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At least 1,567 records · Page 87
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NASA NTRS1975

Theoretical investigations of plasma processes in the ion bombardment thruster

A physical model for a thruster discharge was developed, consisting of a spatially diverging plasma sustained electrically between a small ring cathode and a larger ring anode in a cylindrical chamber with an axial magnetic field. The associated boundary-value problem for the coupled partial differential equations with mixed boundary conditions, which describe the electric potential and the plasma velocity fields, was solved in closed form. By means of quantum-mechanical perturbation theory, a formula for the number S(E) of atoms sputtered on the average by an ion of energy E was derived from first principles. The boundary-value problem describing the diffusion of the sputtered atoms through the surrounding rarefied electron-ion plasma to the system surfaces of ion propulsion systems was formulated and treated analytically. It is shown that outer boundary-value problems of this type lead to a complex integral equation, which requires numerical resolution.

Wilhelm, H. E.↗
NASA NTRS1974

Skylab altimeter applications and scientific results

Analysis of the Skylab S-193 Altimeter Experiment data has produced many significant results. The purpose of this paper is to present some of these results which illustrate the programmatic altimeter technology gained and show the scientific potential of altimetry for various applications, including correlation with existing topographic features, geoids and physical models. It will be shown that there exists a very strong correlation between subsurface topography in the broad ocean area and the shape of the ocean surface as determined from the altimeter. Modeled scattering waveforms will be correlated with Skylab Altimeter waveforms. The agreement of a variety of existing geoid models with the altimeter data will be shown. In addition the calibration, accuracy and stability of the S-193 altimeter instrumentation will be discussed.

Mcgoogan, J. T.↗
NASA NTRS1974

A search for posteclipse brightening of Io in 1973. II

Four eclipse reappearances of Io were observed with an area-scanning photometer during the 1973 apparition of Jupiter. The results of these observations and of the ones reported in the preceding paper are discussed in the context of recent physical models for posteclipse brightening. An evaluation of the relative merits and deficiencies of all observational techniques which have been used to search for posteclipse brightening of Io leads to the conclusion that the reality of this phenomenon remains very much in doubt.

Franz, O. G.↗
NASA NTRS1974

Recent observations of quasi-stellar objects

Some observational results are discussed which have a bearing on the problem of QSO redshifts. A general description of QSO absorption-line spectra is presented, and the spectra of four objects are reviewed. The hypothesis is advanced that production of absorption lines at many different redshifts is due to line-locking. Two QSOs with redshifts greater than 3 are described, and questions raised by the discovery of a double QSO with two different redshifts are discussed. Efforts to determine associations between QSOs and galaxies are outlined, and some questions are posed concerning physical models, effects of strong gravitational fields, and different types of QSOs.

Burbidge, E. M.↗
NASA NTRS1975

Supernova 1972e in NGC 5253

Absolute energy distributions of the Type I supernova 1972e in NGC 5253 extending to about 700 days after maximum light have been obtained. A physical model of the expanding envelope, based on the identification of the feature at 6550 A with H-alpha is proposed. It consists of a differentially expanding atmosphere, with electron density ranging from 10 billion near maximum light to about 10 million 340 days later, illuminated by a photosphere with temperature in the range from 10,000 K to 7000 K. Under these conditions, the identifications of Ca II 8600-A, H, and K lines, the Na D lines, and the Mg I b lines forming P Cygni lines are quite plausible. More than 200 days after maximum, the spectrum is dominated by four features between 4200 and 5500 A. Three of these four features match the blended emissions from over 100 Fe II forbidden lines. If this identification is correct, the envelope requires about 0.01 solar mass of iron, which corresponds to an Fe/H ratio about 20 times higher than the cosmic abundance. Possible identifications of the fourth feature with the Mg I 4571-A line or permitted lines of Fe II are also discussed.

Kirshner, R. P.↗
NASA NTRS1975

Skylab Fluid Mechanics Simulations: Oscillation, Rotation, Collision and Coalescence of Water Droplets Under Low-Gravity Environment

Skylab 4 crew members performed a series of demonstrations showing the oscillations, rotations, as well as collision coalescence of water droplets which simulate various physical models of fluids under low gravity environment. The results from Skylab demonstrations provide information and illustrate the potential of an orbiting space-oriented research laboratory for the study of more sophisticated fluid mechanic experiments. Experiments and results are discussed.

