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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1997

Structural Qualification of Composite Airframes

The development of fundamental approaches for predicting failure and elongation characteristics of fibrous composites are summarized in this document. The research described includes a statistical formulation for individual fiber breakage and fragmentation and clustered fiber breakage, termed macrodefects wherein the aligned composite may represent a structural component such as a reinforcing bar element, a rebar. Experimental work conducted in support of the future exploitation of aligned composite rebar elements is also described. This work discusses the experimental challenges associated with rebar tensile test evaluation and describes initial numerical analyses performed in support of the experimental program.

Kedward, Keith T.↗
NASA NTRS1978

Shear rupture of a directionally solidified eutectic gamma/gamma-prime - alpha /Mo/ alloy

Directionally solidified gamma/gamma-prime - alpha (Mo) eutectic alloys are being evaluated for application as advanced aircraft engine turbine blades. Their excellent high-temperature strength is partly due to their directionally aligned microstructure. However, alloys with such directional structures may display low shear strength at 760 C, the operating temperature of advanced blade roots. The objective of this investigation was to determine the shear rupture strength of the gamma/gamma-prime - alpha eutectic alloy and possibly to improve it by microstructural and heat-treatment variations. Bars of gamma/gamma-prime - alpha alloy containing nominally 5.7% Al and 33.5% Mo by weight with balance Ni were directionally solidified at rates between 10 and 100 mm per hour. Materials were solidified in furnaces with thermal gradients at the liquid-solid interface of 250 or 100 C per cm. A limited number of longitudinal shear rupture tests were conducted at 760 C and 207 MPa in the as-solidified and in several heat-treated conditions. It was found that the shear rupture failures are partly transgranular and that resistance to failure is promoted by good fiber alignment and a matrix structure consisting mainly of gamma-prime. Well-aligned as-solidified specimens sustained the shear stress for an average of 81 hours, while cellular material failed in one hour or less.

Harf, F. H.↗
NASA NTRS1991

An unusually strong Einstein ring in the radio source PKS1830 - 211

High-resolution radio images of PKS1830 - 211 are obtained to study the possibility that the double structure is a gravitationally lensed object. The VLBI observations, taken from interferometric radiotelescope networks, reveal an elliptical ring that connects two bright spots of similar composition. Because the lens and the lensed object are closely aligned, and because of the structure of the two spots, the source is concluded to be a radio Einstein ring. The source is found to be close to the galactic plane, and the lens and the lensed object are extragalactic. The source is also found to be unusually bright, suggesting that it is aligned with a bright background source or amplified by some mechanism related to a source that is not so bright.

Jauncey, D. L.↗
NASA NTRS1995

An IRAS/ISSA Survey of Bow Shocks Around Runaway Stars

We searched for bow shock-like objects like those known around Oph and a Cam near the positions of 183 runaway stars. Based primarily on the presence and morphology of excess 60 micron emission we identify 56 new candidate bow shocks, for which we determine photometric and morphological parameters. Previously only a dozen or so were known. Well resolved structures are present around 25 stars. A comparison of the distribution of symmetry axes of the infrared nebulae with that of their proper motion vectors indicates that these two directions are very significantly aligned. The observed alignment strongly suggests that the structures we see arise from the interaction of stellar winds with the interstellar medium, justifying the identification of these far-infrared objects as stellar wind bow shocks.

IRAS/ISSA Survey↗
NASA NTRS1977

Opto-mechanical subsystem with temperature compensation through isothemal design

An opto-mechanical subsystem for supporting a laser structure which minimizes changes in the alignment of the laser optics in response to temperature variations is described. Both optical and mechanical structural components of the system are formed of the same material, preferably beryllium, which is selected for high mechanical strength and good thermal conducting qualities. All mechanical and optical components are mounted and assembled to provide thorough thermal coupling throughout the subsystem to prevent the development of temperature gradients.

Goodwin, F. E.↗
NASA NTRS2008

High-velocity Interstellar Bullets in IRAS 05506+2414: A Very Young Protostar

We have made a serendipitous discovery of an enigmatic outflow source, IRAS 05506+2414 (hereafter IRAS 05506), as part of a multiwavelength survey of pre-planetary nebulae (PPNs). The HST optical and near-infrared images show a bright compact central source with a jet-like extension, and a fan-like spray of high-velocity (with radial velocities up to 350 km/s) elongated knots which appear to emanate from it. These structures are possibly analogous to the near-IR bullets'' seen in the Orion Nebula. Interferometric observations at 2.6 mm show the presence of a continuum source and a high-velocity CO outflow, which is aligned with the optical jet structure. IRAS 05506 is most likely not a PPN. We find extended NH3 (1,1) emission toward IRAS 05506; these data, together with the combined presence of far-IR emission, H2O and OH masers, and CO and CS J=2-1 emission, strongly argue for a dense, dusty star-forming core associated with IRAS 05506. IRAS 05506 is probably an intermediate-mass or massive protostar, and the very short timescale (200 yr) of its outflows indicates that it is very young. If IRAS 05506 is a massive star, then the lack of radio continuum and the late G to early K spectral type we find from our optical spectra imply that in this object we are witnessing the earliest stages of its life, while its temperature is still too low to provide sufficient UV flux for ionization.

