costs are strictly program dependent) and the direct launch vehicle costs. Only those incremental launch vehicle costs associated directly with the satellite program are considered. For expendable launch vehicles the direct costs include the vehicle investment hardware costs and the launch operations costs. For the reusable STS vehicles the direct costs include only the launch operations, recovery operations, command and control, vehicle maintenance, and propellant support. The costs associated with amortization of reusable vehicle investment, RDT&E range support, etc., are not included.

Source record↗

NASA NTRS1972

Flow direction determination of lava flows.

The flow direction technique, previously applied to ash-flow sheets, can be used to determine direction of movement and locate eruptive centers for lava flows. The method provides statistically stronger and more consistent flow direction data for lava than ash-flow tuff. The accuracy and reliability of the technique was established on the porphyritic basaltic andesite of Mount Taylor, New Mexico, which erupted from a known center, the Mount Taylor Amphitheater. The technique was then applied to volcanic units with unknown sources: the John Kerr Peak Quartz Latite and mid-Tertiary andesite flows in the Mogollon Mountains, both in southwestern New Mexico. The flow direction technique indicated flow patterns and suggested source areas for each rock unit. In the Mogollon Mountains flow direction measurements were supported by independent directional criteria such as dips of cross beds, stratigraphic thickening, facies changes, and megascopic textures.-

Smith, E. I.↗

NASA NTRS1973

Directional diffusion coefficients of solar protons inside and outside the bow shock.

The directional diffusion coefficients of low-energy (greater than or equal to 0.3 MeV) solar protons inside and outside the bow shock are examined during the solar flare event of Jan. 24, 1969. The data are derived from simultaneous observations obtained by Explorer 33 inside the magnetosheath and by Explorer 35 in the interplanetary medium. Although the gross properties of the spin-averaged intensities on a diffusion-type plot appear to be the same in both media, the directional intensities show significant variations. It is shown that directional intensities of low-energy protons can be described reasonably well by anisotropic diffusion with an associated diffusion coefficient. Directional diffusion coefficients are found to differ by a factor of as much as three among different directions in space, and from the spin-averaged diffusion coefficient. This suggests that anisotropic diffusion does indeed take place and that so called 'isotropic' diffusion coefficients derived in the past from spin-averaged intensities may actually be directional diffusion coefficients in cases where substantial anisotropies (greater than 50%) exist.

Verzariu, P.↗

NASA NTRS1984

Directional gear ratio transmissions

Epicyclic gear transmissions which transmit output at a gear ratio dependent only upon the input's direction are considered. A transmission housing envelops two epicyclic gear assemblies, and has shafts extending from it. One shaft is attached to a sun gear within the first epicyclic gear assembly. Planet gears are held symmetrically about the sun gear by a planet gear carrier and are in mesh with both the sun gear and a ring gear. Two unidirectional clutches restrict rotation of the first planet gear carrier and ring gear to one direction. A connecting shaft drives a second sun gear at the same speed and direction as the first planet gear carrier while a connecting portion drives a second planet gear carrier at the same speed and direction as the first ring gear. The transmission's output is then transmitted by the second ring gear to the second shaft. Input is transmitted at a higher gear ratio and lower speed for all inputs in the first direction than in the opposite direction.

Lafever, A. E.↗

NASA NTRS1984

Directional properties of circular Maria: Interpretation in the context of a testable gravitational capture model for lunar origin

Photogeologic study of circular lunar maria and mare-filled craters reveals that several of these features are either elliptical and/or asymmetrical. The ellipticity/asymmetry is interpreted as a directional property which may have genetic significance. Most maria indicate movement in easterly directions. The directional properties of these features are consistent with at least three models of mare basin formation: (1) mare basins were excavated by a swarm of fragments from a tidally disrupted planetary body impacting from a westerly direction; (2) mare basins were excavated by lunar satellites whose orbits decayed sufficiently to cause the satellites to impact onto the lunar surface from a westerly direction; and (3) basins were formed by the impact of large spheroids of lunar basalt onto the lunar surface from a westerly direction.

Malcuit, R. J.↗

NASA NTRS1985

Flow direction measurement criteria and techniques planned for the 40- by 80-/80- x 120-foot wind tunnel integrated systems tests

A study was performed in order to develop the criteria for the selection of flow direction indicators for use in the Integrated Systems Tests (ISTs) of the 40 by 80/80 by 120 Foot Wind Tunnel System. The problems, requirements, and limitations of flow direction measurement in the wind tunnel were investigated. The locations and types of flow direction measurements planned in the facility were discussed. A review of current methods of flow direction measurement was made and the most suitable technique for each location was chosen. A flow direction vane for each location was chosen. A flow direction vane that employs a Hall Effect Transducer was then developed and evaluated for application during the ISTs.

