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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

In-flight comparisons of boundary-layer and wake measurement probes for incompressible flow

The results are presented of in-flight comparisons of a number of boundary-layer and wake measurement probes suitable for low-speed flight-test investigations. The tested boundary-layer probes included a traversing total-pressure probe and a hot-film probe mounted on an internally-mounted drive mechanism, a curved and a straight boundary-layer rake, and a traversing hot-film probe with an externally-mounted drive mechanism. The wake measuring devices included a traversing, self-aligning probe, a wake rake, and an integrating wake rate. The boundary-layer data are compared with a common reference velocity profile and comments given regarding the accuracy of the static-pressure and total-pressure measurements. Discussions on the various calibration presentations used with hot-wire and hot-film sensors and various aspects of improving the accuracy of hot-film sensor results are given in the appendix of this report.

Mertaugh, L. J., Jr.↗
NASA NTRS1971

Comparison of SLAR images and small-scale, low-sun aerial photographs.

A comparison of side-looking airborne radar (SLAR) images and black and white aerial photos of similar scale and illumination of an area in the Mojave Desert of California shows that aerial photos yield far more information about geology than do SLAR images because of greater resolution, tonal range, and geometric fidelity, and easier use in stereo. Nevertheless, radar can differentiate some materials or surfaces that aerial photos cannot; thus, they should be considered as complementary, rather than competing tools in geologic investigations. The most significant advantage of SLAR, however, is its freedom from the stringent conditions of weather, date, and time that are required by small-scale aerial photos taken with a specified direction and angle of illumination. Indeed, in low latitudes, SLAR is the only way to obtain small-scale images with low illumination from certain directions; moreover, in areas of nearly continuous cloudiness, radar may be the only practical source of small-scale images.

Clark, M. M.↗
NASA NTRS1972

Comparison of characteristics and shock capturing methods with application to the space shuttle vehicle.

A critical appraisal is presented of a method-of-characteristics (MOC) and a shock-capturing technique (SCT) for calculating three-dimensional supersonic flow. Detailed comparisons are made for pointed and blunt-nosed bodies with a thick delta wing at angles of attack from 5 to 15 deg. The results show excellent agreement between the methods and with experiment. It is established that the SCT code yields accurate shock locations and jump conditions. The SCT code typically requires about three times as many mesh points as the MOC code in order to resolve the bow shock, but the total computation time is about the same. On a point for point basis, the SCT code is about four times faster than the MOC code.

Rakich, J. V.↗
NASA NTRS1972

Comparison of characteristic schemes for three-dimensional, steady, isentropic flow.

The accuracy and order of three bicharacteristic schemes based on the numerical method of characteristics for three-dimensional, steady, isentropic, supersonic flow were compared based on numerical calculations. Comparisons were made for source flows and for flows in contoured axisymmetric nozzles. The first two methods were tetrahedral line networks, differing only in the manner in which the cross derivatives were evaluated. The third method was the pentahedral line network proposed by Butler. All three methods were numerically stable and produced efficient and accurate solutions. A discussion of the relative merits and disadvantages of each method is presented.

Cline, M. C.↗
NASA NTRS1972

Chemical composition of sawdust from lunar rock 12013 and comparison of a Java tektite with the rock.

Abundances of 11 major and minor elements and 11 trace elements have been determined by instrumental neutron activation analysis of two Apollo 12013 rock fragments, a sample of rock 12013,17 sawdust, and a Java tektite (J2). Although the abundances of major elements in tektite J2 are similar to those of rock 12013, comparison of the minor and trace elements shows that no fragment of sawdust or rock 12013 that has been analyzed to date is chemically similar to tektite glass. Rock sawdust is representative of ?whole rock' composition only if the amount of contamination from the sawing process is known. After appropriate correction for saw wire contamination, analyses of sawdust yield fairly accurate averaged elemental compositions of complex clastic lunar and other rocks.

Showalter, D. L.↗
NASA NTRS1971

A comparison of orthogonal transformations for digital speech processing.

Discrete forms of the Fourier, Hadamard, and Karhunen-Loeve transforms are examined for their capacity to reduce the bit rate necessary to transmit speech signals. To rate their effectiveness in accomplishing this goal the quantizing error (or noise) resulting for each transformation method at various bit rates is computed and compared with that for conventional companded PCM processing. Based on this comparison, it is found that Karhunen-Loeve provides a reduction in bit rate of 13.5 kbits/s, Fourier 10 kbits/s, and Hadamard 7.5 kbits/s as compared with the bit rate required for companded PCM. These bit-rate reductions are shown to be somewhat independent of the transmission bit rate.