star formation↗
NASA NTRS1994

New very large array maps of highly core-dominated BL Lacs: Testing unified schemes

We present high dyanamic range (greater than 10 000:1) Very Large Array (VLA) maps of four highly c ore-dominated BL Lacertae Objects (PKS 0735+178, OJ 287, 1219+285, and PKS 1413+135), all of which had been reported previously as being unresolved at dynamic ranges approximately 1000-3000:1. Three of the four possess low surface brightness kpc-scale structure, with extended power levels and morphologies reasonable for FR 1 radio galaxies. One, however (PKS 1413+135), possesses no extended structure at dynamic range 9600:1. Only one of the sources exhibits aligned Very Large Array (VLA) and VLBI structures, suggesting bends on approximately 100-1000 pc size scales for the other two. As these four have extremely high core-to-extended flux ratios (f approximately 30-5000), they provide an extreme test of unified schemes. By deriving minimum constraints for the beaming angles and Lorentz factors which are required to produce the degrees of core dominance seen, we find that the three extended radio sources are consistent with being beamed FR 1 radio galaxies. PKS 1413+135 remains an anomalous BL Lac Object for several reasons.

Perlman, Eric S.↗
NASA NTRS1990

The structure of the Birkeland current system in the post-midnight plasma sheet

Simultaneous magnetic field and particle measurements by AMPTE/CCE with magnetic field measurements by GOES 5 and GOES 6 clarify the structure of the field-aligned Birkeland current system in the post-midnight region of the near-earth magnetotail. The current system, consisting of current sheets flowing in opposite directions, exists near the boundary region of the plasma sheet; its magnetic effects are confined mainly to the current region because of its structure. During the expansion phase of substorms, a single-sheet current system is formed in the boundary region of the plasma sheet and net current is probably present in one meridian plane. This type of current system produces the substorm-related transient variations in the azimuthal (D) component normally observed at synchronous orbit.

Nagai, Tsugunobu↗
NASA NTRS1982

Large scale X-ray and radio structures associated with compact extragalactic sources

Knots of X-ray emission have been detected within 20 arcmin of five compact sources initially selected from the MPIfR north polar 5 GHz survey. Two of the knots have also been detected at centimeter wavelengths and probably have nonthermal spectra. They appear to be associated with the compact sources since the probability of serendipitous discovery at the observed flux levels is low. While the apparent association may be due to colocation of the sources in a distant supercluster, it is suggested on the basis of overall alignment, and possible correlations with structures in the respective central sources, that the association may be similar to that found in extended radio sources. The observed emission may thus be due to synchrotron or inverse Compton radiation, the energy being supplied by jets from the central source.

Biermann, P.↗
NASA NTRS1994

Numerical simulation of large-scale field-aligned current generation from finite-amplitude magnetosonic waves

A two-dimensional numerical simulation of finite-amplitude magnetohydrodynamic (MHD) magnetosonic waves is performed under a finite-velocity background convection condition. Isothermal cases are considered for simplicity. External dissipation is introduced by assuming that the field-aligned currents are generated in proportion to the accumulated charges. The simulation results are as follows: Paired field-aligned currents are found from the simulated waves. The flow directions of these field-aligned currents depend on the angle between the background convection and the wave normal, and hence two pairs of field-aligned currents are found from a bowed wave if we look at the overall structure. The majority of these field-aligned currents are closed within each pair rather than between two wings. These features are not observed under slow background convection. The result could be applied to the cusp current system and the substorm current system.

Yamauchi, M.↗
NASA NTRS1984

High-altitude observations of an intense inverted V event

Inverted-V events which generally occur in the pre-midnight sector over the auroral zone are frequently associated with reversals in the convection electric field. Such a reversal is observed by the University of Iowa quasispherical LEPEDEA on board ISEE 1 at an altitude of 13 RE on May 1, 1978. The bulk of the plasma shows a large shear over a five-minute interval. The associated change in the convection electric field is 5.1 mV/m. Large values of the field-aligned current are simultaneously detected. The potential structure appears to extend to the satellite location. Using a theoretical model, the field-aligned current due to the electric field discontinuity has been calculated. The magnitude of the parallel potential drop and width of the inverted-V region agree well with observation.

Huang, C. Y.↗
NASA NTRS1995

Electron structures in the cusp/cleft region observed by DE 2 satellite

Narrow enhancements of electron precipitation, with energy and flux well above typical values, have been observed with Dynamics Explorer 2 (DE 2) in the cusp/cleft region. The electron flux in the energy range 0.2-1 keV was 2 orders of magnitude higher in these structures than in the magnetosheath and were seen in approximaetly 80% of DE 2 cusp crossings at ionospheric altitudes. Typically, there was more than one electron structure in each cusp crossing. The position of these structures showed a systematic variation: for poleward ion dispersion (energy decreases with increasing latitude), electron structures were seen more often on the equatorial boundary of the cusp, while for equatorward ion dispersion (energy decreases with decreasing latitude), electron structures were more often seen on the poleward boundary. This suggests that the electron structures are associated with newly reconnected field lines. The electron spectra suggest that field-aligned acceleration processes could produce the electron structures, first near the boundary of the cusp/cleft during the reconnection of field lines and then in the cusp/cleft during the motion of reconnected flux tubes through the polar ionosphere.

