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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 595 records · Page 33
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

U.S. commercial space activities - Returning the U.S. to preeminence in space

The current status of NASA's activities related to the commercial development of space is reviewed with particular reference to the emerging new commercial space activities and the post-Challenger policy developments affecting space commerce. The discussion covers the development of U.S. private sector launching capabilities, cooperative agreements with the private sector, the NASA technology utilization program, the technology applications activities of the Office of Commercial Programs, and the activities of the Centers for the Commercial Development of Space program.

Stone, Barbara A.↗
NASA NTRS1987

A generalized method for optimization of active noise controllers in three-dimensional spaces

In this investigation, the formulation of an optimal active noise controller for harmonic, enclosed sound fields is derived using an indirect boundary element method. The optimal active noise controller is defined as the volume velocity secondary source strengths which minimize the sound pressure level at a number of discrete interior locations. In addition, the formulation of an optimal active noise controller which minimizes the free field power radiated from a generalized, distributed noise source is presented. Representative results are provided for each formulation. For the enclosure problem, results are given for global control, local control, and passive wall treatments using a rectangular cavity. For the free field radiation problem, the effects of secondary source location and quantity on active noise controller performance are investigated. The relative merits of each formulation are discussed.

Mollo, C. G.↗
NASA NTRS1987

Far-infrared variability in active galactic nuclei

Pointed IRAS observations of 20 active galaxies were examined in the first detailed search for far-infrared variability in a large sample of active galaxies. Less extensive survey data were also checked for evidence of strong 6-month variability in 45 active galaxies. The far-infrared fluxes of three highly polarized objects ('blazars') appeared to vary by up to a factor of 2 on time scales of a few months. No convincing cases of variability greater than about 15 percent (rms) were found in any of the normal quasars or Seyfert galaxies studied. It appears that blazars tend to be variable at all wavelengths, but most active galaxies may be systematically less variable in the far-infrared than at higher frequencies.

Edelson, R. A.↗
NASA NTRS1989

Recent activities within the Aeroservoelasticity Branch at the NASA Langley Research Center

The objective of research in aeroservoelasticity at the NASA Langley Research Center is to enhance the modeling, analysis, and multidisciplinary design methodologies for obtaining multifunction digital control systems for application to flexible flight vehicles. Recent accomplishments are discussed, and a status report on current activities within the Aeroservoelasticity Branch is presented. In the area of modeling, improvements to the Minimum-State Method of approximating unsteady aerodynamics are shown to provide precise, low-order aeroservoelastic models for design and simulation activities. Analytical methods based on Matched Filter Theory and Random Process Theory to provide efficient and direct predictions of the critical gust profile and the time-correlated gust loads for linear structural design considerations are also discussed. Two research projects leading towards improved design methodology are summarized. The first program is developing an integrated structure/control design capability based on hierarchical problem decomposition, multilevel optimization and analytical sensitivities. The second program provides procedures for obtaining low-order, robust digital control laws for aeroelastic applications. In terms of methodology validation and application the current activities associated with the Active Flexible Wing project are reviewed.

Noll, Thomas E.↗
NASA NTRS1989

Control surface spanwise placement in active flutter suppression systems

All flutter suppression systems require sensors to detect the movement of the lifting surface and to activate a control surface according to a synthesized control law. Most of the work performed to date relates to the development of control laws based on predetermined locations of sensors and control surfaces. These locations of sensors and control surfaces are determined either arbitrarily, or by means of a trial and error procedure. The aerodynamic energy concept indicates that the sensors should be located within the activated strip. Furthermore, the best chordwise location of a sensor activating a T.E. control surface is around the 65 percent chord location. The best chordwise location for a sensor activating a L.E. surface is shown to lie upstream of the wing (around 20 percent upstream of the leading edge), or alternatively, two sensors located along the same chord should be used.

Nissim, E.↗
NASA NTRS1989

Electron Spin Resonance (ESR) detection of active oxygen species and organic phases in Martian soils

The presence of active oxygen species (O(-), O2(-), O3(-)) and other strong oxidants (Fe2O3 and Fe3O4) was invoked in interpretations of the Viking biological experiments and a model was also suggested for Martian surface chemistry. The non-biological interpretations of the biological results gain futher support as no organic compounds were detected in the Viking pyrolysis-gas chromatography mass spectrometer (GCSM) experiments at concentrations as low as 10 ppb. Electron spin resonance (ESR) measures the absorption of microwaves by a paramagnetic and/or ferromagnetic center in the presence of an external field. In many instances, ESR has the advantage of detailed submicroscopic identification of the transient species and/or unstable reaction intermediates in their environments. Since the higly active oxygen species (O(-), O2(-), O3(-), and R-O-O(-)) are all paramagnetic in nature, they can be readily detected in native form by the ESR method. Active oxygen species likely to occur in the Martian surface samples were detected by ESR in UV-irradiated samples containing MgO. A miniaturized ESR spectrometer system can be developed for the Mars Rover Sample Return Mission. The instrument can perform the following in situ Martian samples analyses: detection of active oxygen species; characterization of Martian surface chemistry and photooxidation processes; and searching for organic compounds in the form of free radicals preserved in subsoils, and detection of microfossils with Martian carbonate sediments.

