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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS2008

Microshutter Arrays for NIRSpec

A primary science goal of the 6.5 m James Webb Space Telescope is to characterize the epoch of initial galaxy formation, observing the early universe as the first stars and galaxies formed. This program will be accomplished using a combination of multi-band imaging and spectroscopic measurements of large numbers of faint galaxies. Since these objects are in general sparse on the sky, a multi object spectrometer is required to enable this critical scientific investigation. We have developed a microshutter array which is to be used as a high contrast, remotely programmable field selector for the Near Infrared Spectrometer (NIRSpec) on JWST. This device allows slits to be opened at the locations of selected galaxies in the field of view, and blocks sources and background light from the rest of the field. The first flight design devices have been built and were tested. These first generation of devices were used to demonstrate performance and flight readiness. Devices of the same design will be delivered to ESA for installation into the NIRSpec instrument.

Silverberg, Robert↗
NASA NTRSDate not supplied

Alternating Direct Current (ADC) - A New Form of Efficient Energy Transmission for Renewable Sources

The prevailing means of electricity transmission through Direct Current (DC) and Alternating Current (AC) have limitations with the ongoing transition to renewable energy sources. In this paper, we discuss a new form of energy transmission, known as alternating direct current (ADC) that would provide significant savings in energy consumption and simultaneously allow efficient utilization of generated energy from renewable sources. This has become possible due to substantial advances in electronics and optoelectronics technologies since the days of Nicholas Tesla and Thomas Edison. Recent efforts of implementing the proposed ADC technology in relevant environments have demonstrated significant energy savings and opened a new paradigm that has the potential to provide an efficient and cost-effective pathway for transition to renewable energy sources even with the existing grid infrastructure. The DC power utilization initiated by Thomas Edison can be stored in battery arrangement but suffers from line loss and hence is not efficient for long-distance transmission. However, AC power developed by Nicholas Tesla cannot be stored efficiently but can be transmitted over long distances due to significantly lower line losses. Currently, AC power is the global foundational energy transmission methodology used because of events that took place over 135 years ago involving Edison, Tesla, and Westinghouse, and since AC power can transmit long distances from power plants to end users efficiently and economically. Approximately 50 years ago, the introduction of semiconductors in commercialized electronic devices and products changed the way people consume electricity. Also, over the years, the expanding adaptation of nonlinear loads has adversely affected the grid stability and hence the utility infrastructures worldwide.

AC↗
NASA NTRS1994

Response of a grounded dielectric slab to an impulse line source using leaky modes

This paper describes how expansions in leaky (or improper) modes may be used to represent the continuous spectrum in an open radiating waveguide. The technique requires a thorough knowledge of the life history of the improper modes as they migrate from improper to proper Riemann surfaces. The method is illustrated by finding the electric field resulting from an impulsively forced current located in the free space above a grounded dielectric slab.

DIELECTRIC SLABS↗
NASA NTRS2008

Fast Electromechanical Switches Based on Carbon Nanotubes

Electrostatically actuated nanoelectromechanical switches based on carbon nanotubes have been fabricated and tested in a continuing effort to develop high-speed switches for a variety of stationary and portable electronic equipment. As explained below, these devices offer advantages over electrostatically actuated microelectromechanical switches, which, heretofore, have represented the state of the art of rapid, highly miniaturized electromechanical switches. Potential applications for these devices include computer memories, cellular telephones, communication networks, scientific instrumentation, and general radiation-hard electronic equipment. A representative device of the present type includes a single-wall carbon nanotube suspended over a trench about 130 nm wide and 20 nm deep in an electrically insulating material. The ends of the carbon nanotube are connected to metal electrodes, denoted the source and drain electrodes. At bottom of the trench is another metal electrode, denoted the pull electrode (see figure). In the off or open switch state, no voltage is applied, and the nanotube remains out of contact with the pull electrode. When a sufficiently large electric potential (switching potential) is applied between the pull electrode and either or both of the source and drain electrodes, the resulting electrostatic attraction bends and stretches the nanotube into contact with the pull electrode, thereby putting the switch into the "on" or "closed" state, in which substantial current (typically as much as hundreds of nanoamperes) is conducted. Devices of this type for use in initial experiments were fabricated on a thermally oxidized Si wafer, onto which Nb was sputter-deposited for use as the pull-electrode layer. Nb was chosen because its refractory nature would enable it to withstand the chemical and thermal conditions to be subsequently imposed for growing carbon nanotubes. A 200- nm-thick layer of SiO2 was formed on top of the Nb layer by plasma-enhanced chemical vapor deposition. In the device regions, the SiO2 layer was patterned to thin it to the 20-nm trench depth. The trenches were then patterned by electron- beam lithography and formed by reactive- ion etching of the pattern through the 20-nm-thick SiO2 to the Nb layer.

