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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Different bonding type along each crystallographic axis: Computational study of poly($\textit{p}$-phenylene terephthalamide)

Poly($\textit{p}$-phenylene terephthalamide) (PPTA) exhibits van der Waals (vdW) bonding along its $\textit{a}$ axis, hydrogen bonding along its $\textit{b}$ axis, and covalent bonding along its $\textit{c}$ axis. In this work, we explore the structural and mechanical properties of PPTA using density functional theory with various functionals including LDA, PBE, PBE+rVV10L, SCAN, and SCAN+rVV10, compared with available experiments. The hierarchy of nonempirical semilocal functionals LDA, PBE, and SCAN (not fitted to any multicenter bonded system) includes differing amounts of intermediate-range vdW interaction. rVV10 is the long-range vdW correction. (rVV10L differs from rVV10 only in the value of a range parameter.) Among the tested functionals, SCAN shows the best performance for the lattice parameters of PPTA along the two crystal directions involving vdW or hydrogen-bond interaction. The equilibrium lattice constants obtained by SCAN and PBE+rVV10L are closest to experimental data, while SCAN+rVV10 slightly overbinds the system. We study the mechanical response of PPTA by applying strain along three lattice directions. Due to the inclusion of vdW interaction, SCAN, PBE+rVV10L, and SCAN+rVV10 all exhibit correct bonding strain-energy curves. On the contrary, PBE strongly underestimates the vdW interaction needed to resist uniaxial stretching along the $\textit{a}$ axis. The Young's modulus and yield strength of PPTA are computed and compared with previous results. The experimental values are much smaller than the computed ones, mainly due to the fact that the PPTA fiber samples used for measurements are mechanically weaker than the perfect molecular crystals considered in the simulations. Interestingly, when a compressive uniaxial stress of 25 Kbar is applied along the $\textit{b}$ axis, a structural phase transition, in which the hydrogen bonds reform along one diagonal of the $\textit{ab}$ rectangle, is predicted by SCAN.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Nb₃Sn Coating of Twin Axis Cavity for SRF Applications

The twin axis cavity with two identical accelerating beams has been proposed for energy recovery linac (ERL) applications. Nb₃Sn is a superconducting material with a higher critical temperature and a higher critical field as compared to Nb, which promises a lower operating cost due to higher quality factors. Two niobium twin axis cavities were fabricated at JLab and were proposed to be coated with Nb₃Sn. Due to their more complex geometry, the typical coating process used for basic elliptical cavi-ties needs to be improved to coat these cavities. This development advances the current coating system at JLab for coating complex cavities. Two twin axis cavities were coated recently for the first time. This contribution dis-cusses initial results from coating of twin axis cavities, RF testing and witness sample analysis with an overview of the current challenges towards high performance Nb₃Sn coated twin axis cavities.

43 PARTICLE ACCELERATORS↗
DOE OSTI2024

The Evolution of Galaxies and Clusters at High Spatial Resolution with Advanced X-ray Imaging Satellite (AXIS)

Stellar and black hole feedback heat and disperse surrounding cold gas clouds, launching gas flows off circumnuclear and galactic disks, producing a dynamic interstellar medium. On large scales bordering the cosmic web, feedback drives enriched gas out of galaxies and groups, seeding the intergalactic medium with heavy elements. In this way, feedback shapes galaxy evolution by shutting down star formation and ultimately curtailing the growth of structure after the peak at redshift 2–3. To understand the complex interplay between gravity and feedback, we must resolve both the key physics within galaxies and map the impact of these processes over large scales, out into the cosmic web. The Advanced X-ray Imaging Satellite (AXIS) is a proposed X-ray probe mission for the 2030s with arcsecond spatial resolution, large effective area, and low background. AXIS will untangle the interactions of winds, radiation, jets, and supernovae with the surrounding interstellar medium across the wide range of mass scales and large volumes driving galaxy evolution and trace the establishment of feedback back to the main event at cosmic noon. This white paper is part of a series commissioned for the AXIS Probe mission concept; additional AXIS white papers can be found at the AXIS website.

