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At least 901 records · Page 50
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NASA NTRS2013

High-Energy X-rays from J174545.5-285829, the Cannonball: a Candidate Pulsar Wind Nebula Associated with Sgr a East

We report the unambiguous detection of non-thermal X-ray emission up to 30 keV from the Cannonball, a few arcsecond long diffuse X-ray feature near the Galactic Center, using the NuSTAR X-ray observatory. The Cannonball is a high-velocity (v(proj) approximately 500 km s(exp −1)) pulsar candidate with a cometary pulsar wind nebula (PWN) located approximately 2' north-east from Sgr A*, just outside the radio shell of the supernova remnant Sagittarius A (Sgr A) East. Its non-thermal X-ray spectrum, measured up to 30 keV, is well characterized by a Gamma is approximately 1.6 power law, typical of a PWN, and has an X-ray luminosity of L(3-30 keV) = 1.3 × 10(exp 34) erg s(exp −1). The spectral and spatial results derived from X-ray and radio data strongly suggest a runaway neutron star born in the Sgr A East supernova event. We do not find any pulsed signal from the Cannonball. The NuSTAR observations allow us to deduce the PWN magnetic field and show that it is consistent with the lower limit obtained from radio observations.

stars: neutron↗
NASA NTRS1973

Startup analysis for a high temperature gas loaded heat pipe

A model for the rapid startup of a high-temperature gas-loaded heat pipe is presented. A two-dimensional diffusion analysis is used to determine the rate of energy transport by the vapor between the hot and cold zones of the pipe. The vapor transport rate is then incorporated in a simple thermal model of the startup of a radiation-cooled heat pipe. Numerical results for an argon-lithium system show that radial diffusion to the cold wall can produce large vapor flow rates during a rapid startup. The results also show that startup is not initiated until the vapor pressure p sub v in the hot zone reaches a precise value proportional to the initial gas pressure p sub i. Through proper choice of p sub i, startup can be delayed until p sub v is large enough to support a heat-transfer rate sufficient to overcome a thermal load on the heat pipe.

Sockol, P. M.↗
NASA NTRS1974

Seasonal and magnetic storm related changes in the thermosphere induced by eddy mixing

The possibility of explaining the seasonal and magnetic storm related changes in the thermosphere and the ionosphere through variation in eddy diffusion coefficient in the lower thermosphere is investigated theoretically by obtaining simultaneous numerical solutions of the relevant continuity, momentum, and energy balance equations in a self-consistent manner. It is shown that various important features of thermospheric seasonal behavior, including the winter helium bulge phenomenon, can be explained by assuming effective reduction in the winter-time eddy diffusion coefficient by about one order of magnitude. Storm related changes in the thermal structure and composition are seen to arise by assuming a downward shift of turbulence. Physical implications and relative significance of variation in thermospheric turbulence and large scale inter-hemispheric circulation are briefly discussed.

Sinha, A. K.↗
NASA NTRS1977

Design, fabrication, and evaluation of charge-coupled devices with aluminum-anodized-aluminum gates

A 4-phase, 49 1/2 bit CCD shift register was designed and fabricated using two levels of aluminum metallization with anodic Al2O3 insulation separating the layers. Test circuitry was also designed and constructed. A numerical analysis of an MOS-RC transmission line was made and results are given to characterize performance for various conductivities. The electrical design of the CCD included a low-noise dual-gate input and a balanced floating diffusion output circuit. Metallization was accomplished both by low voltage DC sputtering and thermal evaporation. The audization was according to published procedures using a buffered tartaric acid bath. Approximately 20 wafers were processed with 50 complete chips per wafer. All devices failed by shorting between the metal levels at some point. Experimental procedures eliminated temperature effects from sintering and drying, anodic oxide thickness, edge effects, photoresist stripping procedures, and metallization techniques as the primary causes of failure. It was believed from a study of SEM images that protuberances (hillocks) grow up from the first level metal through the oxide either causing a direct short or producing a weak, highly stressed insulation point which fails at low voltage. The cause of these hillocks is unknown; however, they have been observed to grow during temperature excursions to 470 C.