Zell, P. T.↗

NASA NTRS1985

Directivity optimization of a reflector antenna with cluster feeds - A closed-form solution

The directivity of a reflector antenna deteriorates as the feed moves away from the focal point for beam scanning. This deterioration can be substantially reduced if a cluster feed instead of single feed is used to control a beam. A closed-form solution is presented for the cluster excitation to achieve the optimum directivity. For an offset 108 lambda parabolic reflector scanning 10 beamwidths, the optimum directivity achieved by a 19-element (seven-element) cluster is 12 dB (8 dB) higher than that of a single element. Comparison of the optimum-directivity design and the popular conjugate field matching design is made. When the cluster spacing d is greater than 1 lambda, it is found that the optimum directivity is higher than that of conjugate field matching (CFM) scheme by an insignificant amount, although the excitations of two designs can be quite different. For d less than 0.5 lambda, the optimum design may exhibit the supergain phenomenon, namely extremely high directivities achieved by an oscillatory cluster excitation.

Lam, P. T.↗

NASA NTRS1986

Predicting crack growth direction in unidirectional composites

The purpose of this study is to gain a better understanding of the parameters affecting crack growth direction in unidirectional composite materials. To achieve this, the effect of anisotropy and biaxial, far field, loading on the direction of crack growth in unidirectional off-axis composite materials is investigated. Specific emphasis is placed on defining the crack-tip-stress field and finding a consistent criterion for predicting the direction of crack growth. An anisotropic crack-tip-stress analysis was implemented using three criteria (the normal stress ratio theory, the tensor polynomial failure criterion, and the strain energy density theory) to predict the direction of crack extension in unidirectional off-axis graphite-epoxy. The theoretically predicted crack extension directions were then compared with experimental results. It was determined that only the normal stress-ratio criterion correctly predicts the direction of crack extension.

Gregory, M. A.↗

NASA NTRS1988

Attenuated direct and scattered wave propagation on simulated land mobile satellite service paths in the presence of trees

Measurements were made of direct path with no trees, attenuated direct, and tree scattered signal levels at 1.3 GHz. Signals were received in two small groves of mixed hardwood trees. In the groves studied, average total signal levels were about 13 dB below adjacent no-trees locations, with attenuated direct signal levels about 14.6 dB below the no-trees case and scattered signals about 17.3 dB below the no-trees case. A simple model for land mobile satellite service (LMSS) propagation in groves of trees is proposed. The model assumes a constant scattered signal contribution at 17 dB below no-trees levels added to an attenuated direct signal which varies, depending on the number and density of trees in the direct path. When total signal levels are strong, the attenuated direct signal dominates. When total signal levels are more than 15 dB below no-trees levels, the scattered signals dominate.

Campbell, Richard L.↗

NASA NTRS1989

Direction of movement effects under transformed visual/motor mappings

Performance in a discrete aiming task was compared under several transformed visual/motor mappings: rotations by 45, 90, 135, and 180 deg and reflections about the horizontal and the vertical midlines. Eight aiming targets were used, corresponding to eight directions of movement: up, down, right, left, up-right, down-left, up-left, and down-right. Direction of movements were characterized in terms of separable visual and motor components, and two kinds of direction of movement effects were considered. First, a direction of movement effect paralleling that seen in rapid aiming under the usual nontransformed mapping. Second, because rotations, but not reflections, are physically realizable 2-D transformations, a visual/motor control system which is sensitive to physical constraints should perform reflections, but not rotations, in a piecemeal fashion. Results supported the hypothesis that a motor factor having to do with complexity of limb movement accounts for differences in movement accuracy between right and left oblique directions. Direction of movement effects were more evident in reflections than in rotations, and were consistent with the hypothesis that the visual/motor-control system seeks a physically realizable 2-D rotation solution to reflections. Results also suggested that reversal of two orthogonal basis dimensions is far less difficult than reversing only one and leaving the other intact.

Cunningham, H. A.↗

NASA NTRS1989

Visual direction as a metric of virtual space

Two experiments examine the abilities of ten subjects to visualize directions shown on a perspective display. Subjects indicated their perceived directions by adjusting a head-mounted cursor to correspond to the direction depicted on the display. This task is required of telerobotic operators who use map-like pictures of their workspace to determine the direction of objects seen by direct view. Results show significant open-loop judgment biases that may be composed of errors arising from misinterpretation of the map geometry and overestimation of gaze direction.