47 OTHER INSTRUMENTATION↗
NASA NTRS1975

Past orientation of the lunar spin axis

The orientation of the lunar spin axis is traced from the early history of the earth-moon system to the present day. Tides raised on the earth by the moon have caused an expansion of the lunar orbit. Tides raised on the moon by the earth have de-spun the moon to synchronous rotation and driven its spin axis to a Cassini state - that is, in a coprecessing configuration, coplanar with the lunar orbit normal and the normal to the Laplacian plane (which is at present coincident with the normal to the ecliptic). This combination of events has resulted in a complex history for the lunar spin axis. For much of the period during which its orbital semimajor axis expanded between 30 and 40 earth radii, the obliquity of the moon was of order 25 to 50 deg. In fact, for a brief period the obliquity periodically attained a value as high as 77 deg; that is, the spin axis of the moon was only 13 deg from lying in its orbit plane.

Ward, W. R.↗
NASA NTRS1978

Fracture modes in off-axis fiber composites

Criteria were developed for identifying, characterizing, and quantifying fracture modes in high-modulus graphite-fiber/resin unidirectional composites subjected to off-axis tensile loading. Procedures are described which use sensitivity analyses and off-axis data to determine the uniaxial strength of fiber composites. It was found that off-axis composites fail by three fracture modes which produce unique fracture surface characteristics. The stress that dominates each fracture mode and the load angle range of its dominance can be identified. Linear composite mechanics is adequate to describe quantitatively the mechanical behavior of off-axis composites. The uniaxial strengths predicted from off-axis data are comparable to these measured in uniaxial tests.

Sinclair, J. H.↗
NASA NTRS1979

Fracture modes in off-axis fiber composites

Criteria have been developed for identifying, characterizing, and quantifying fracture modes in high-modulus graphite-fiber/resin unidirectional composites subjected to off-axis tensile loading. Procedures are described which use sensitivity analyses and off-axis data to determine the uniaxial strength of fiber composites. It was found that off-axis composites fail by three fracture modes which produce unique fracture surface characteristics. The stress that dominates each fracture mode and the load angle range of its dominance can be identified. Linear composite mechanics is adequate to describe quantitatively the mechanical behavior of off-axis composites. The uniaxial strengths predicted from off-axis data are comparable to those measured in uniaxial tests.

Sinclair, J. H.↗
NASA NTRS1983

Focal axis resolver for offset reflector antennas

Method and apparatus for determining the focal axis of an asymmetrical antenna such as an offset paraboloid reflector whose physical rim is not coincident with the boundary of the electrical aperture but whose focal point is known is provided. A transmitting feed horn array consisting of at least two feed horn elements is positioned asymmetrically on either side of an estimated focal axis which is generally inclined with respect to the boresight axis of the antenna. The feed horn array is aligned with the estimated focal axis so that the phase centers (CP sub 1, CP sub 2) of the two feed horn elements are located on a common line running through the focal point (F) orthogonally with respect to the estimated focal axis.

Schmidt, R. F.↗
NASA NTRS1986

Shear characterization of unidirectional composites with the off-axis tension test

The influence of end constraints on accurate determination of the intralaminar shear modulus G(12) from an off-axis tension test is examined both analytically and experimentally. The Pagano-Halpin (1968) model is employed to illustrate that, when the effect of end constraints is properly considered, the exact expression for G(12) is obtained. When the effect of end constraints is neglected, expressions for the apparent shear modulus G(12)-asterisk and apparent Young's modulus Exx-asterisk are obtained. Numerical comparison for various off-axis configurations and aspect ratios is carried out using typical material properties for graphite/polyimide unidirectional composites. It is demonstrated that the end-constraint effect influences accurate determination of G(12) more adversely than it affects the laminate Young's modulus E(xx) in the low off-axis range. Experimental results obtained from off-axis tests on unidirectional graphite/polyimide specimens confirm the above. Based on the presented analytical and experimental evidence, the 45-deg off-axis coupon is proposed for the determination of the intralaminar shear modulus G(12).

Pindera, M.-J.↗
NASA NTRS1988

Three axis attitude control system

A three-axis attitude control system for an orbiting body comprised of a motor driven flywheel supported by a torque producing active magnetic bearing is described. Free rotation of the flywheel is provided about its central axis and together with limited angular torsional deflections of the flywheel about two orthogonal axes which are perpendicular to the central axis. The motor comprises an electronically commutated DC motor, while the magnetic bearing comprises a radially servoed permanent magnet biased magnetic bearing capable of producing cross-axis torques on the flywheel. Three body attitude sensors for pitch, yaw and roll generate respective command signals along three mutually orthogonal axes (x, y, z) which are coupled to circuit means for energizing a set of control coils for producing torques about two of the axes (x and y) and speed control of the flywheel about the third (z) axis. An energy recovery system, which is operative during motor deceleration, is also included which permits the use of a high-speed motor to perform effectively as a reactive wheel suspended in the magnetic bearing.

