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At least 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Star formation in the inner galaxy: A far-infrared and radio study of two H2 regions

Far-infrared and radio continuum maps have been made of the central 6' of the inner-galaxy HII regions G30.8-0.0 (in the W43 complex) and G25.4-0.2, along with radio and molecular line measurements at selected positions. The purpose of this study is an effort to understand star formation in the molecular ring at 5 kpc in galactic radius. Measurements at several far infrared wavelengths allow the dust temperature structures and total far infrared fluxes to be determined. Comparison of the radio and infrared maps shows a close relationship between the ionized gas and the infrared-emitting material. There is evidence that parts of G30.8 are substantially affected by extinction, even at far-infrared wavelengths. Using radio recombination line and CO line data for G25.4-0.2, the distance ambiguity for this source is resolved. The large distance previously ascribed to the entire complex is found to apply to only one of the two main components. The confusion in distance determination is found to result from an extraordinary near-superposition of two bright HII regions. Using the revised distances of 4.3 kpc for G25.4SE and 12 kpc for G25.4NW, it is found that the latter, which is apparently the fainter of the two sources, is actually the more luminous. The ratio of total luminosity to ionizing luminosity is very similar to that of HII regions in the solar circle. Assuming a coeval population of ionizing stars, a normal initial mass function is indicated.

Lester, D. F.↗
NASA NTRS1990

An ionic mechanism of carbon formation in flames

The formation of incipient carbon in flames can be described by a series of elementary reactions in which the precursor of soot is the ion C3H3+, and the major building block is acetylene, polyacetylenes, or other hydrocarbon fragments which are present in large concentrations. The precursor ion is produced by nonequilibrium chemi-ionization reactions, e.g., CH asterisk plus C2H2 yields C3H3+ plus e or CH asterisk plus O yields CHO+ plus e. The CHO+ rapidly becomes C3H3+ through a series of ion-molecule reactions. The chemi-ion then adds acetylene in a series of very rapid ion-molecule reactions producing larger and larger ions. Ions isomerize very rapidly to produce aromatic structures overcoming one of the major problems with neutral species mechanisms: how to form the first carbon ring. As the ions grow, their recombination rate coefficients with electrons increase so that the larger ions are removed by dissociative recombination. These neutral species now continue to grow by the addition of more acetylene producing even larger neutral species. With increasing size, the rate coefficient of electron attachment also increases causing the reaction to become important, depending upon the temperature. Appreciable concentrations of negative ions shift the ion removal process to dissociative ion-ion recombination, which is orders of magnitude slower than ion-electron recombination. As the neutral species continue to grow by the addition of acetylene, they gradually take on the aspect of particles, i.e., the bulk properties of the substance CxHy dominate over the chemical properties. This change is gradual and there is no distinct size at which a large molecule becomes a particle. Evidence will be presented supporting the above mechanism. All of the ions proposed in the mechanism have been observed by mass spectrometric analysis, their concentrations measured, and their rates of reaction in the flame environment determined to be adequate to account for the observed rate of formation of carbon.

Calcote, H. F.↗
NASA NTRS1990

Two-dimensional caustics in disturbed galaxy disks

Interacting galaxies show a variety of exotic morphologies besides the well-known rings, spirals, and bars. It is proposed that many of these can be identified with nonlinear, caustic structures that are known from singularity theory to be generic in collisionless dynamics. A simple kinematical model for stellar orbits is used to study the development of caustic waveforms in galaxy disks following impulsive, nonaxially symmetric (collisional) disturbances. It appears that all of the caustics known from singularity theory to be generic in two dimensions develop as transient forms in at least some moderately off-center collisions. The more complex, higher-order caustics appear most prominently in the more off-center and larger amplitude disturbances. It is further suggested that the highly nonlinear caustic waves may play an important role in interaction-induced star formation.

Struck-Marcell, Curtis↗
NASA NTRS1992

The large impact process inferred from the geology of lunar multiring basins

The nature of the impact process has been inferred through the study of the geology of a wide variety of impact crater types and sizes. Some of the largest craters known are the multiring basins found in ancient terrains of the terrestrial planets. Of these features, those found on the Moon possess the most extensive and diverse data coverage, including morphological, geochemical, geophysical, and sample data. The study of the geology of lunar basins over the past 10 years has given us a rudimentary understanding of how these large structures have formed and evolved. The topics covered include basin morphology, basin ejecta, basin excavation, and basin ring formation.