Gassaway, J. D.↗
NASA NTRS1983

Simple energy balance model resolving the seasons and the continents - Application to the astronomical theory of the ice ages

An analysis is undertaken of the properties of a one-level seasonal energy balance climate model having explicit, two-dimensional land-sea geography, where land and sea surfaces are strictly distinguished by the local thermal inertia employed and transport is governed by a smooth, latitude-dependent diffusion mechanism. Solutions of the seasonal cycle for the cases of both ice feedback exclusion and inclusion yield good agreements with real data, using minimal turning of the adjustable parameters. Discontinuous icecap growth is noted for both a solar constant that is lower by a few percent and a change of orbital elements to favor cool Northern Hemisphere summers. This discontinuous sensitivity is discussed in the context of the Milankovitch theory of the ice ages, and the associated branch structure is shown to be analogous to the 'small ice cap' instability of simpler models.

North, G. R.↗
NASA NTRS1988

SINDA - Systems Improved Numerical Differencing Analyzer

Capability to model fluid flow added. SINDA is software system developed to solve physical problems governed by diffusion-type equations mathematically modeled by lumped-parameter representations. Used as general thermal analyzer with resistor-and-capacitor-network representations. Adapted to wide range of problems represented by Fourier, Possion, Laplace, or other differential equations. Written in FORTRAN and Assembler.

Damico, Steve J.↗
NASA NTRS1993

Origin and maintenance of the oxygen torus in Saturn's magnetosphere

Observations of thermal ions in Saturn's inner magnetosphere suggest distributed local sources rather than diffusive mass loading from a source located further out. We suggest that the plasma is produced and maintained mainly by 'self-sputtering' of E ring dust. Sputtered particles are 'picked up' by the planetary magnetospheric field and accelerated to corotation energies (of the order of 8 eV/amu). The sputter yield for oxygen on ice at, for example, 120 eV is about 5, which implies that an avalanche of self-sputtering occurs. The plasma density is built up until it is balanced by local losses, presumably pitch angle scattering into the loss cone and absorption in the planet's ionosphere. The plasma density determines the distribution of dust in the E ring through plasma drag. Thus a feedback mechanism between the plasma and the E ring dust is established. The model accounts for the principal plasma observations and simultaneously the radial optical depth profile of the E ring.

Morfill, G. E.↗
NASA NTRS2014

MODIS Instrument Operation and Calibration Improvements

Terra and Aqua MODIS have successfully operated for over 14 and 12 years since their respective launches in 1999 and 2002. The MODIS on-orbit calibration is performed using a set of on-board calibrators, which include a solar diffuser for calibrating the reflective solar bands (RSB) and a blackbody for the thermal emissive bands (TEB). On-orbit changes in the sensor responses as well as key performance parameters are monitored using the measurements of these on-board calibrators. This paper provides an overview of MODIS on-orbit operation and calibration activities, and instrument long-term performance. It presents a brief summary of the calibration enhancements made in the latest MODIS data collection 6 (C6). Future improvements in the MODIS calibration and their potential applications to the S-NPP VIIRS are also discussed.

Aqua↗
NASA NTRS2019

Determination of Response Versus Scan Angle for the NOAA-20 Visible Infrared Imaging Radiometer Suite Thermal Emissive Bands from Emissive Radiation Measurements During the Pitch Maneuver

The visible infrared imaging radiometer suite (VIIRS) is a key sensor onboard the newly launched (November 18, 2017) NOAA-20 satellite and the early launched Suomi National Polar-orbiting Partnership (SNPP) satellite. The two VIIRS sensors are nearly identical in design. Its onboard calibration components include a solar diffuser (SD) and a SD stability monitor for the reflective solar bands, a V-grooved blackbody for the thermal emissive bands (TEBs),and a space view port for background subtraction. These onboard calibrators are located at fixed scan angles. The response versus scan angle (RVS) of the VIIRS scan mirror was characterized prelaunch in lab-ambient conditions and is currently used to calibrate the on-orbit response for all scan angles relative to the calibrator's scan angle. A spacecraft-level pitch maneuver was scheduled during the initial intensive calibration and validation testing for both the NOAA-20 and SNPP. The pitch maneuver provided a rare opportunity for VIIRS to make observations of deep space over the entire range of the scan angle, which can be used to characterize the TEBRVS. We analyze the NOAA-20 pitch maneuver data and assess the derived TEB RVS. A comparison between the RVS determined by the pitch maneuver observations and prelaunch lab measurements is conducted for each band, detector, and mirror side of the half-angle mirror.