Studer, Philip A.↗
NASA NTRS1987

Improved beta-SiC heteroepitaxial films using off-axis Si substrates

All beta-SiC films grown using on-axis (001) Si substrates that have been examined with transmission electron microscopy exhibit a high density of interfacial twins, stacking faults, and antiphase disorder. The antiphase boundaries can be decorated by chemical etching, sputter etching, wet oxidation, and beta-SiC growth in the presence of diborane. All traces of antiphase disorder are eliminated when the heteroepitaxial growth is carried out on vicinal (001) Si substrates that are tilted 2 deg about a 110-line axis. In addition, growth on the off-axis Si produces beta-SiC films that are significantly smoother than on-axis films. The density of stacking faults is apparently unaffected by growth on the off-axis substrates.

Powell, J. A.↗
NASA NTRS1989

Concentration of off-axis radiation by solar concentrators for space power

Four types of off-axis radiation are discussed. These are: (1) small off-axis angles during walk-off, (2) large off-axis angles, (3) an extended off-axis source such as Earth albedo, and (4) miscellaneous off-axis sources including radio frequency sources and local point sources. A code named OFFSET has been developed to represent the solar concentrator being developed for Space Station Freedom. It is a detailed, ray-tracing model which represents 50 ray originating points on the Sun and reflections from 10 points on each of the 456 concentrator facets. Results of this code are generally similar to the PIXEL results although there are small differences due to the more detailed representations of the Sun and concentrator that were used in the OFFSET code.

Jefferies, Kent S.↗
NASA NTRS1993

Concepts for conformal and body-axis attitude information for spatial awareness presented in a helmet-mounted display

A piloted simulation study has been conducted to evaluate two methods of presenting attitude information in a helmet-mounted display (HMD) for spatial awareness in a fighter airplane. One method, the body-axis concept, displayed the information relative to the body axis of the airplane. The quantitative results of this study favored the body-axis concept. Although no statistically significant differences were noted for either the pilots' understanding of roll attitude or target position, the pilots made pitch judgment errors three times more often with the conformal display. The subjective results showed the body-axis display did not cause attitude confusion, a prior concern with this display. In the posttest comments, the pilots overwhelmingly selected the body-axis display as the display of choice.

Jones, Denise R.↗
NASA NTRS1994

Effect of delta tabs on mixing and axis switching in jets from asymmetric nozzles

The effect of delta tabs on mixing and the phenomenon of axis switching in free air jets from various asymmetric nozzles was studied experimentally. Flow visualization and Pitot probe surveys were carried out with a set of small nozzles (D = 1.47 cm) at a jet Mach number, Mj = 1.63. Hot wire measurements for streamwise vorticity were carried out with larger nozzles (D = 6.35 cm) at Mj = 0.31. Jet mixing with the asymmetric nozzles, as indicated by the mass fluxes downstream, was found to be higher than that produced by a circular nozzle. The circular nozzle with four delta tabs, however, produced fluxes much higher than that produced by a asymmetric nozzles themselves or by most of the tab configurations tried with them. Even higher fluxes could be obtained with only a few cases, e.g., with 3:1 rectangular nozzle with two large delta tabs placed on the narrow edges. In this case, the jet 'fanned out' at a large angle after going through one axis switch. The axis switching could be either stopped or augmented with suitable choice of the tab configurations. Two mechanisms are identified governing the phenomenon. One, as described in Ref. 12 and referred to here as the omega(sub Theta)-induced dynamics, is due to differential induced velocities of different segments of a rolled up azimuthal vortical structure. The other is the omega(sub x)-induced dynamics due to the induced velocities of streamwise vortex pairs in the flow. While the former dynamics are responsible for rapid axis switching in periodically forced jets, the effect of the tabs is governed mainly by the latter. It is inferred that both dynamics are active in a natural asymmetric jet issuing from a nozzle having an upstream contraction. The tendency for axis switching caused by the omega(sub Theta)-induced dynamics is resisted by the omega(sub x)-induced dynamics, leading to a delayed or no switch over in that case. In jets from orifices and in screeching jets, the omega(sub Theta)-induced dynamics dominate causing a faster switch over.