Otha H Vaughan, Jr↗
NASA NTRS1974

Seasat and floating ice

Data collected by SEASAT would be useful in developing predictive physical models for the drift and deformation of sea ice, for estimating the heat budget of the polar seas, for the optimum routing of shipping through pack ice areas, for the design of both offshore structures and shipping capable of surviving in heavy pack ice, and for the tracking of large icebergs and ice islands. The instrument package for SEASAT-A is particularly useful for studying sea ice in that the Coherent Imaging Radar (CIR), the Scanning Multifrequency Microwave Radiometer (SMMR) and the Compressed Pulse Radar Altimeter (CPRA) are not limited by the presence of clouds.

Weeks, W. F.↗
NASA NTRS1976

Computation of turbulent boundary layers on curved surfaces, 1 June 1975 - 31 January 1976

An accurate method was developed for predicting effects of streamline curvature and coordinate system rotation on turbulent boundary layers. A new two-equation model of turbulence was developed which serves as the basis of the study. In developing the new model, physical reasoning is combined with singular perturbation methods to develop a rational, physically-based set of equations which are, on the one hand, as accurate as mixing-length theory for equilibrium boundary layers and, on the other hand, suitable for computing effects of curvature and rotation. The equations are solved numerically for several boundary layer flows over plane and curved surfaces. For incompressible boundary layers, results of the computations are generally within 10% of corresponding experimental data. Somewhat larger discrepancies are noted for compressible applications.

Wilcox, D. C.↗
NASA NTRS1976

Some results of cislunar plasma research

The main results of plasma cislunar investigations, carried out during Luna-19 and Luna-22 spacecraft flights by means of dual frequency dispersion interferrometry, are briefly outlined. It is shown that a thin layer of plasma, with a height of several tens of kilometers and a maximum concentration of the order 1,000 electrons/cu cm exists above the solar illuminated lunar surface. A physical model of the formation and existence of such a plasma in cislunar space is proposed, taking into account the influence of local magnetic areas on the moon.

Vyshlov, A. S.↗
NASA NTRS1975

Quantization and symmetry in periodic coverage patterns with applications to earth observation

Analytical approaches based on an idealized physical model and concepts from number theory show that in periodic coverage patterns, uniquely defined by their revolution numbers R (orbital) and N (rotational), the subnodal points are earth-fixed, and they divide the equator into R equal segments of length s. The ascending subsatellite trace crosses each point once (only) each period. The descending subnodal points coincide with the ascending points if the integers N and R have like parity, and bisect the intervals between them if opposite. The interval between consecutive unidirectional crossings is Ns. Symmetries extend the equatorial results to all parallels of latitude. Complete periodic patterns of traces exhibit an overall symmetry, with trace intersections confined to discrete coordinate values which are quantized in longitude (basic s-unit) and symmetric in latitude.

King, J. C.↗
NASA NTRS1975

On the sputtering of binary compounds

A simple physical model is presented to describe some aspects of the sputtering of compound targets. In particular, expressions are developed for the partial sputtering yields for binary systems in terms of the elemental sputtering rates, the stoichiometric concentrations and surface binding energy. The partial yields depend non-linearly on the bulk target concentrations. Comparison of the theoretical predictions with the data on sputtering of PtSi, NiSi and Cu3Au indicates that the general features are well described.

Haff, P. K.↗
NASA NTRS1975

Quantization and symmetry in periodic coverage patterns with applications to earth observation

The general orbit-coverage problem in a simplified physical model is investigated by application of numerical approaches derived from basic number theory. A system of basic and general properties is defined by which idealized periodic coverage patterns may be characterized, classified, and delineated. The principal common features of these coverage patterns are their longitudinal quantization, determined by the revolution number R, and their overall symmetry.

