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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Negative pressures and melting point depression in oxide-coated liquid metal droplets

Negative pressures and melting point depression in oxide-coated liquid metal droplets are studied. The calculation presented show the existence of large negative pressures when the oxide coating is thick enough. The change in the melting point caused by these negative pressures should be considered in studies of homogeneous crystal nucleation. Furthermore, since the negative pressure raises the entropy of the melt, it increases the entropy loss at the crystal-melt interface; the resulting increase of the surface tension could have a large effect on the homogeneous nucleation frequency.

Spaepen, F.↗
NASA NTRS1980

X-ray bright points and the solar cycle

The shape of the Sun's activity spectrum is such that the majority of all magnetic flux emerging at the surface comes in the form of bright points, i.e., regions living less than two days. Examination of soft X-ray data obtained from 1970 to 1978 shows that the number of bright points appears to be anticorrelated with traditional activity indices, such as sunspot number; the anticorrelation persists after corrections are made for obscuration by active regions. Comparison of X-ray data with KPNO magnetograms shows that to within a factor of two, the average total amount of magnetic flux emerging over the full Sun is constant through the entire period of observation. The Solar cycle therefore appears to be more an oscillation in the wavenumber distribution of emerging flux than of the total quantity of magnetic flux produced.

Golub, L.↗
NASA NTRS1980

Performance characteristics of point-focusing distributed-receiver solar Brayton systems

Due to variations in solar insolation, it may be necessary or desirable to operate solar energy systems continuously in off-design conditions. Design of solar energy systems should maximize system efficiency at the design point and throughout the range of operational solar insolation. The solar power system considered in this study consists of a point-focusing concentrator, a cavity receiver, an open cycle gas turbine engine, and a heat regenerator. A computer simulation model was developed to predict off-design system performance. Results showing system efficiency and associated subsystems interface requirements are presented for various turbine inlet temperatures and engine speeds.

El Gabalawi, N.↗
NASA NTRS1980

X-ray bright points and the solar cycle

Soft X-ray filtergrams show the presence of large numbers of small closed regions of coronal emission. These features, called X-ray bright points, correspond to small short-lived regions of emerging magnetic flux. X-ray data for 1970 to 1978 shows that the number of X-ray bright points appears to be anticorrelated with traditional activity indices such as sunspot number. A comparison of X-ray data with KPNO magnetograms shows that to within a factor of 2 the average total amount of magnetic flux emerging over the full sun is constant through the entire period of observation. The solar cycle therefore appears to be an oscillation in the wavenumber distribution of emerging flux than of the total quantity of the magnetic flux produced.

Golub, L.↗
NASA NTRS1980

A precision pointing system for Shuttle experiment payloads

This paper describes the Annular Suspension and Pointing System (ASPS) being developed to support NASA Shuttle payloads in the 1980s. The ASPS employs a unique magnetic suspension system to isolate Shuttle payloads from Orbiter disturbances and provide vernier control of the payload's attitude, thereby allowing extremely accurate and stable pointing. A description of the system design, configurations, and performance goals is given. Component and system development testing of the full-size ASPS Engineering Development Model is described, and hardware photographs and test configurations are presented.

Van Riper, R.↗
NASA NTRS1980

A new Saturnian satellite near Dione's L4 point

A new satellite of Saturn, discovered by Laques and Lecacheux, has been observed on a number of occasions and its orbit determined. It has a period of 2.73614 plus or minus 0.00006 days and occupies the leading Lagrangian point (L4) of Dione. A second object has been observed on one night which may be near the L5 point of Dione.

Reitsema, H. J.↗
NASA NTRS1980

Motion about the stable libration points in the linearized, restricted three-body problem

The motion of a point particle in the neighborhood of a triangular libration point (L sub 4 or L sub 5) in the linearized, restricted problem of three bodies in the plane is described. The derivation of the equations of motion is standard. From these equations, three invariants of the motion are obtained; the Jacobi integral is expressed linearly in terms of two of these. The trajectories for varied initial conditions are drawn, and a complete geometric description of the particle motion is given in elementary terms. Each trajectory has an exterior boundary curve; its equation is found. An approximation to this boundary curve was known; the two curves are compared graphically. For certain initial conditions, there is an interior region from which the trajectory is excluded; the equation of the boundary of this region is given.

