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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Measurements of transition probabilities for spin-changing lines of atomic ions used in diagnostics of astrophysical plasmas

The intensities of ultraviolet, spin-changing, 'intersystem' lines of low-Z atomic ions are frequently used in determinations of electron densities and temperatures in astrophysical plasmas as well as in measurements of element abundances in the interstellar gas. The transition probabilities (A-values) of these lines, which are about five orders of magnitude weaker than allowed lines, have not been measured heretofore and various calculations produce A-values for these lines that differ by as much as 50 percent A radio-frequency ion trap has been used for the first measurements of transition probabilities for intersystem lines seen in astronomical spectra. The measurement procedure is discussed and results for Si III, O III, N II, and C III are reviewed and compared to calculated values. Discrepancies exist; these indicate that some of the calculated A-values may be less reliable than has been beleived and that revisions to the electron densities determined for some astrophysical plasmas may be required.

Smith, P. L.↗
NASA NTRS1985

KPD 0005 + 5106 - A post-PG 1159 type object?

In a survey for galactic plane UV-excess objects, a very hot (T about 100,000 K) helium-rich white dwarf is discovered. The object is spectroscopically similar to the pulsating PG 1159 class of stars, with narrow emission lines of He II, probably C IV, and possibly C III/N III, although there is no trace of carbon absorptions. Not all features have positive identifications. This object appears to be an old PG 1159 star, having ceased pulsations during evolution toward a more typical DO white dwarf.

Downes, R. A.↗
NASA NTRS1987

A comparison of active vibration control techniques - Output feedback vs optimal control

The paper presents analytical and experimental comparison of two control laws for a laboratory structure designed to simulate large space structures. The first control law is the standard linear quadratic law, which is optimal but requires model reduction for practical implementation. The second control law is a new simple direct feedback control law designed to minimize control forces while guaranteeing stability. The optimal control law was found to be only slightly better than the direct feedback law even in terms of the quadratic performance index. Moreover, the optimal control law provided almost no margin of stability for the unmodeled modes while the direct feedback law provided significant stability margins to all modes. The above results were verified experimentally using a digital implementation of the control laws. Excellent agreement between the analytical prediction and experimental measurements was observed.

Martinovic, Zoran N.↗
NASA NTRS1987

An analytical and experimental investigation of output feedback vs. linear quadratic regulator

This paper presents analytical and experimental comparison of three control laws for a laboratory structure designed to simulate large space structures. The first control law is the standard time invariant linear quadratic regulator with state estimation, which requires model reduction for practical implementation. This model reduction is associated with the so-called spillover instability. Two new simple direct output feedback control laws guaranteeing stability are proposed. One minimizes the maximum control force, and the other minimizes the same quadratic performance index as the linear quadratic regulator. The three control laws are found to give comparable performance for the modes retained in the reduced model. However, the standard linear quadratic regulator with state estimation provides almost no margin of stability for the unmodeled modes, while the simpler direct feedback laws provide significant stability margins to all modes. The analytical results were verified experimentally using a digital implementation of the control laws. Good agreement between the analytical predictions and experimental measurements was observed.

Martinovic, Zoran N.↗
NASA NTRS1987

The rapidly oscillating Ap stars as a test of stellar chromospheric heating mechanisms

The discovery of a new class of magnetic stars, the rapidly pulsating Ap stars, provides an unprecedented tool for the study of Alfvenic heating in the outer atmospheres of main sequence stars. In this note, it is noted that detection of rotationally-modulated chromospheric emission line fluxes, especially the Ca II, Mg II, and C IV lines, can provide a novel method for investigating the role of Alfvenic heating in stellar atmospheres.

Shore, S. N.↗
NASA NTRS1987

Driving mechanism of unsteady separation shock motion in hypersonic interactive flow

Wall pressure fluctuations were measured under the steady separation shock waves in Mach 5 turbulent interactions induced by unswept circular cylinders on a flat plate. The wall temperature was adiabatic. A conditional sampling algorithm was developed to examine the statistics of the shock wave motion. The same algorithm was used to examine data taken in earlier studies in the Princeton University Mach 3 blowdown tunnel. In these earlier studies, hemicylindrically blunted fins of different leading-edge diameters were tested in boundary layers which developed on the tunnel floor and on a flat plate. A description of the algorithm, the reasons why it was developed and the sensitivity of the results to the threshold settings, are discussed. The results from the algorithm, together with cross correlations and power spectral density estimates suggests that the shock motion is driven by the low-frequency unsteadiness of the downstream separated, vortical flow.