Zaman, K. B. M. Q.↗
NASA NTRS1999

Morphology and Structure of ZnO Films Synthesized by Off-Axis Sputtering Deposition

ZnO is a wide-band-gap oxide material that has many applications. A new potential application of ZnO material is for light emitting devices since its structure and electrical properties are similar to that of the GaN material (a blue laser candidate). It also is a good substrate for fabricating GaN-based devices. Off-axis sputtering technique has revealed great potential in synthesizing excellent oxide materials because the negative ion bombardment is greatly reduced when adatoms condense on substrates. The surface of films grown by off-axis sputtering will be much smoother than that produced in a regular sputtering configuration. A growth mechanism is studied by investigating the morphology and structure of ZnO films under different growth conditions and orientations. ZnO films are deposited on (0001) sapphire and quartz substrates by off- axis sputtering deposition at various oxygen/argon mixture ratios and pressures and at different temperatures. All films reveal highly textured structures on quartz substrates and epitaxial growth on sapphire substrates. Two off-axis configurations, vertical and horizontal orientations are conducted to study the process of film growth, surface morphology, and film structure. X-ray diffraction, scanning probe microscopy, and electrical measurements are used to characterize these films. Detailed results will be discussed in the presentation. Keywords: ZnO, Photonic material, Off-axis sputtering, Growth mechanism

Zhu, Shen↗
NASA NTRS2000

Semi-Major Axis Knowledge and GPS Orbit Determination

In recent years spacecraft designers have increasingly sought to use onboard Global Positioning System receivers for orbit determination. The superb positioning accuracy of GPS has tended to focus more attention on the system's capability to determine the spacecraft's location at a particular epoch than on accurate orbit determination, per se. The determination of orbit plane orientation and orbit shape to acceptable levels is less challenging than the determination of orbital period or semi-major axis. It is necessary to address semi-major axis mission requirements and the GPS receiver capability for orbital maneuver targeting and other operations that require trajectory prediction. Failure to determine semi-major axis accurately can result in a solution that may not be usable for targeting the execution of orbit adjustment and rendezvous maneuvers. Simple formulas, charts, and rules of thumb relating position, velocity, and semi-major axis are useful in design and analysis of GPS receivers for near circular orbit operations, including rendezvous and formation flying missions. Space Shuttle flights of a number of different GPS receivers, including a mix of unfiltered and filtered solution data and Standard and Precise Positioning Service modes, have been accomplished. These results indicate that semi-major axis is often not determined very accurately, due to a poor velocity solution and a lack of proper filtering to provide good radial and speed error correlation.

Carpenter, J. Russell↗
NASA NTRS2000

Semi-Major Axis Knowledge and GPS Orbit Determination

In recent years spacecraft designers have increasingly sought to use onboard Global Positioning System receivers for orbit determination. The superb positioning accuracy of GPS has tended to focus more attention on the system's capability to determine the spacecraft's location at a particular epoch than on accurate orbit determination, per se. The determination of orbit plane orientation and orbit shape to acceptable levels is less challenging than the determination of orbital period or semi-major axis. It is necessary to address semi-major axis mission requirements and the GPS receiver capability for orbital maneuver targeting and other operations that require trajectory prediction. Failure to determine semi-major axis accurately can result in a solution that may not be usable for targeting the execution of orbit adjustment and rendezvous maneuvers. Simple formulas, charts, and rules of thumb relating position, velocity, and semi-major axis are useful in design and analysis of GPS receivers for near circular orbit operations, including rendezvous and formation flying missions. Space Shuttle flights of a number of different GPS receivers, including a mix of unfiltered and filtered solution data and Standard and Precise Positioning, Service modes, have been accomplished. These results indicate that semi-major axis is often not determined very accurately, due to a poor velocity solution and a lack of proper filtering to provide good radial and speed error correlation.