Kaul, Anupama↗
NASA NTRS2008

Simulating the Gradually Deteriorating Performance of an RTG

Degra (now in version 3) is a computer program that simulates the performance of a radioisotope thermoelectric generator (RTG) over its lifetime. Degra is provided with a graphical user interface that is used to edit input parameters that describe the initial state of the RTG and the time-varying loads and environment to which it will be exposed. Performance is computed by modeling the flows of heat from the radioactive source and through the thermocouples, also allowing for losses, to determine the temperature drop across the thermocouples. This temperature drop is used to determine the open-circuit voltage, electrical resistance, and thermal conductance of the thermocouples. Output power can then be computed by relating the open-circuit voltage and the electrical resistance of the thermocouples to a specified time-varying load voltage. Degra accounts for the gradual deterioration of performance attributable primarily to decay of the radioactive source and secondarily to gradual deterioration of the thermoelectric material. To provide guidance to an RTG designer, given a minimum of input, Degra computes the dimensions, masses, and thermal conductances of important internal structures as well as the overall external dimensions and total mass.

Wood, Eric G.↗
NASA NTRS1986

The megageomorphology of the radar rivers of the eastern Sahara

The Eastern Sahara is devoid of surface drainage; this unusual characteristic distinguishes its morphology from that of most other desert regions where running water dominates landscape development. A map derived from SIR-A/B and LANDSAT images and the literature, shows the major presently known paleodrainages in the Eastern Sahara. This compilation permits consideration of the key questions: Where did the radar rivers come from and where did they go? Analysis of SIR-A data led McCauley et al. to suggest that the radar rivers, because of their southwestward trends, once flowed into the Chad basin. This key North African feature is a regional structural low formed in the Early Cretaceous in response to initial opening of the South Atlantic. The problem of the origin of headwaters for the radar rivers was less tractable. The idea that the source areas of the radar rivers might originally have been the same as those later captured by the Nile was proposed tentatively. A more extensive review of the Cenozoic tectonic history of North Africa reveals no reason now to suppose that the Central African tributaries of the present Nile were ever connected to the large alluvial valleys in southwestern Egypt and northwestern Sudan. formed in the Early Cretaceous in response to initial opening of the South Atlantic. The problem of the origin of headwaters for the radar rivers was less tractable. The idea that the source areas of the radar rivers might originally have been the same as those (The Ethiopian Highlands) later captured by the Nile was proposed tentatively. A more extensive review of the Cenozoic tectonic history of North Africa reveals no reason now to support that the Central African tributaries of the present Nile were ever connected to the large alluvial valleys in southwestern Egypt and northwestern Sudan.

Mccauley, John F.↗
NASA NTRS2010

Magnetic Topology of Coronal Hole Linkages

In recent work, Antiochos and coworkers argued that the boundary between the open and closed field regions on the Sun can be extremely complex with narrow corridors of open ux connecting seemingly disconnected coronal holes from the main polar holes, and that these corridors may be the sources of the slow solar wind. We examine, in detail, the topology of such magnetic configurations using an analytical source surface model that allows for analysis of the eld with arbitrary resolution. Our analysis reveals three important new results: First, a coronal hole boundary can join stably to the separatrix boundary of a parasitic polarity region. Second, a single parasitic polarity region can produce multiple null points in the corona and, more important, separator lines connecting these points. Such topologies are extremely favorable for magnetic reconnection, because it can now occur over the entire length of the separators rather than being con ned to a small region around the nulls. Finally, the coronal holes are not connected by an open- eld corridor of finite width, but instead are linked by a singular line that coincides with the separatrix footprint of the parasitic polarity. We investigate how the topological features described above evolve in response to motion of the parasitic polarity region. The implications of our results for the sources of the slow solar wind and for coronal and heliospheric observations are discussed.

Titov, V. S.↗
NASA NTRS1974

System for stabilizing cable phase delay utilizing a coaxial cable under pressure

Stabilizing the phase delay of signals passing through a pressurizable coaxial cable is disclosed. Signals from an appropriate source at a selected frequency, e.g., 100 MHz, are sent through the controlled cable from a first cable end to a second cable end which, electrically, is open or heavily mismatched at 100 MHz, thereby reflecting 100 MHz signals back to the first cable end. Thereat, the phase difference between the reflected-back signals and the signals from the source is detected by a phase detector. The output of the latter is used to control the flow of gas to or from the cable, thereby controlling the cable pressure, which in turn affects the cable phase delay.