Ellis, Stephen R.↗

NASA NTRS1991

A critical comparison of second order closures with direct numerical simulation of homogeneous turbulence

Recently, several second order closure models have been proposed for closing the second moment equations, in which the velocity-pressure gradient (and scalar-pressure gradient) tensor and the dissipation rate tensor are two of the most important terms. In the literature, these correlation tensors are usually decomposed into a so called rapid term and a return-to-isotropy term. Models of these terms have been used in global flow calculations together with other modeled terms. However, their individual behavior in different flows have not been fully examined because they are un-measurable in the laboratory. Recently, the development of direct numerical simulation (DNS) of turbulence has given us the opportunity to do this kind of study. With the direct numerical simulation, we may use the solution to exactly calculate the values of these correlation terms and then directly compare them with the values from their modeled formulations (models). Here, we make direct comparisons of five representative rapid models and eight return-to-isotropy models using the DNS data of forty five homogeneous flows which were done by Rogers et al. (1986) and Lee et al. (1985). The purpose of these direct comparisons is to explore the performance of these models in different flows and identify the ones which give the best performance. The modeling procedure, model constraints, and the various evaluated models are described. The detailed results of the direct comparisons are discussed, and a few concluding remarks on turbulence models are given.

Shih, Tsan-Hsing↗

NASA NTRS1991

Visual direction as a metric of virtual space

Two experiments examine the abilities of ten subjects to visualize directions shown on a perspective display. Subjects indicated their perceived directions by adjusting a head-mounted cursor to correspond to the direction depicted on the display. This task is required of telerobotic operators who use map-like pictures of their workspace to determine the direction of objects seen by direct view. Results show significant open loop judgment biases that may be composed of errors arising from misinterpretation of the map geometry and overestimation of gaze direction.

Ellis, Stephen R.↗

NASA NTRS1993

Critical comparison of second-order closures with direct numerical simulations of homogeneous turbulence

Recently, several models have been proposed for closing the second-moment equations, in which the velocity-pressure gradient tensor and the dissipation rate tensor are two of the most important terms. In the literature, these correlation tensors are usually decomposed into a so-called rapid term and a return-to-isotropy term. Models of these terms have been used in global flow calculations together with other modeled terms. However, their individual behaviors in different flows have not been fully examined because they are unmeasurable in the laboratory. Recently, the development of direct numerical simulation (DNS) of turbulence has given us the possibility to do this kind of study. With direct numerical simulation, we may use the solution to calculate exactly the values of these correlation terms and then directly compare them with the values from their modeled formulations. In this paper, we make direct comparisons of five representative rapid models and eight return-to-isotropy models using the DNS data of 45 homogeneous flows, which were done by Rogers et al. (1986) and Lee and Reynolds (1985). The purpose of these direct comparisons is to explore the performance of these models in different flows and identify the ones that give the best performance. The paper also describes the modeling procedure, model constraints, and the various evaluated models. The detailed results of the direct comparisons are discussed, and a few concluding remarks on turbulence models are given.

Shih, Tsan-Hsing↗

NASA NTRS1995

Inferring the thermal-infrared hemispheric emission from a sparsely-vegetated surface by directional measurements

The thermal-infrared (longwave) emission from a vegetated terrain is generally anisotropic, i.e., the emission temperature varies with the view direction. If a directional measurement of temperature is considered to be equal to the effective temperature of the hemispheric emission, then the estimate of the latter can be significantly in error. The view-direction (zenith angle theta(sub eq) at which the emission equivalence does hold is determined in our modeling study. In a two-temperature field-of-view (soil and plants), theta(sub eq) falls in a narrow range depending on plant density and canopy architecture. Theta(sub eq) does not depend on soil and (uniform) plant temperatures nor on their ratio, even though the pattern of emission vs. the view direction depends crucially on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimuth) with horizontal facets, theta(sub eq) ranges from 48 to 53 deg depending on the optical density of the vertical elements alone. When plant elements are modeled as small spheres, theta(sub eq) lies between 53 to 57 deg (for the same values of the canopy optical density). Only for horizontal leaves (a truly planophile canopy) is the temperature measured from any direction equal to the temperature of the hemispheric emission. When the emission temperature changes with optical depth within the canopy at a specified rate, theta(sub eq) depends to some extent on that rate. For practically any sparsely vegetated surface, a directional measurement at the zenith angle of 50 deg offers an appropriate evaluation of the hemispheric emission, since the error in the estimate will, at most, only slightly exceed 1% (around 4 W/sq m). Estimates of the hemispheric emission through a nadir measurement, on the other hand, can be in error in some cases by about 10%, i.e., on the order of 40 W/sq m.