Wu, Aisheng↗
NASA NTRS2002

New High-Performance SiC Fiber Developed for Ceramic Composites

Sylramic-iBN fiber is a new type of small-diameter (10-mm) SiC fiber that was developed at the NASA Glenn Research Center and was recently given an R&D 100 Award for 2001. It is produced by subjecting commercially available Sylramic (Dow Corning, Midland, MI) SiC fibers, fabrics, or preforms to a specially designed high-temperature treatment in a controlled nitrogen environment for a specific time. It can be used in a variety of applications, but it currently has the greatest advantage as a reinforcement for SiC/SiC ceramic composites that are targeted for long-term structural applications at temperatures higher than the capability of metallic superalloys. The commercial Sylramic SiC fiber, which is the precursor for the Sylramic-iBN fiber, is produced by Dow Corning, Midland, Michigan. It is derived from polymers at low temperatures and then pyrolyzed and sintered at high temperatures using boron-containing sintering aids (ref. 1). The sintering process results in very strong fibers (>3 GPa) that are dense, oxygen-free, and nearly stoichiometric. They also display an optimum grain size that is beneficial for high tensile strength, good creep resistance, and good thermal conductivity (ref. 2). The NASA-developed treatment allows the excess boron in the bulk to diffuse to the fiber surface where it reacts with nitrogen to form an in situ boron nitride (BN) coating on the fiber surface (thus the product name of Sylramic-iBN fiber). The removal of boron from the fiber bulk allows the retention of high tensile strength while significantly improving creep resistance and electrical conductivity, and probably thermal conductivity since the grains are slightly larger and the grain boundaries cleaner (ref. 2). Also, as shown in the graph, these improvements allow the fiber to display the best rupture strength at high temperatures in air for any available SiC fiber. In addition, for CMC applications under oxidizing conditions, the formation of an in situ BN surface layer creates a more environmentally durable fiber surface not only because a more oxidation-resistant BN is formed, but also because this layer provides a physical barrier between contacting fibers with oxidation-prone SiC surface layers (refs. 3 and 4). This year, Glenn demonstrated that the in situ BN treatment can be applied simply to Sylramic fibers located within continuous multifiber tows, within woven fabric pieces, or even assembled into complex product shapes (preforms). SiC/SiC ceramic composite panels have been fabricated from Sylramic-iBN fabric and then tested at Glenn within the Ultra-Efficient Engine Technology Program. The test conditions were selected to simulate those experienced by hot-section components in advanced gas turbine engines. The results from testing at Glenn demonstrate all the benefits expected for the Sylramic-iBN fibers. That is, the composites displayed the best thermostructural performance in comparison to composites reinforced by Sylramic fibers and by all other currently available high-performance SiC fiber types (refs. 3 and 5). For these reasons, the Ultra-Efficient Engine Technology Program has selected the Sylramic-iBN fiber for ongoing efforts aimed at SiC/SiC engine component development.