Clements, P. A.↗
NASA NTRS1974

A scale model wind tunnel study of dispersion in the Cleveland area. Laboratory simulation of lake breeze effects on diffusion from ground level emissions

A wind tunnel simulation of the diffusion patterns in a sea breeze was attempted. The results indicate that the low level onshore flow was well simulated for neutral, stable, unstable, and elevated inversion conditions. Velocity, turbulence, shear stress, and temperature data were taken, and the spread of emissions from ground level sources was investigated. Comparison is made with theoretical predictions by E. Inoue and with the open, homogeneous plane field results of Pasquill. Agreement with the predictions by Inoue is good, and the comparison with Pasquill's results shows that the wind tunnel flows are shifted two categories towards more stable. The discrepancy may be explained as a matter of averaging time.

Hoydysh, W. G.↗
NASA NTRS1975

A wind-tunnel study of sea breeze effects

A wind-tunnel simulation of the diffusion patterns in a sea breeze has been attempted. No attempt was made to reproduce the recirculation that characterizes a sea breeze, but the results indicate that the low-level onshore flow was well simulated for neutral, stable, unstable, and elevated inversion conditions. Velocity, turbulence, shear stress, and temperature data were taken, and the spread of emissions from ground-level sources was investigated. Comparison is made with theoretical predictions by Inoue (1960) and with the open, countryside results of Pasquill. Agreement with the predictions by Inoue is good. The comparison with Pasquill's results shows that the wind-tunnel flows are shifted two categories towards more stable. The discrepancy may be explained as a lack of mesoscale turbulence in the wind tunnel.

Ogawa, Y.↗
NASA NTRS1978

Douglas Aircraft cabin fire tests

Program objectives are outlined as follows: (1) examine the thermal and environmental characteristics of three types of fuels burned in two quantities contained within a metal lavatory; (2) determine the hazard experienced in opening the door of a lavatory containing a developed fire; (3) select the most severe source fuel for use in a baseline test; and (4) evaluate the effect of the most severe source upon a lavatory constructed of contemporary materials. All test were conducted in the Douglas Cabin Fire Simulator.

Klinck, D.↗
NASA NTRS1981

Acoustic ground impedance meter

A compact, portable instrument was developed to measure the acoustic impedance of the ground, or other surfaces, by direct pressure-volume velocity measurement. A Helmholz resonator, constructed of heavy-walled stainless steel but open at the bottom, is positioned over the surface having the unknown impedance. The sound source, a cam-driven piston of known stroke and thus known volume velocity, is located in the neck of the resonator. The cam speed is a variable up to a maximum 3600 rpm. The sound pressure at the test surface is measured by means of a microphone flush-mounted in the wall of the chamber. An optical monitor of the piston displacement permits measurement of the phase angle between the volume velocity and the sound pressure, from which the real and imaginary parts of the impedance can be evaluated. Measurements using a 5-lobed cam can be made up to 300 Hz. Detailed design criteria and results on a soil sample are presented.

Zuckerwar, A. J.↗
NASA NTRS1983

Acoustic ground impedance meter

A compact, portable instrument was developed to measure the acoustic impedance of the ground, or other surfaces, by direct pressure-volume velocity measurement. A Helmholtz resonator, constructed of heavy-walled stainless steel but open at the bottom, is positioned over the surface having the unknown impedance. The sound source, a cam-driven piston of known stroke and thus known volume velocity, is located in the neck of the resonator. The cam speed is a variable up to a maximum 3600 rpm. The sound pressure at the test surface is measured by means of a microphone flush-mounted in the wall of the chamber. An optical monitor of the piston displacement permits measurement of the phase angle between the volume velocity and the sound pressure, from which the real and imaginary parts of the impedance can be evaluated. Measurements using a 5-lobed cam can be made up to 300 Hz. Detailed design criteria and results on a soil sample are presented. Previously announced in STAR as N82-17476

Zuckerwar, A. J.↗
NASA NTRS1989

A review of high-speed, convective, heat-transfer computation methods

The objective of this report is to provide useful engineering formulations and to instill a modest degree of physical understanding of the phenomena governing convective aerodynamic heating at high flight speeds. Some physical insight is not only essential to the application of the information presented here, but also to the effective use of computer codes which may be available to the reader. A discussion is given of cold-wall, laminar boundary layer heating. A brief presentation of the complex boundary layer transition phenomenon follows. Next, cold-wall turbulent boundary layer heating is discussed. This topic is followed by a brief coverage of separated flow-region and shock-interaction heating. A review of heat protection methods follows, including the influence of mass addition on laminar and turbulent boundary layers. Also discussed are a discussion of finite-difference computer codes and a comparison of some results from these codes. An extensive list of references is also provided from sources such as the various AIAA journals and NASA reports which are available in the open literature.