Mittleman, D.↗
NASA NTRS1981

CCD photometry of 1218+304 1219+28 and 1727+50: Point sources, associated nebulosity and broadband spectra

Visual and far red surface photometry were obtained of two X-ray emitting BL Lacertae objects, 1218+304 (2A1219+305) and 1727+50 (Izw 187), as well as the highly variable object 1219+28 (ON 231, W Com). The intensity distribution for 1727+50 can be modeled using a central point source plus a de Vaucouleurs intensity law for an underlying galaxy. The broad band spectral energy distribution so derived is consistent with what is expected for an elliptical galaxy. The spectral index of the point source is alpha = 0.97. Additional VLBI and X-ray data are also reported for 1727+50. There is nebulosity associated with the recently discovered object 1218+304. No nebulosity is found associated with 1219+28. A comparison of the results with observations at X-ray and radio frequencies suggests that all the emission from 1727+50 and 1218+304 can be interpreted as due solely to direct synchrotron emission. If this is the case, the data further imply the existence of relativistic motion effects and continuous particle injection.

Weistrop, D.↗
NASA NTRS1981

On the use of off-nadir pointing for increased temporal resolution of Earth observing satellite systems

The change in radiance expected at a satellite in a LANDSAT type orbit by pointing the sensor across track was examined with simulated data. The simulation incorporated the bidirectional reflectance distribution function of a spherical geometry grass canopy and scattering under clear, light haze and heavy haze atmospheric conditions. The results indicate that if the sensor pointed up to + or - 37 deg off nadir (up to three tracks east and west) through a clear atmosphere, between 50% and 75% of the orbital path between 60 deg N and 60 deg S would have two or more off-nadir views within + or - 5% of nadir. Although increased atmospheric scattering causes large variations in across track radiance, it appears possible, using various combinations of views depending on atmospheric conditions, that at least two views could be obtained with radiances within + or - 5% of each other over atmospheric visibilities from 23 km to 4 km.

Schnetzler, C. C.↗
NASA NTRS1980

Calculation of power spectrums from digital time series with missing data points

Two algorithms are developed for calculating power spectrums from the autocorrelation function when there are missing data points in the time series. Both methods use an average sampling interval to compute lagged products. One method, the correlation function power spectrum, takes the discrete Fourier transform of the lagged products directly to obtain the spectrum, while the other, the modified Blackman-Tukey power spectrum, takes the Fourier transform of the mean lagged products. Both techniques require fewer calculations than other procedures since only 50% to 80% of the maximum lags need be calculated. The algorithms are compared with the Fourier transform power spectrum and two least squares procedures (all for an arbitrary data spacing). Examples are given showing recovery of frequency components from simulated periodic data where portions of the time series are missing and random noise has been added to both the time points and to values of the function. In addition the methods are compared using real data. All procedures performed equally well in detecting periodicities in the data.

Murray, C. W., Jr.↗
NASA NTRS1980

Pointing and control for planetary spacecraft - The first twenty years

The evolution of guidance and control systems for United States planetary and unmanned lunar spacecraft over the last 20 years is traced. The characteristics of the guidance and control systems used on spacecraft from the Range lunar impactor to the planned Galileo Jupiter orbiter and entry probe are surveyed, with attention given to the uses of three-axis stabilized, spin-stabilized and dual-spin designs. System performance trends that have evolved to meet the increasing science and mission requirements of the spacecraft are considered in the areas of attitude references, control consumables, dynamics and system modeling, thrust vector control, optical navigation, manuever turns, maneuver velocity control, instrument pointing, and antenna pointing. Hardware trends in optical sensors, inertial sensors, processing electronics, electromechanical devices, and system testing and reliability are also reviewed. The achievements represented by these advances are emphasized, and it is predicted that future developments will be in the areas of increased control system autonomy and performance requirements.

Pace, G. D.↗
NASA NTRS1980

Planetary spacecraft pointing and control - The next 20 years

Pointing and control systems recommended for NASA planetary exploration missions of the next twenty years are discussed. The functional and operational requirements and design constraints imposed on the driving control system by the mission and science goals of the spacecraft are examined. Criteria for the selection of a pointing and control system type (spin, dual-spin, three-axis active or momentum bias) are outlined, including payload functional characteristics, orbital characteristics, vehicle configuration and mission duration, and candidate system types proposed for preliminary survey, detailed remote observation, deep space close scrutiny, near-sun close scrutiny and sample return missions are indicated. Trends in control system design are discussed, and key enabling technology areas that will require additional or new development are considered, with particular emphasis on laser and fiber optics gyros, CCD arrays and DMA signal/power transmission, bearings and motor technologies, directed at three-axis active and dual-spin implementations.