King, J. C.↗
NASA NTRS1975

Statistical properties of the interplanetary microscale fluctuations

Results are reported for a statistical study of short-period fluctuations in the interplanetary plasma velocity and magnetic field. The data base used consists of measurements of the interplanetary plasma and magnetic field by Pioneer 6 with a time resolution of 72 sec and by Mariner 5 with a resolution of 5 min. The analysis is conducted to characterize the parent population from which all individual microscale events on these time scales are drawn. The microscale changes are grouped according to their energy densities relative to the energy density of the background magnetic field, and it is found that each grouping exhibits certain statistical properties within the limits of observational uncertainty. It is noted that these statistical properties are purely observational and independent of any physical model which may be used to interpret them. The results are discussed in the context of MHD discontinuity theory for a thermally anisotropic plasma.

Belcher, J. W.↗
NASA NTRS1976

A study on the forced vibration of a Timoshenko beam

By using Galerkin's variational method an approximate solution is developed for a system of two differential equations with linear partial derivatives of the second order. This system of differential equations corresponds to the physical model, known in the literature as the Timoshenko Beam. The results obtained are applied to two particular cases representing respectively: the case of a beam with a rectangular section, with a constant height and a basis with a linear variation; and the case of a beam with a constant basis and a height with cubic variation.

Zainea, B.↗
NASA NTRS1977

Effects of unsteady aerodynamics on rotor aeroelastic stability

The effects of unsteady aerodynamics on the stability characteristics of helicopter rotor blades were studied. A simple physical model of each blade was used together with Theodorsen, Loewy, and quasi-steady aerodynamics to derive the equations of motion. The stability analysis comparing the effects of using each of the three theories revealed some significant differences between the Loewy and Theodorsen results. These included increases and decreases in lead-lag damping, localized around integer lead-lag frequencies. It was also shown that the standard method of multi-blade coordinates must be modified for use in conjunction with Loewy aerodynamics.

Kunz, D. L.↗
NASA NTRS1976

On the sputtering of binary compounds

A simple physical model is presented to describe some aspects of the sputtering of compound targets. In particular, expressions are developed for the partial sputtering yields for binary systems in terms of the elemental sputtering rates, the stoichiometric concentrations, and surface binding energy. The partial yields depend nonlinearly on the bulk target concentrations. Comparison of the theoretical predictions with the data on sputtering of PtSi, NiSi, and Cu3Au indicates that the general features are well described.

Haff, P. K.↗
NASA NTRS1976

Concept and analytical basis for revistas - A fast, flexible computer/graphic system for generating periodic satellite coverage patterns

The generation of satellite coverage patterns is facilitated by three basic strategies: use of a simplified physical model, permitting rapid closed-form calculation; separation of earth rotation and nodal precession from initial geometric analyses; and use of symmetries to construct traces of indefinite length by repetitive transposition of basic one-quadrant elements. The complete coverage patterns generated consist of a basic nadir trace plus a number of associated off-nadir traces, one for each sensor swath edge to be delineated. Each trace is generated by transposing one or two of the basic quadrant elements into a circle on a nonrotating earth model sphere, after which the circle is expanded into the actual 'helical' pattern by adding rotational displacements to the longitude coordinates. The procedure adapts to the important periodic coverage cases by direct insertion of the characteristic integers N and R (days and orbital revolutions, respectively, per coverage period).

King, J. C.↗
NASA NTRS1977

A scanning multichannel microwave radiometer for Nimbus-G and SeaSat-A

A scanning multichannel microwave radiometer (SMMR) has been designed for the Nimbus-G spacecraft and incorporated also into the SeaSat-A payload for the primary purpose of determining sea surface temperatures and wind stress on a nearly all-weather basis. Observations of microwave polarization components will be made at wavelengths of 0.8, 1.4, 1.7, 2.8, and 4.6 cm over a swath 822 km wide below the Nimbus-G and 595 km wide below the SeaSat-A spacecraft. The smallest spatial resolution cell is about 20 km at a wavelength of 0.8 cm, and proportionately larger at the other wavelengths. Using algorithms based on a combination of experimental data and physical models for converting the observed brightness temperatures, the indicated accuracy of the results (excluding conditions of significant rainfall) are within 1 K for sea surface temperature and 2 m/s for surface wind speeds, over a range from 0-50 m/s.

Gloersen, P.↗