DiCarlo, James A.↗
NASA NTRS2011

Metallization for Yb14MnSb11-Based Thermoelectric Materials

Thermoelectric materials provide a means for converting heat into electrical power using a fully solid-state device. Power-generating devices (which include individual couples as well as multicouple modules) require the use of ntype and p-type thermoelectric materials, typically comprising highly doped narrow band-gap semiconductors which are connected to a heat collector and electrodes. To achieve greater device efficiency and greater specific power will require using new thermoelectric materials, in more complex combinations. One such material is the p-type compound semiconductor Yb14MnSb11 (YMS), which has been demonstrated to have one of the highest ZT values at 1,000 C, the desired operational temperature of many space-based radioisotope thermoelectric generators (RTGs). Despite the favorable attributes of the bulk YMS material, it must ultimately be incorporated into a power-generating device using a suitable joining technology. Typically, processes such as diffusion bonding and/or brazing are used to join thermoelectric materials to the heat collector and electrodes, with the goal of providing a stable, ohmic contact with high thermal conductivity at the required operating temperature. Since YMS is an inorganic compound featuring chemical bonds with a mixture of covalent and ionic character, simple metallurgical diffusion bonding is difficult to implement. Furthermore, the Sb within YMS readily reacts with most metals to form antimonide compounds with a wide range of stoichiometries. Although choosing metals that react to form high-melting-point antimonides could be employed to form a stable reaction bond, it is difficult to limit the reactivity of Sb in YMS such that the electrode is not completely consumed at an operating temperature of 1,000 C. Previous attempts to form suitable metallization layers resulted in poor bonding, complete consumption of the metallization layer or fracture within the YMS thermoelement (or leg).

Firdosy, Samad↗
NASA NTRS1988

Boundary shape identification problems in two-dimensional domains related to thermal testing of materials

The identification of the geometrical structure of the system boundary for a two-dimensional diffusion system is reported. The domain identification problem treated here is converted into an optimization problem based on a fit-to-data criterion and theoretical convergence results for approximate identification techniques are discussed. Results of numerical experiments to demonstrate the efficacy of the theoretical ideas are reported.

Banks, H. T.↗
NASA NTRS1989

Boundary shape identification problems in two-dimensional domains related to thermal testing of materials

The identification of the geometrical structure of the system boundary for a two-dimensional diffusion system is reported. The domain identification problem treated here is converted into an optimization problem based on a fit-to-data criterion and theoretical convergence results for approximate identification techniques are discussed. Ressults of numerical experiments to demonstrate the efficacy of the theoretical ideas are reported.

Banks, H. T.↗
NASA NTRS1998

An Integrated Tool for the Coupled Thermal and Mechanical Analysis of Pyrolyzing Heatshield Materials

Materials that pyrolyze at elevated temperature have been commonly used as thermal protection materials in hypersonic flight, and advanced pyrolyzing materials for this purpose continue to be developed. Because of the large temperature gradients that can arise in thermal protection materials, significant thermal stresses can develop. Advanced applications of pyrolytic materials are calling for more complex heatshield configurations, making accurate thermal stress analysis more important, and more challenging. For non-pyrolyzing materials, many finite element codes are available and capable of performing coupled thermal-mechanical analyses. These codes do not, however, have a built-in capability to perform analyses that include pyrolysis effects. When a pyrolyzing material is heated, one or more components of the original virgin material pyrolyze and create a gas. This gas flows away from the pyrolysis zone to the surface, resulting in a reduction in surface heating. A porous residue, referred to as char, remains in place of the virgin material. While the processes involved can be complex, it has been found that a simple physical model in which virgin material reacts to form char and pyrolysis gas, will yield satisfactory analytical results. Specifically, the effects that must be modeled include: (1) Variation of thermal properties (density, specific heat, thermal conductivity) as the material composition changes; (2) Energy released or absorbed by the pyrolysis reactions; (3) Energy convected by the flow of pyrolysis gas from the interior to the surface; (4) The reduction in surface heating due to surface blowing; and (5) Chemical and mass diffusion effects at the surface between the pyrolysis gas and edge gas Computational tools for the one-dimensional thermal analysis these materials exist and have proven to be reliable design tools. The objective of the present work is to extend the analysis capabilities of pyrolyzing materials to axisymmetric configurations, and to couple thermal and mechanical analyses so that thermal stresses may be efficiently and accurately calculated.