Spudis, Paul D.↗
NASA NTRS1993

Pressurized modules for Space Station Freedom

Crew workstations, storage, and facilities in the SSF Habitation module along with station housekeeping and supporting subsystems (power, thermal, and life support) are considered. The U.S. Laboratory and Habitation modules are based on a common structural design. The pressurized logistics modules (PLMs) have maximum commonality with the other pressurized modules including common ring frames and waffle grid skin; multilayer insulation and debris shield; and one common endcone.

Hopson, George D.↗
NASA NTRS2001

Development and Validation of High Performance Unshrouded Centrifugal Impeller

The feasibility of using a two-stage unshrouded impeller turbopump to replace the current three-stage reusable launch vehicle engine shrouded impeller hydrogen pump has been evaluated from the standpoint of turbopump weight reduction and overall payload improvement. These advantages are a by-product of the higher tip speeds that an unshrouded impeller can sustain. The issues associated with the effect of unshrouded impeller tip clearance on pump efficiency and head have been evaluated with one-dimensional tools and full three-dimensional rotordynamic fluid reaction forces and coefficients have been established through time dependent computational fluid dynamics (CFD) simulation of the whole 360 degree impeller with different rotor eccentricities and whirling ratios. Unlike the shrouded impeller, the unshrouded impeller forces are evaluated as the sum of the pressure forces on the blade and the pressure forces on the hub using the CFD results. The turbopump axial thrust control has been optimized by adjusting the first stage impeller backend wear ring seal diameter and diverting the second stage backend balance piston flow to the proper location. The structural integrity associated with the high tip speed has been checked by analyzing a 3D-Finite Element Model at maximum design conditions (6% higher than the design speed). This impeller was fabricated and tested in the NASA Marshall Space Flight Center water-test rig. The experimental data will be compared with the analytical predictions and presented in another paper. The experimental data provides validation data for the numerical design and analysis methodology. The validated numerical methodology can be used to help design different unshrouded impeller configurations.

Chen, Wei-Chung↗
NASA NTRS1999

Composite Nozzle/Thrust Chambers Analyzed for Low-Cost Boosters

The Low Cost Booster Technology Program is an initiative to minimize the cost of future liquid engines by using advanced materials and innovative designs, and by reducing engine complexity. NASA Marshall Space Flight Center s 60K FASTRAC Engine is one example where these design philosophies have been put into practice. This engine burns a liquid kerosene/oxygen mixture. It uses a one-piece, polymer composite thrust chamber/nozzle that is constructed of a tape-wrapped silica phenolic liner, a metallic injector interface ring, and a filament-wound epoxy overwrap. A cooperative effort between NASA Lewis Research Center s Structures Division and Marshall is underway to perform a finite element analysis of the FASTRAC chamber/nozzle under all the loading and environmental conditions that it will experience during its lifetime. The chamber/nozzle is a complex composite structure. Of its three different materials, the two composite components have distinctly different fiber architectures and, consequently, require separate material model descriptions. Since the liner is tape wrapped, it is orthotropic in the nozzle global coordinates; and since the overwrap is filament wound, it is treated as a monoclinic material. Furthermore, the wind angle on the overwrap varies continuously along the length of the chamber/nozzle.

Sullivan, Roy M.↗
NASA NTRS1988

Diode end pumped laser and harmonic generator using same

A second harmonic, optical generator is disclosed in which a laser diode produces an output pumping beam which is focused by means of a graded, refractive index rod lens into a rod of lasant material, such as Nd:YAG, disposed within an optical resonator to pump the lasant material and to excite the optical resonator at a fundamental wavelength. A non-linear electro-optic material such as MgO:LiNbO.sub.3 is coupled to the excited, fundamental mode of the optical resonator to produce a non-linear interaction with the fundamental wavelength producing a harmonic. In one embodiment, the gain medium and the non-linear material are disposed within an optical resonator defined by a pair of reflectors, one of which is formed on a face of the gain medium and the second of which is formed on a face of the non-linear medium. In another embodiment, the non-linear, electro-optic material is doped with the lasant ion such that the gain medium and the non-linear doubling material are co-extensive in volume. In another embodiment, a non-linear, doubling material is disposed in an optical resonator external of the laser gai medium for improved stability of the second harmonic generation process. In another embodiment, the laser gain medium andthe non-linear material are bonded together by means of an optically transparent cement to form a mechanically stable, monolithic structure. In another embodiment, the non-linear material has reflective faces formed thereon to define a ring resonator to decouple reflections from the non-linear medium back to the gain medium for improved stability.