Tsay, Fun-Dow↗
NASA NTRS1987

Star formation in active galaxies and quasars

The observational evidence for a causal or statistical link between star formation and active galactic nuclei are reviewed. The chief difficulty is in quantitatively ascertaining the star formation rate in active galaxies: most of the readily observable manifestations of star formation superficially resemble those of an active nucleus. Careful multi-wavelength spatially-resolved observations demonstrate that many Seyfert galaxies are undergoing star formation. A survey of CO emission from Seyferts (interpreted in conjunction with IRAS data) suggests that type 2 Seyferts have unusually high rates of star formation, but type 1 Seyferts do not. Recent work also suggests that many powerful radio galaxies may be actively forming stars: radio galaxies with strong emission-lines often have blue colors and strong far-infrared emission. Determining the star formation rate in the host galaxies of quasars is especially difficult. Multicolor imaging and long-slit spectroscopy suggests that many of the host galaxies of radio-loud quasars are blue, and a cold interstellar medium has been detected in some quasar hosts.

Heckman, Timothy M.↗
NASA NTRS1988

Structural models for nickel electrode active mass

Raman spectroscopic data allow one to distinguish nickel electrode active mass, alpha and beta phase materials. Discharges active mass is not isostructural with beta-Ni(OH)2. This is contrary to the generally accepted model for the discharged beta phase of active mass. It is concluded that charged active mass displays a disordered and nonstoichiometric, nonclose packed structure of the R3 bar m, NiOOH structure type. Raman spectral data and X ray diffraction data are analyzed and shown to be consistent with this structural model.

Cornilsen, B. C.↗
NASA NTRS1989

Active modes of the Pacific ITCZ

ITCZ intensity across the Pacific for the 1975-1976 season is studied. Analysis of the data reveal: zonal bands of convective development; isolated patches of stronger convection; and several zonal bands which merge as tropical intraseasonal oscillations. The ITCZ intensity at 160 deg W is compared with the intensity averaged all the way across the Pacific. Composite outgoing longwave radiation anomaly maps for 22 days of local active ITCZ at 160 deg W, 26 days of zonally averaged active ITCZ, and 16 days of both local and zonal active ITCZ are presented and examined. It is noted that: systematic, yet different, patterns, movement, and temporal behavior correspond to local and zonally extensive convection; intensifying zonal convection moves southward; local convection occurs at two preferential latitudes; and local convection does not mean there is a generally active ITCZ.

Hayes, P. M.↗
NASA NTRS1990

The application of active controls technology to a generic hypersonic aircraft configuration

Analytical methods are described for the prediction of aerothermoelastic stability of hypersonic aircraft including active control systems. Thermal loads due to aerodynamic heating were applied to the finite element model of the aircraft structure and the thermal effects on flutter were determined. An iterative static aeroelastic trim analysis procedure was developed including thermal effects. And active control technology was assessed for flutter suppression, ride quality improvement, and gust load alleviation to overcome any potential adverse aeroelastic stability or response problems due to aerodynamic heating. A generic hypersonic aircraft configuration was selected which incorporates wing flaps, ailerons, and all moveable fins to be used for active control purposes. The active control system would use onboard sensors in a feedback loop through the aircraft flight control computers to move the surfaces for improved structural dynamic response as the aircraft encounters atmospheric turbulence.

Gilbert, M. G.↗
NASA NTRS1990

An approach to rescheduling activities based on determination of priority and disruptivity

A constraint-based scheduling system called SPIKE is being used to create long term schedules for the Hubble Space Telescope. Feedback for the spacecraft or from other ground support systems may invalidate some scheduling decisions and those activities concerned must be reconsidered. A function rescheduling priority is defined which for a given activity performs a heuristic analysis and produces a relative numerical value which is used to rank all such entities in the order that they should be rescheduled. A function disruptivity is also defined that is used to place a relative numeric value on how much a pre-existing schedule would be changed in order to reschedule an activity. Using these functions, two algorithms (a stochastic neural network approach and an exhaustive search approach) are proposed to find the best place to reschedule an activity. Prototypes were implemented and preliminary testing reveals that the exhaustive technique produces only marginally better results at much greater computational cost.

Sponsler, Jeffrey L.↗
NASA NTRS1990

Elements of active vibration control for rotating machinery

The success or failure of active vibration control is determined by the availability of suitable actuators, modeling of the entire system including all active elements, positioning of the actuators and sensors, and implementation of problem-adapted control concepts. All of these topics are outlined and their special problems are discussed in detail. Special attention is given to efficient modeling of systems, especially for considering the active elements. Finally, design methods for and the application of active vibration control on rotating machinery are demonstrated by several real applications.