Dolling, D. S.↗
NASA NTRS1989

Late emission from supernovae - A window on stellar nucleosynthesis

Monte Carlo techniques are used to study the energy deposition of gamma rays from the radioactive decay of Co-56 in Type Ib and Type II SN. It is found that above an electron fraction of about 0.1, direct excitations are negligible, and the line emission arises from thermal processes. The results indicate that the emission is dominated by neutral and singly ionized lines, including forbiden O I, forbidden Ca II, forbidden C I, and forbidden Si I. The late spectrum is shown to be useful as a diagnostic of the nucleosynthetic structure of the ejecta. The present model has been applied to observations of the Type Ib SN 1985F, and it is suggested that Type Ib SN come from stars of greater than 25 solar masses.

Fransson, Claes↗
NASA NTRS1989

LQG and direct rate feedback control with model reduction on a flexible laboratory grid structure

This paper presents experimental and theoretical comparisons of three control laws applied to a complex laboratory structure. A reduced finite element model was generated for designing the control systems and then corrected based on measured mode shapes and frequencies. A standard time-invariant linear quadratic regulator with state estimation was investigated first. Two simple direct rate feedback control laws both guaranteeing stability were also designed using the reduced model. One minimizes the maximum control force and the other minimizes the same quadratic performance index as the linear quadratic regulator. The three control laws have comparable performance indices with the direct rate feedback designs having better spillover properties. Experimental results for all designs were obtained with digital implementation. It was shown that the performance of the control system designed on the basis of the corrected finite element model agreed better with experimental results than the performance of the control system designed on the basis of the uncorrected model.

Schamel, G. C., II↗
NASA NTRS1990

The Vehicle Control Systems Branch at the Marshall Space Flight Center

This paper outlines the responsibility of the Vehicle Control Systems Branch at the Marshall Space Flight Center (MSFC) to analyze, evaluate, define, design, verify, and specify requirements for advanced launch vehicles and related space projects, and to conduct research in advanced flight control concepts. Attention is given to branch responsibilities which include Shuttle-C, Shuttle-C Block II, Shuttle-Z, lunar cargo launch vehicles, Mars cargo launch vehicles, orbital maneuvering vehicle, automatic docking, tethered satellite, aeroassisted flight experiment, and solid rocket booster parachute recovery system design.

Barret, Chris↗
NASA NTRS1991

Active vibration control with model correction on a flexible laboratory grid structure

This paper presents experimental and computational comparisons of three active damping control laws applied to a complex laboratory structure. Two reduced structural models were used with one model being corrected on the basis of measured mode shapes and frequencies. Three control laws were investigated, a time-invariant linear quadratic regulator with state estimation and two direct rate feedback control laws. Experimental results for all designs were obtained with digital implementation. It was found that model correction improved the agreement between analytical and experimental results. The best agreement was obtained with the simplest direct rate feedback control.

Schamel, George C., II↗
NASA NTRS1992

Variability of emission lines in the planetary nebulae IC 4997 and NGC 6572

New IUE high-dispersion data and recent ground-based spectra of the planetary nebulae IC 4997 and NGC 6572 are compared with equivalent IUE observations taken in 1980 and optical data taken in 1974-1990. Significant changes are detected in many emission-line fluxes in IC 4997, and primarily in He II and C IV for NGC 6572. These changes are interpreted as due to recent temperature increases in the central stars of the planetaries which are known to belong to a small set which have shown variability in the past.

Feibelman, Walter A.↗
NASA NTRS1993

CP Puppis - No ordinary old nova

We present spectrophotometric data for the old nova CP Puppis (Nova Puppis 1942) that demonstrates the simultaneous existence of two distinct periods for the system. We find that the photometric period of 0.06834(7) day is 11 percent longer than the spectroscopic period. We also present Doppler tomographic images of CP Pup that indicate the existence of an accretion disk in the system. Finally, we interpret the observational evidence in terms of a DQ Herculis model and an SU Ursae Majoris model for CP Pup. While the evidence mildly supports both interpretations, it is insufficient for us to conclusively classify CP Pup as either a DQ Her system or an SU UMa system.

White, James C., II↗