Byer, Robert L.↗
NASA NTRSDate not supplied

Key Observations and Discoveries from the Vector Electric Field Investigation on the C/NOFS Satellite

Launched in 2008 and operating for 7.5 years, the Air Force Communication /Navigation Outage Forecasting System (C/NOFS) satellite included the Vector Electric Field Investigation (VEFI) designed and built at NASA’s Goddard Space Flight Center. VEFI successfully met its objectives: 1) investigate the role of ambient electric fields in initiating nighttime density depletions and turbulence; 2) determine the quasi-DC electric fields associated with abrupt density depletions, and 3) quantify the spectrum of the irregularities associated with density depletions, providing many key observations and discoveries including: -- Global (low latitude) vector DC electric fields at 16 s/sec revealing variations with local time, longitude, and season between extremely low and moderate solar activity-- Reversed zonal E x B drifts below the F-region ledge at sunset and simultaneous observations of large scale Kelvin-Helmholtz instabilities as seeds of spread-F -- Large expanses of quasi-coherent kilometer-scale vector wave observations (electric field and density) below 450 km and their discovery as a source of scintillations using the C/NOFS GPS -- Reversed zonal DC electric fields and simultaneous observations of afternoon counter electrojet-- Enhanced zonal DC electric fields at sunrise and full vector plasma flow continuity at the terminator-- Vector electric field and density irregularities extending to meter-scales within equatorial plasma depletions -- Intense electric field structure within the equatorial ionosphere at night in the absence of density depletions -- First observations of the westward equatorial electrojet in post-midnight ionosphere and possible association with downward meridional drifts-- Spaceborne Dst observations and implications for ring current asymmetries-- Vector electric and magnetic field Poynting flux within depletions and TIDs-- Measurements of ionospheric reflectance and Alfvenic waves associated with TIDs-- Observations of Alfven resonators -- ULF magnetic field structure within the nightime equatorial ionosphere-- Discovery of Schumman resonances in space and implications of ELF radiowave propagation-- Vector observations of 50-60 Hz powerline radiation without harmonics-- Ion cyclotron resonance absorption lines associated with ELF hiss and the identification of ambient ions-- Parallel electric fields associated with lower hybrid waves driven by thunderstorm lightning-related sferics-- Discovery of Z-mode radiation in the equatorial ionosphere including its associated with density depletions-- Optical lightning detector waveforms and electric field sferics observed up to altitudes of 800 km

Robert F Pfaff↗
NASA NTRS2015

Pyrotechnically Actuated Gas Generator Utilizing Aqueous Methanol

A gas-generating device was developed to supplement the ram-air inflation of a supersonic ballute. The device is designed to initially pressurize the ballute following deployment, exposing and orienting its ram-air inlets to free-stream air for complete inflation. The supplemental pressurization decreases the total inflation time, and increases the likelihood of a successful inflation. The device contains a reservoir filled with an aqueous mixture of methanol that, when released in to the interior of the ballute, rapidly vaporizes due to the low ambient pressure. Upon activation of the device, a pair of redundant ring mechanisms initiate pyrotechnic charges that pressurize and rupture the reservoir, resulting in ejection of the methanol in to the ballute. In addition to its role in inflation, the device serves as the structural connection to the ballute. Analytical models were developed for the inflation capability of the device, which were verified using vacuum chamber testing of developmental hardware. Static, deployment, and environmental testing demonstrated the functionality of the ring mechanism and reservoir under several temperature and pressure conditions. Finally, the device was successfully operated during the first Supersonic Flight Dynamics Test (SFDT) of NASA's Low Density Supersonic Decelerator (LDSD) project. The design architecture is scalable to accommodate different quantities of gas generation, can be adjusted to operate in a variety of temperature and atmospheric pressure regimes, and provides a robust device that may be installed with minimal risk to personnel or hardware.