Otterman, J.↗

NASA NTRS1995

The underlying magnetic field direction in Ulysses observations of the southern polar heliosphere

Between May 1993 and January 1995, the Ulysses spacecraft has probed the southern polar heliosphere at latitudes greater than 30 deg S, reaching a maximum latitude of 80.2 deg S in September 1994. Using hourly averages of the data obtained by the magnetometer experiment on Ulysses we have studied the underlying direction of the magnetic field threading this region of the heliosphere, away from the influence of the magnetic sector structure which complicates similar analyses in the ecliptic plane. We have constructed histograms of the measured magnetic field direction using the simple Parker spiral model field direction as a reference. We find that throughout this region the meridional angle between the field vector and the Parker model direction has a distribution which is symmetric and has a most probable value consistent with the model. At latitudes below about 60 deg S the azimuthal angle distribution also has a most probable value consistent with the model but this distribution is highly asymmetric with a greater number of observations of field lines less tightly wound than the expected spiral direction. At latitudes greater than 60 deg S the most probable value of the azimuthal angle is found to have become nearly 30 more tightly wound than the expected direction, but due to the asymmetric distribution still with a greater number of observations less tightly wound than expected. We consider possible causes of both the asymmetry in the distributions and the shift in the most probable value at the highest latitudes, one of which may be the presence of large amplitude, long period Alfven waves in the magnetic field originating from the Sun's southern polar coronal hole.

Forsyth, R. J.↗

NASA NTRS1961

Direct Measurements of Interplanetary Dust Particles in the Vicinity of Earth

The direct measurements made by the Explorer VIII satellite provide the first sound basis for analyzing all available direct measurements of the distribution of interplanetary dust particles. The model average distribution curve established by such an analysis departs significantly from that predicted by the (uncertain) extrapolation of results from meteor observations. A consequence of this difference is that the daily accretion of interplanetary particulate matter by the earth is now considered to be mainly dust particles of the direct measurements range of particle size. Almost all the available direct measurements obtained with microphone systems on rockets, satellites, and spacecraft fit directly on the distribution curve defined by Explorer VIII data. The lack of reliable datum points departing significantly from the model average distribution curve means that available direct measurements show no discernible evidence of an appreciable geocentric concentration of interplanetary dust particles.

McCracken, C. W.↗

NASA NTRS1998

Spaceborne Simulations of Two Direct-Detection Doppler Lidar Techniques

Direct-detection (or incoherent) lidar is now a proven technique for measuring winds in the atmosphere. Over the last few years, several types of direct-detection lidar have evolved. These methods rely on Fabry-Perot interferometers(also termed etalons) or other narrow-passband filters to provide the required spectral resolution. One method, now called the edge (EDG) technique, uses a sharply-sloping filter and measures changes in the filter transmission caused by Doppler shifting of the laser wavelength. A variation of the EDG method, called the double-edge (DEDG) technique, uses two filters. The molecular DEDG method was first demonstrated by Chanin et al. for stratospheric measurements and more recently Korb et al. successfully demonstrated the aerosol DEDG through the troposphere. A second method, here termed the multi-channel (MC) technique, measures Doppler shifts by observing angular displacement of a Fabry-Perot fringe in a spatially resolving detector. The EDG technique thus employs the Fabry-Perot to convert the frequency shift into an amplitude signal, while the MC technique uses the Fabry-Perot to resolve the spectral signature which is then fitted to determine the centroid. The focus of this presentation is on the DEDG and MC methods because these are viewed as the current state of the art in direct-detection lidar. Successful ground-based demonstrations of direct-detection wind measurements have resulted in proposals for spaceborne systems. With this new emphasis on spaceborne systems comes the need for accurate prediction of spaceborne direct-detection Doppler lidar performance. Previously, the EDG and MC methods have been compared although only for aerosol Doppler systems. A recent paper by McGill and Spinhirne compares the DEDG and MC methods in a non-system specific manner for both the aerosol and molecular Doppler systems. The purpose of this presentation is to extend the previous work of McGill and Spinhirne to examine the performance of spaceborne profiling systems. Particular emphasis will be placed on the molecular systems, as these are viewed as the strength of direct-detection Doppler lidar.

McGill, Matthew J.↗