Campanella, S. J.↗
NASA NTRS1971

Tests and comparisons of gravity models.

Optical observations of the GEOS satellites were used to obtain orbital solutions with different sets of geopotential coefficients. The solutions were compared before and after modification to high order terms (necessary because of resonance) and were then analyzed by comparing subsequent observations with predicted trajectories. The most important source of error in orbit determination and prediction for the GEOS satellites is the effect of resonance found in most published sets of geopotential coefficients. Modifications to the sets yield greatly improved orbits in most cases. The results of these comparisons suggest that with the best optical tracking systems and gravity models, satellite position error due to gravity model uncertainty can reach 50-100 m during a heavily observed 5-6 day orbital arc. If resonant coefficients are estimated, the uncertainty is reduced considerably.

Marsh, J. G.↗
NASA NTRS1971

International comparison of radiometers - Techniques and results.

Description of an international comparison of radiometers made on May 10 through 14, 1971, with 24 instruments, each one representing the calibration standard of a different country or a major laboratory. All were pyranometers, viewing the sun and sky, thus bypassing the conventional method of transferring the standard from Angstrom pyrheliometers. This is the first time that pyranometers from all around the globe have been brought together in order to verify the scale on which to report solar radiation data. A highly accurate data collection system measured simultaneously the output of each instrument and that of a reference standard at 20-sec intervals. Over 300,000 radiometric signals were recorded under varying atmospheric conditions, as well as data on turbidity, temperature, and other relevant parameters. Computer programs were developed for overnight analysis of the data and distribution to participants on the following day. Among major findings are that instruments of different types and independent calibration such as those of Eppley, U. S. A., World Radiation Center, Switzerland, and Melbourne, Australia, show better agreement than anticipated, that certain national standards have to be revised by as much as 16% and that the cosine factor is highly dependent on the type of instrument.

Thekaekara, M. P.↗
NASA NTRS1972

Further comparison of theory and experiment for decay of homogeneous turbulence.

An analysis of the decay of homogeneous turbulence from a given initial state is compared with the experiment of Lin and Huang (1970). Comparisons were made for decay of turbulent energy, decay of energy-transfer spectra, decay of three-dimensional turbulent-energy spectra, and decay of higher-order spectral quantities V, R, and S, where these respectively represent functionals of three-, four-, and five-point spectral quantities. Good agreement between theory and experiment is noted.

Deissler, R. G.↗
NASA NTRS1962

Comparison of the Effectiveness of Flares With That of Fins for Stabilizing Low Fineness Ratio Bodies at Mach Numbers From 0.6 to 5.8

An experimental and analytical investigation has been made of the effectiveness of flares and of fins for providing aerodynamic stability of low-fineness-ratio bodies near zero angle of attack. Wind-tunnel tests were performed at Mach numbers from 0.6 to 3.8 to measure the normal force, center of pressure, and drag of bodies consisting of conical noses in combination with cylindrical midsections and finned or flared aftersections. This study included an investigation of the effects of nose bluntness, midbody length, flare angle (from 0° to 20°), and fin leading-edge bluntness. The results showed that for the same plan-form area, flares were more effective than fins, but for the same drag, fins were more effective than flares. Flow separation, which occurred ahead of a flare at certain Mach numbers, resulted in an increase in the stabilizing effectiveness of the flare and a decrease in the drag. However, flow separation led to large undesirable shifts in the center of pressure. Blunt fins were found to be more effective than sharp fins, with the vertical as well as the horizontal fins contributing to the effectiveness. Comparisons of the analytical with the experimental results indicated that the analytical method employed gave good estimates of the stabilizing effectiveness of both fins and flares throughout the supersonic Mach number range.