IA↗
DOE OSTI2021

Preferential hole defect formation in monolayer W Se 2 by electron-beam irradiation

Monolayer transition-metal dichalcogenides (TMDCs) have been extensively studied due to their wide range of physical properties and applications. It has been demonstrated that the electron beam in a transmission electron microscope (TEM) or scanning TEM (STEM) generates chalcogen vacancies that agglomerate into dispersed linelike or holelike defects. Here we employ a STEM electron beam and demonstrate that, in WSe 2 , beam-induced chalcogen vacancies initially form certain dispersed multivacancy structures, as seen in TMDCs in prior work. However, with suitable control of the STEM focused electron beam, these multivacancies gradually evolve into a dense network of ten-, 12-, 14-, and 16-member ring round holes, whereas the same process leads predominantly to chalcogen-vacancy line defects in other trigonal-prismatic TMDCs. Density functional theory calculations find that the underlying atomic-scale processes lead preferentially to defect structures that lower the total energy so that we are able to track the formation of the observed multivacancy complexes, which then lead to the formation of dense large round holes in WSe 2 . The same processes in WS 2 , however, are energetically unfavorable, while linear multivacancy defects are preferred, as observed. The demonstrated control of the formation of unique high-density round holes in WSe 2 has potential for applications such as atomic and molecular sieving.

36 MATERIALS SCIENCE↗
NASA NTRS2003

Structural Model for the Flip-Flop Action in Thiamin Pyrophosphate-Dependent Human Pyruvate Dehydrogenase

The derivative of vitamin B1 thiamin pyrophosphate (TPP) is a cofactor of enzymes performing catalysis in pathways of energy production, including (i) decarboxylation of alpha-keto acids followed by (ii) transketolation. These enzymes have shown a common mechanism of TPP activation by imposing an active V-conformation of this coenzyme that brings the N4 atom of the aminopyrimidine ring to the distance required for the intramolecular C-H N hydrogen-bonding with the C2- atom of the thiazolium ring. The reactive C2 atom of TPP is the nucleophile that attacks the carbonyl carbon of different substrates used by the TPP-dependent enzymes. The structure of the heterotetrameric human pyruvate dehydrogenase (Elp) recently determined in our laboratory (1) revealed the association pattern of the subunits and the specifics of two chemically equivalent cofactor binding sites. Dynamic nonequivalence of these two cofactor sites directs the flip-flop action of this enzyme, depending upon which two active sites effect each other (2). The crystal structure derived from the holo-form of Elp provided the basis for the model of the flip-flop action of Elp in which different steps of the catalytic reaction are performed in each of the two cofactor sites at any given moment, where these steps are governed by the concerted shuttle-like motion of the subunits. It is further proposed that balancing a hydrogen-bond network and related cofactor geometry determine the continuity of catalytic events.

Ciszak, Ewa↗
NASA NTRS1993

Partially Grooved Domain Stabilization Structures for Vertical Bloch Line Memory

Bias field stability ranges were measured and numerically simulated for magnetic domains ingarnets stabilized by partially grooved rectangular and ring grooves. Simulation results agreefavorably with experimental results when finite slope effects of the groove walls are included. Asbias fields increase, rectangular and ring domains both destabilize through stripe head recession. Asbias fields decrease, destabilization in rectangular domains occurs by runout, while destabilization inring domains occurs by midstripe domain buckling. While ring domains are stable at lower biasfields than rectangular domains, bias field stability ranges are approximately equal. Hence, for thesame partial grooving depth, rectangular domains are preferred because they offer higher storagedensity potential in Vertical Block Line (VBL) storage arrays as long as bit propagation margins atstripe ends are sufficient.

Katti, R. R.↗
DOE OSTI2022

Precise equilibrium structures of 1 H - and 2 H -1,2,3-triazoles (C 2 H 3 N 3 ) by millimeter-wave spectroscopy

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (r e SE ) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a “best theoretical estimate” (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Finally, agreement between BTE and r e SE values is discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Comparison of the Energy-Transfer Rates in Structural and Spectral Variants of the B800–850 Complex from Purple Bacteria