Carpenter, J. Russell↗
NASA NTRS1995

Eye movements and motion perception induced by off-vertical axis rotation (OVAR) at small angles of tilt after spaceflight

The nystagmus and motion perception of two astronauts were recorded during Earth-vertical axis rotation and during off-vertical axis rotation (OVAR) before and after 7 days of spaceflight. Postflight, the peak velocity and duration of per- and postrotatory nystagmus during velocity steps about the Earth-vertical axis were the same as preflight values. During OVAR at constant velocity (45/s, tilt angles successively 5, 10, and 15 degrees), the mean horizontal slow-phase eye velocity (bias), produced by the 'velocity storage mechanism' in the vestibular system, and the peak-to-peak amplitude (modulation) in horizontal eye velocity and position, generated from the output of otolith afferents, were also the same before as after flight. There were, however, changes in the vertical eve position and in the perceived body motion during OVAR. The angle of the perceived body path described as a cone was larger in both astronauts postflight. One astronaut experienced either a large cone angle with its axis upright, or a smaller cone angle with its axis tilted backwards, accompanied by an upward vertical eye drift. These results suggest an increase in the sensitivity of the otolithic system after spaceflight and a longer period of readaptation to Earth's gravity for otolith-induced responses than for canal-induced responses. Our data support the hypothesis that just after spaceflight the CNS generally interprets changes in the otolith signals to be due to translation rather than to tilt.

manned↗
NASA NTRS2006

Modification of Eye Movements and Motion Perception during Off-Vertical Axis Rotation

Constant velocity Off-Vertical Axis Rotation (OVAR) imposes a continuously varying orientation of the head and body relative to gravity. The ensuing ocular reflexes include modulation of both torsional and horizontal eye movements as a function of the varying linear acceleration along the lateral plane, and modulation of vertical and vergence eye movements as a function of the varying linear acceleration along the sagittal plane. Previous studies have demonstrated that tilt and translation otolith-ocular responses, as well as motion perception, vary as a function of stimulus frequency during OVAR. The purpose of this study is to examine normative OVAR responses in healthy human subjects, and examine adaptive changes in astronauts following short duration space flight at low (0.125 Hz) and high (0.5 Hz) frequencies. Data was obtained on 24 normative subjects (14 M, 10 F) and 14 (13 M, 1F) astronaut subjects. To date, astronauts have participated in 3 preflight sessions (n=14) and on R+0/1 (n=7), R+2 (n= 13) and R+4 (n= 13) days after landing. Subjects were rotated in darkness about their longitudinal axis 20 deg off-vertical at constant rates of 45 and 180 deg/s, corresponding to 0.125 and 0.5 Hz. Binocular responses were obtained with video-oculography. Perceived motion was evaluated using verbal reports and a two-axis joystick (pitch and roll tilt) mounted on top of a two-axis linear stage (anterior-posterior and medial-lateral translation). Eye responses were obtained in ten of the normative subjects with the head and trunk aligned, and then with the head turned relative to the trunk 40 deg to the right or left of center. Sinusoidal curve fits were used to derive amplitude, phase and bias of the responses over several cycles at each stimulus frequency. Eye responses during 0.125 Hz OVAR were dominated by modulation of torsional and vertical eye position, compensatory for tilt relative to gravity. While there is a bias horizontal slow phase velocity (SPV), the modulation of horizontal and vergence SPV is negligible at this lower stimulus frequency. Eye responses during 0.5 Hz OVAR; however, are characterized by modulation of horizontal and vergence SPV, compensatory for translation in the lateral and sagittal planes, respectively. Neither amplitude nor bias velocities were significantly altered by head-on-trunk position. The phases of the ocular reflexes, on the other hand, shifted towards alignment with the head. During the lower frequency OVAR, subjects reported the perception of progressing along the edge of a cone. During higher frequency OVAR, subjects reported the perception of progressing along the edge of an upright cylinder. In contrast to the eye movements, the phase of both perceived tilt and translation motion is not altered by stimulus frequency. Preliminary results from astronaut data suggest that the ocular responses are not substantially altered by short-duration spaceflight. However, compared to preflight averages, astronauts reported greater amplitude of both perceived tilt and translation at low and high frequency, respectively, during early post-flight testing. We conclude that the neural processing to distinguish tilt and translation linear acceleration stimuli differs between eye movements and motion perception. The results from modifying head-on-trunk position are consistent with the modulation of ocular reflexes during OVAR being primarily mediated by the otoliths in response to the sinusoidally varying linear acceleration along the interaural and naso-occipital head axis. While the tilt and translation ocular reflexes appear to operate in an independent fashion, the timing of perceived tilt and translation influence each other. We conclude that the perceived motion path during linear acceleration in darkness results from a composite representation of tilt and translation inputs from both vestibular and somatosensory systems.

Wood, S. J.↗