Tauber, Michael E.↗
NASA NTRS1989

A review of high-speed, convective, heat-transfer computation methods

The objective is to provide useful engineering formulations and to instill a modest degree of physical understanding of the phenomena governing convective aerodynamic heating at high flight speeds. Some physical insight is not only essential to the application of the information presented here, but also to the effective use of computer codes which may be available to the reader. Given first is a discussion of cold-wall, laminar boundary layer heating. A brief presentation of the complex boundary layer transition phenomenon follows. Next, cold-wall turbulent boundary layer heating is discussed. This topic is followed by a brief coverage of separated flow-region and shock-interaction heating. A review of heat protection methods follows, including the influence of mass addition on laminar and turbulent boundary layers. Next is a discussion of finite-difference computer codes and a comparison of some results from these codes. An extensive list of references is also provided from sources such as the various AIAA journals and NASA reports which are available in the open literature.

Tauber, Michael E.↗
NASA NTRS1997

Characteristics of Interstellar and Circumstellar Dust

This paper will review our current knowledge of circumstellar and interstellar dust from an astronomical point of view. About half of the interstellar dust volume consists of amorphous silicates. The remainder has to be made up of an carbonaceous component such as graphite, amorphous carbon (i.e., soot), Polycyclic Aromatic Hydrocarbon molecules (PAHs), and/or organic grain mantles (i.e., mixed polymers). The observational evidence for these components will be reviewed and their relative importance assessed. The emphasis will be on recent observations using the Infrared Space Observatory. Most of these dust components are formed in the outflows from stars in the late stages of their evolution (i.e., red giants, planetary nebulae, novae, supergiants, Wolf Rayet stars, and supernovae). Indeed, observation of such objects indicate an even richer spectrum of stardust components, including also SiC, MgS, and aluminates and crystalline silicates. These observations will be briefly discussed. The stardust budget of the galaxy will be reviewed and the relative importance of the various birth sites assessed. Finally, in recent years, isotopic composition studies have shown that some circumstellar and interstellar dust grains have been incorporated into solar system bodies such as planetary dust particles and meteorites without totally losing their identity. Among the components identified are SiC, graphite, diamonds, PAHs, aluminum oxides, as well as various trace element carbides. Studies of this kind have opened up a new window on the composition and structure of interstellar dust. These different sources of information on interstellar and circumstellar dust will be briefly contrasted.

Tielens, A. G. G. M.↗
NASA NTRS2003

Recent Accomplishments in Laser-Photovoltaic Wireless Power Transmission

Wireless power transmission can be accomplished over long distances using laser power sources and photovoltaic receivers. Recent research at AMOS has improved our understanding of the use of this technology for practical applications. Research by NASA, Boeing, the University of Alabama-Huntsville, the University of Colorado, Harvey Mudd College, and the Naval Postgraduate School has tested various commercial lasers and photovoltaic receiver configurations. Lasers used in testing have included gaseous argon and krypton, solid-state diodes, and fiber optic sources, at wavelengths ranging from the visible to the near infra-red. A variety of Silicon and Gallium Arsenide photovoltaic have been tested with these sources. Safe operating procedures have been established, and initial tests have been conducted in the open air at AMOS facilities. This research is progressing toward longer distance ground demonstrations of the technology and practical near-term space demonstrations.

Fikes, John C.↗
NASA NTRS2005

Long-Lag, Wide-pulse Gamma-Ray Bursts

The best available probe of the early phase of gamma-ray burst (GRB) jet attributes is the prompt gamma-ray emission, in which several intrinsic and extrinsic variables determine observed GRB pulse evolution, including at least: jet opening angle, profiles of Lorentz factor and matter/field density, distance of emission region from central source, and viewing angle. Bright, usually complex bursts have many narrow pulses that are difficult to model due to overlap. However, the relatively simple, long spectral lag, wide-pulse bursts may have simpler physics and are easier to model. We have analyzed the temporal and spectral behavior of wide pulses in 24 long-lag bursts from the BATSE sample, using a pulse model with two shape parameters - width and asymmetry - and the Band spectral model with three shape parameters. We find that pulses in long-lag bursts are distinguished both temporally and spectrally from those in bright bursts: the pulses in long spectral lag bursts are few in number, and approximately 100 times wider (10s of seconds), have systemtically lower peaks in nu*F(nu), harder low-energy spectra and softer high-energy spectra. These five pulse descriptors are essentially uncorrelated for our long-lag sample, suggesting that at least approximately 5 parameters are needed to model burst temporal and spectral behavior, roughly commensurate with the theoretical phase space. However, we do find that pulse width is strongly correlated with spectral lag; hence these two parameters may be viewed as mutual surrogates. The prevalence of long-lag bursts near the BATSE trigger threshold, their predominantly low nu*F(nu) spectral peaks, and relatively steep upper power-law spectral indices indicate that Swiift will detect many such bursts.

Norris, J. P.↗