Mcglinchey, L. F.↗
NASA NTRS1981

Deformation of a line-element in the phase space at the triangular libration point

The flow in the projection of the phase space into the configuration space is presented in the neighborhood of a neutrally (or critically) stable equilibrium point in the restricted problem of three bodies. The projection is a line-element every point of which has zero initial velocity. After the elapse of various times the mapping (the rotations and elongations) of the line-element is described showing chaotic behavior.

Szebehely, V.↗
NASA NTRS1981

H2O frost point detection on Mars

The Viking Mars landers contain meteorological instrumentation to measure wind, temperature, and pressure but not atmospheric water content. The landings occurred during local summer, and it was observed that the nocturnal temperature decrease at sensor height (1.6 m) did not exhibit a uniform behavior at either site. It was expected that the rate of decrease would gradually slow, leveling off near sunrise. Instead, a leveling occurred several hours earlier. Temperature subsequently began a more rapid decrease which slowed by sunrise. This suggested that the temperature sensors may be detecting the frost point of water vapor. Analysis of alternative hypotheses demonstrates that none of these are viable candidates. The frost point interpretation is consistent with other lander and orbiter observations, with terrestrial experience, and with modeling of Mars' atmospheric behavior. It thus appears that the meteorology experiment can help provide a basis toward understanding the distribution and dynamics of Martian water vapor.

Ryan, J. A.↗
NASA NTRS1981

Optimum shape of a Kirkpatrick-Baez X-ray reflector supported at discrete points for on-axis performance

A method is proposed for optimizing the on-axis resolution of a Kirkpatrick-Baez reflecting element. The proposed procedure provides a way to determine the location of each of the 11,200 support points (175 different points plus repeats) by using a computer-assisted structural-optical software package. This automated procedure will make it possible to determine the optical characteristics of an entire telescope module consisting of seventy mirrors.

Cohen, L. M.↗
NASA NTRS1981

Sound diffraction around screens and wedges for arbitrary point source locations

A development is presented for the Green's function for a point source in the vicinity of a rigid wedge. The diffraction contributions to the Green's function for arbitrary source and listener location is expressed in a form which can be readily evaluated using the Laguerre technique for numerical integration. The present approach offers the advantages of efficient numerical evaluation and of relatively straightforward reduction to well-known analytical approximations in limiting cases. Comparisons with previously obtained experimental and numerical results obtained by Ambaud and Bergassoli (1972) are presented. The comparison with the experimental results is excellent; the advantages of the present numerical technique, vis a vis that of Ambaud and Bergassoli, are pointed out.

Hadden, W. J., Jr.↗
NASA NTRS1981

Experiment pointing with magnetic suspension

A need has been identified for a payload auxilliary pointing system onboard the Space Shuttle which provides sub-arcsecond stability in the Shuttle disturbance environment. This paper describes such a pointing system, presently being developed by Sperry Flight Systems for the NASA Langley Research Center. At the core of the design is a non-contacting magnetic suspension which provides a high degree of isolation between payload and carrier. Design concepts and control laws will be discussed. Also, test results from full-scale protoflight hardware and planned system refinements will be presented.

Hamilton, B. J.↗
NASA NTRS1981

On a fourth order accurate implicit finite difference scheme for hyperbolic conservation laws. II - Five-point schemes

This paper presents a family of two-level five-point implicit schemes for the solution of one-dimensional systems of hyperbolic conservation laws, which generalized the Crank-Nicholson scheme to fourth order accuracy (4-4) in both time and space. These 4-4 schemes are nondissipative and unconditionally stable. Special attention is given to the system of linear equations associated with these 4-4 implicit schemes. The regularity of this system is analyzed and efficiency of solution-algorithms is examined. A two-datum representation of these 4-4 implicit schemes brings about a compactification of the stencil to three mesh points at each time-level. This compact two-datum representation is particularly useful in deriving boundary treatments. Numerical results are presented to illustrate some properties of the proposed scheme.

Harten, A.↗