Leland H Jorgensen↗
NASA NTRS1972

Comparison of neutron spectra measured with three sizes of organic liquid scintillators using differentiation analysis

Proton recoil distributions were obtained by using organic liquid scintillators of different size. The measured distributions are converted to neutron spectra by differentiation analysis for comparison to the unfolded spectra of the largest scintillator. The approximations involved in the differentiation analysis are indicated to have small effects on the precision of neutron spectra measured with the smaller scintillators but introduce significant error for the largest scintillator. In the case of the smallest cylindrical scintillator, nominally 1.2 by 1.3 cm, the efficiency is shown to be insensitive to multiple scattering and to the angular distribution to the incident flux. These characteristics of the smaller scintillator make possible its use to measure scalar flux spectra within media high efficiency is not required.

Shook, D. F.↗
NASA NTRS1972

A Computer Aided Broad Band Impedance Matching Technique Using a Comparison Reflectometer

An improved broadband impedance matching technique was developed. The technique is capable of resolving points in the waveguide which generate reflected energy. A version of the comparison reflectometer was developed and fabricated to determine the mean amplitude of the reflection coefficient excited at points in the guide as a function of distance, and the complex reflection coefficient of a specific discontinuity in the guide as a function of frequency. An impedance matching computer program was developed which is capable of impedance matching the characteristics of each disturbance independent of other reflections in the guide. The characteristics of four standard matching elements were compiled, and their associated curves of reflection coefficient and shunt susceptance as a function of frequency are presented. It is concluded that an economical, fast, and reliable impedance matching technique has been established which can provide broadband impedance matches.

Gordy, R. S.↗
NASA NTRS1973

Preliminary cost and mission value comparisons for planetary probes delivered by advanced propulsion systems

The three advanced propulsion systems analyzed are an advanced chemical system, an improved solid-core nuclear rocket engine with a 25-kilowatt auxiliary powerplant, and a nuclear-electric system. The comparison of these systems is made on the basis of transportation cost divided by the expected value of the data returned to earth. The analysis shows that for the Mercury Orbiter mission and for missions to the outer planets with a high data requirement, the nuclear-electric system emerges as the best system. For the Venus Orbiter mission, the advanced chemical propulsion system is best.

Hrach, F. J.↗
NASA NTRS1972

Space shuttle auxiliary propulsion system design study. Phase B report: Candidate RCS concept comparisons

The competing auxiliary propulsion concepts for the reusable space shuttle vehicle are defined. The concepts are compared on the basis of selection criteria such as weight, reliability, and technology requirements. Propulsion systems using both cryogenic oxygen-hydrogen and earth storable propellants were considered. Three high value oxygen-hydrogen reaction control system concepts were evaluated. The final comparisons demonstrate that all three concepts are viable design approaches. The flexibility and growth potential of the parallel concept are considered to provide an advantage over the series concept.

Orton, G. F.↗
NASA NTRS1973

The luminosity function of quasars and its evolution: A comparison of optically selected quasars and quasars found in radio catalogs

The luminosity function of quasars and its evolution are discussed, based on comparison of available data on optically selected quasars and quasars found in radio catalogs. It is assumed that the red shift of quasars is cosmological and the results are expressed in the framework of the Lambda = 0, Q sub Q = 1 cosmological model. The predictions of various density evolution laws are compared with observations of an optically selected sample of quasars and quasar samples from radio catalogs. The differences between the optical luminosity functions, the red shift distributions and the radio to optical luminosity ratios of optically selected quasars and radio quasars rule out luminosity functions where there is complete absence of correlation between radio and optical luminosities. These differences also imply that Schmidt's (1970) luminosity function, where there exists a statistical correlation between radio and optical luminosities, although may be correct for high red shift objects, disagrees with observation at low red shifts. These differences can be accounted for by postulating existence of two classes (1 and 2) of objects.

Petrosian, V.↗
NASA NTRS1972

A comparison of CMG steering laws for High Energy Astronomy Observatories (HEAOs)

A comparison of six selected control moment gyro steering laws for use on the HEAO spacecraft is reported. Basic equations are developed to project the momentum and torque of four skewed, single gimbal CMGs into vehicle coordinates. In response to the spacecraft attitude error signal, six algorithms are derived for controlling the CMG gimbal movements. HEAO performance data are obtained using each steering law and compared on the basis of such factors as accuracy, complexity, singularities, gyro hang-up and failure adaption. Moreover, each law is simulated with and without a magnetic momentum management system. The performance of any steering law is enhanced by the magnetic system. Without magnetics, the gimbal angles get large and there are significant differences in steering law performances due to cross coupling and nonlinearities. The performance of the pseudo inverse law is recommended for HEAO.

Davis, B. G.↗