Pronchick, Stephen W.↗
NASA NTRS1998

3-D Modeling of Double-Diffusive Convection During Directional Solidification of a Non-Dilute Alloy with Application to the HgCdTe Growth Under Microgravity Conditions

A numerical calculation for a non-dilute alloy solidification was performed using the FIDAP finite element code. For low growth velocities plane front solidification occurs. The location and the shape of the interface was determined using melting temperatures from the HgCdTe liquidus curve. The low thermal conductivity of the solid HgCdTe causes thermal short circuit through the ampoule walls, resulting in curved isotherms in the vicinity of the interface. Double-diffusive convection in the melt is caused by radial temperature gradients and by material density inversion with temperature. Cooling from below and the rejection at the solid-melt interface of the heavier HgTe-rich solute each tend to reduce convection. Because of these complicating factors dimensional rather then non-dimensional modeling was performed. Estimates of convection contributions for various gravity conditions was performed parametrically. For gravity levels higher then 1 0 -7 of earth's gravity it was found that the maximum convection velocity is extremely sensitive to gravity vector orientation and can be reduced at least by factor of 50% for precise orientation of the ampoule in the microgravity environment. The predicted interface shape is in agreement with one obtained experimentally by quenching. The results of 3-D modeling are compared with previous 2-D finding. A video film featuring melt convection will be presented.

Bune, Andris V.↗
NASA NTRS1995

Polarimetry of the Interstellar Medium

The talk will review what is known about the composition of ices and organics in the dense and diffuse interstellar media (ISM). Mixed molecular ices make up a significant fraction of the solid materials in dense molecular clouds and it is now known that thermal and radiation processing of these ices results in the production of more complex organic species, some of which may survive transport into forming stellar systems and the diffuse ISM. Molecular species identified in interstellar ices include H2O, CH3OH, CO, CH4, CO2, and somewhat surprisingly, H2. Theoretical and laboratory studies of the processing of interstellar analog ices containing these species indicate that species like HCO, H2CO, CH3, and NH3 are readily made and should also be present. The irradiation of mixed molecular ices containing these species, when followed by warming, leads to the production of a large variety of more complex species, including ethanol (CH3CH2OH), formamide (HC(=O)NH2), acetamide (CH3C(=O)NH2), nitriles or isonitriles (R-CN or R-NC hexamethylenetetramine (HMT; C6H12N4), a number of polymeric species related to polyoxymethylene [POM,(-CH2O-)n], and ketones {R-C(=O)-R'}. Spectral studies of dust in the diffuse ISM indicate the presence of fairly complex organics, some of which may be related to the organics produced in dense molecular clouds. Spectral comparisons indicate that the diffuse ISM organics may be quite similar to meteoritic kerogens, i.e. they may consist largely of aromatic moieties interlinked by short aliphatic bridges. Interestingly, recent evidence indicates that the galactic distribution of this material closely matches that of silicates, but does not correlate directly with visual extinction. This implies that a large fraction of the visual extinction is caused by a material other than these organics and silicates and that this other material has a significantly different distribution within the galaxy.

Sandford, Scott↗
NASA NTRS1976

Refinement of Promising Coating Compositions for Directionally Cast Eutectics

The successful application of high creep strength, directionally solidified gamma/gamma prime-delta (Ni-19.7Cb-6Cr-2.5Al) eutectic superalloy turbine blades requires the development of suitable coatings for airfoil, root and internal blade surfaces. In order to improve coatings for the gamma/gamma prime-delta alloy, the current investigation had the goals of (1) refining promising coating compositions for directionally solidified eutectics, (2) evaluating the effects of coating/ substrate interactions on the mechanical properties of the alloy, and (3) evaluating diffusion aluminide coatings for internal surfaces. Burner rig cyclic oxidation, furnace cyclic hot corrosion, ductility, and thermal fatigue tests indicated that NiCrAlY+Pt(63 to 127 micron Ni-18Cr-12Al-0.3Y + 6 micron Pt) and NiCrAlY(63 to 127 micron Ni-18Cr-12Al-0.3Y) coatings are capable of protecting high temperature gas path surfaces of eutectic alloy airfoils. Burner rig (Mach 0.37) testing indicated that the useful coating life of the 127 micron thick coatings exceeded 1000 hours at 1366 K (2000 deg F). Isothermal fatigue and furnance hot corrosion tests indicated that 63 micron NiCrAlY, NiCrAlY + Pt and platinum modified diffusion aluminide (Pt + Al) coating systems are capable of protecting the relatively cooler surfaces of the blade root. Finally, a gas phase coating process was evaluated for diffusion aluminizing internal surfaces and cooling holes of air-cooled gamma/gamma prime-delta turbine blades.

Strangman, T. E.↗