Photosynthetic light harvesting can occur with a remarkable near-unity quantum efficiency. The B800–850 complex, also known as light-harvesting complex 2 (LH2), is the primary light-harvesting complex in purple bacteria and has been extensively studied as a model system. The bacteriochlorophylls of the B800–850 complex are organized into two concentric rings, known as the B800 and B850 rings. However, depending on the species and growth conditions, the number of constituent subunits, the pigment geometry, and the absorption energies vary. While the dynamics of some B800–850 variants have been exhaustively characterized, others have not been measured. Furthermore, a direct and simultaneous comparison of how both structural and spectral differences between variants affect these dynamics has not been performed. In this work, we utilize ultrafast transient absorption measurements to compare the B800 to B850 energy-transfer rates in the B800–850 complex as a function of the number of subunits, geometry, and absorption energies. The nonameric B800–850 complex from Rhodobacter (Rb.) sphaeroides is 40% faster than the octameric B800–850 complex from Rhodospirillum (Rs.) molischianum, consistent with structure-based predictions. In contrast, the blue-shifted B800–820 complex from Rs. molischianum is only 20% faster than the B800–850 complex from Rs. molischianum despite an increase in the spectral overlap between the rings that would be expected to produce a larger increase in the energy-transfer rate. These measurements support current models that contain dark, higher-lying excitonic states to bridge the energy gap between rings, thereby maintaining similar energy-transfer dynamics. Altogether, these results demonstrate that energy-transfer dynamics in the B800–850 complex are robust to the spectral and structural variations between species used to optimize energy capture and flow in purple bacteria.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

The luminosity function of ringed galaxies

ABSTRACT We perform an analysis of the luminosity functions (LFs) of two types of ringed galaxies – polar-ring galaxies and collisional ring galaxies – using data from the Sloan Digital Sky Survey (SDSS). Both classes of galaxies were formed as a result of interaction with their environment and they are very rare objects. We constructed LFs of galaxies by different methods and found their approximations by the Schechter function. The luminosity functions of both types of galaxies show a systematic fall-off at low luminosities. The polar structures around bright (Mr ≤ −20m) and red (g − r > +0.8) galaxies are about twice as common as around blue ones. The LF of collisional rings is shifted towards brighter luminosities compared to polar-ring galaxies. We analysed the published data on the ringed galaxies in several deep fields and confirmed the increase in their volume density with redshift: up to z ∼ 1 their density grows as (1 + z)m, where m ≳ 5.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1982

Volumetric analysis of complex lunar craters - Implications for basin ring formation

The crater to basin transition in complex lunar craters is characterized by combining morphological and volumetric analyses of their central peaks with subsurface data from terrestrial complex impact structures which suggest that the amount of uplifted material, as judged from its depth of origin, continues to increase with increasing rim diameter. This latter phenomenon implies that a redistribution of uplifted material away from a centralized peak may occur in the larger craters. The morphological and volumetric changes described occur over a rim diameter range of 51-80 km, which is considerably lower than the previously proposed range for the crater to basin transition of 140-175 km. Evidence is given in support of a crater to basin transition which begins at 51-80 km, and is characterized by a relative reduction in central peak volume and a development of rings of floor roughening which may be precursors of peak ring development.

Hale, W. S.↗
NASA NTRS1992

Failure analysis of thick composite cylinders under external pressure

Failure of thick section composites due to local compression strength and overall structural instability is treated. Effects of material nonlinearity, imperfect fiber architecture, and structural imperfections upon anticipated failure stresses are determined. Comparisons with experimental data for a series of test cylinders are described. Predicting the failure strength of composite structures requires consideration of stability and material strength modes of failure using linear and nonlinear analysis techniques. Material strength prediction requires the accurate definition of the local multiaxial stress state in the material. An elasticity solution for the linear static analysis of thick anisotropic cylinders and rings is used herein to predict the axisymmetric stress state in the cylinders. Asymmetric nonlinear behavior due to initial cylinder out of roundness and the effects of end closure structure are treated using finite element methods. It is assumed that local fiber or ply waviness is an important factor in the initiation of material failure. An analytical model for the prediction of compression failure of fiber composites, which includes the effects of fiber misalignments, matrix inelasticity, and multiaxial applied stresses is used for material strength calculations. Analytical results are compared to experimental data for a series of glass and carbon fiber reinforced epoxy cylinders subjected to external pressure. Recommendations for pretest characterization and other experimental issues are presented. Implications for material and structural design are discussed.

Caiazzo, A.↗