Ulbrich, Heinz↗
NASA NTRS1989

IUE observations of the M dwarfs CM Draconis and Rossiter 137 B - Magnetic activity at saturated levels

IUE observations of two active M dwarfs with known rotation rate or age and presumed to be almost totally convective are presented. The first of these stars, CM Draconis (Gl 630.1), is an old Population II binary with its components in tidally induced rapid rotation (P = 1.27 d, dM 4 + dM 4). The other one, Rossiter 137 B, forms with HD 36705 (AB Dor) a visual pair of young active stars. The activity of CM Dra is due to the forced rotation in a close binary, while Rst 137 B is assumed to rotate fast enough to generate its magnetica activity. These results are compared with those for M dwarfs, particularly AU Mic and YZ CMi, that have known rotational periods and measured ultraviolet emission line fluxes. The chromospheric-coronal saturation levels of cool dwarfs between 0.3 less than B-V less than 1.6 is determined. It is found that saturated F stars have stronger chromospheres than saturated M stars, but the opposite is true for the corona. Results of these observations seem to favor a rotation-dependent distributed dynamo generating magnetic flux in totally convective stars.

Vilhu, O.↗
NASA NTRS1989

The evolution of chromospheric activity of cool giant and subgiant stars

IUE spectra for a large sample of cool subgiant stars are examined, and evidence is found that subgiants in the mass range 1.2-1.6 solar masses undergo a sudden decline in UV transition region emission near B - V = 0.6, which corresponds to spectral type G0 IV. The decline in UV emission coincides with a sharp decrease in stellar rotation rates, and it is suggested that this decay in activity and rotation marks a transformation from acoustic heating in the early F stars to magnetic dynamo-driven activity in the cooler stars, resulting in a strong rotational braking action by stellar wind. For more massive giant stars, there is a similar transformation in the nature of chromospheric activity near B - V = 0.7, or spectral type G0 III, from acoustic heating in the F-type giants to a solarlike dynamo mechanism in the cooler giants. No sign of an abrupt drop in activity near spectral type G5 III at the location of Gray's proposed rotational boundary line is seen.

Simon, Theodore↗
NASA NTRS1990

Solar activity variation of ionospheric plasma temperatures

The present status of understanding and modeling of the variation of electron and ion temperatures with solar activity is reviewed. All atmospheric and ionospheric densities and temperatures are positively correlated with solar activity except the electron temperature, which exhibits a rather complex variation pattern during a solar cycle. The ion temperature at low altitudes closely follows the variation patterns of the neutral temperature. The electron temperature at high altitudes increases with increasing solar activity; the increase is larger during day than during night and exhibits a latitudinal variation. At low altitudes during daytime, tha amplitude of the seasonal variation of the electron temperature increases toward the solar maximum. At 400 km during daytime, the summer electron temperatures for Millstone Hill increase slightly toward higher solar activities, whereas the winter temperatures decrease distinctly. For Arecibo, an opposite trend is noticeable.

Bilitza, D.↗
NASA NTRS1990

Seasonal variations in Be-7 activity in the sediments of Cape Lookout Bight, North Carolina

The short-term sediment-accumulation rates in the interior of the Cape Lookout Bight (North Caroline) were determined using data on Be-7 activity distribution in the surface of sediments of the bight. Lack of a significant bioturbation in this lagoon made it possible to interpret variations in depth-integrated activity profiles of Be-7 as short-term accumulation events. The accumulation rates calculated from Be-7 activity profiles indicate that the delivery of particulate matter to the sediments of Cape Lookout Bight is not constant throughout an annual cycle, with the highest monthly accumulation rates being associated with north/northeast storm activity. Inputs were found to be highest during the late winter/early spring season, when the storm frequency is greatest.

Canuel, E. A.↗
NASA NTRS1990

Bridge feedback for active damping augmentation

A method is described for broadband damping augmentation of a structural system in which the active members (with feedback control) were developed such that their mechanical input impedance can be electrically adjusted to maximize the energy dissipation rate in the structural system. The active member consists of sensors, an actuator, and a control scheme. A mechanical/electrical analogy is described to model the passive structures and the active members in terms of their impedance representation. As a result, the problem of maximizing dissipative power is analogous to the problem of impedance matching in the electrical network. Closed-loop performance was demonstrated for single- and multiple-active-member controlled truss structure.

Chen, G.-S.↗
NASA NTRS1990

Numerical evaluation of the performance of active noise control systems

This paper presents a generalized numerical technique for evaluating the optimal performance of active noise controllers. In this technique, the indirect BEM numerical procedures are used to derive the active noise controllers for optimal control of enclosed harmonic sound fields where the strength of the noise sources or the description of the enclosure boundary may not be known. The performance prediction for a single-input single-output system is presented, together with the analysis of the stability and observability of an active noise-control system employing detectors. The numerical procedures presented can be used for the design of both the physical configuration and the electronic components of the optimal active noise controller.

Mollo, C. G.↗