Escoubet, C. P.↗
NASA NTRS1974

Current system in an auroral substorm

An instrumented sounding rocket was launched at 0040 LT into the expansive phase of an auroral substorm on March 16, 1973, from Poker Flat, Alaska. This report describes coordinated measurements of electric field, magnetic field, and energetic electrons. By comparing the observed magnetic field with that predicted by model currents, a current system, which includes field-aligned currents, is deduced. The model indicates three currents: a southward current, a westward current, and two field-aligned currents. A southward electric field of 60 mV/m identifies the southward current as a Pedersen current and the westward current as a Hall current. The field-aligned current measured from particle detectors shows a structure similar to that in the magnetic model. The total field-aligned electron current measured by particle detectors agrees within 10% of the current model.

Kintner, P. M.↗
NASA NTRS1988

An approach to optical structures control

The stabilization of a large, spaceborne Cassegrain telescope is examined. Modal gain factors and known characteristics of disturbances are used to determine which structural model affect line of sight (LOS) the most and are candidates for active control. THe approach is to: (1) actively control and maintain alignment of optical components; (2) place structural control actuators for optimum impact on the selected modes for active vibration control; and (3) feed back the best available estimate of LOS error for direct LOS control. Local analog loops are used for lower bandwidth control and multivariable digital control for lower bandwidth control. The control law is synthesized in the frequency domain using the characteristic gain approach. Robustness is measured by employing conicity, which is an outgrowth of the positivity approach to robust feedback system design. The feasibility of the design approach will be demonstrated by conducting a lab experiment on a structure similar to a scaled version of the telescope.

Iwens, R. P.↗
NASA NTRS2022

The Polstar High Resolution Spectropolarimetry MIDEX Mission

The Polstar mission will provide a space-borne 60 cm spectropolarimeter operating at ultraviolet (UV) wavelengths, capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter and inform massive star evolution, affect the stellar remnant population, and stir and enrich the interstellar medium (ISM). These will be achieved by investigating the dynamical geometries in the winds and disks of hot stars, the composition and magnetic alignment of interstellar dust, and the star-forming accretion disks of UV-bright stars at an important transition boundary. Together these areas map out a kind of two-way interface between massive stars and their effect on our galaxy, wherein the stellar winds enrich the ISM with metals and kinetic energy, preconditioning their environment and the stellar endpoints prior to undergoing supernova. The ISM dust in turn reveals the composition and magnetic environment leading to new star formation, and the accretion disks of Herbig Ae/Be stars reveal how the ISM gas returns to make new massive stars. Polstar will combine high-resolution spectroscopy in the time domain with high-precision UV polarimetry. Doppler-shifted UV resonance line opacity will provide information about circumstellar kinematics, while polarization gives complementary geometric information about unseen structures. The composition and magnetic alignment of the smallest interstellar dust grains provides a probe of the ISM utilizing radiative alignment theory (RAT). Polstar will operate in the far-UV (FUV) at 122–200 nm at high spectral resolution of around R ∼ 30k, and at FUV and near-UV (NUV) wavelengths of 122–320 nm at lower spectral resolutions of 0.1 - 1k. Detection of polarization levels as weak as 0.1% are expected, with a temporal cadence ranging from 5–10 minutes for most wind variability studies, to hours or days for sampling rotation, to days or weeks for sampling binary orbits, to months to a year for sampling substructure in the inner regions of protoplanetary disks. Sub-meter-class aperture is well suited to access this wide array of time domain science, made possible by restricting to a few hundred bright, massive stars, necessarily extincted by a small to moderate column of interstellar dust, informing both the attributes of the stars and the ISM through which they are seen. As such, the focus is on our own galaxy and its evolutionary drivers, but a few targets in the Magellanic clouds offer the potential to extend this understanding to low-metallicity environments.

Paul A Scowen↗
NASA NTRS2017

A Mobile Robot for Locomotion Through a 3D Periodic Lattice Environment

This paper describes a novel class of robots specifically adapted to climb periodic lattices, which we call 'Relative Robots'. These robots use the regularity of the structure to simplify the path planning, align with minimal feedback, and reduce the number of degrees of freedom (DOF) required to locomote. They can perform vital inspection and repair tasks within the structure that larger truss construction robots could not perform without modifying the structure. We detail a specific type of relative robot designed to traverse a cuboctahedral (CubOct) cellular solids lattice, show how the symmetries of the lattice simplify the design, and test these design methodologies with a CubOct relative robot that traverses a 76.2 mm (3 in.) pitch lattice, MOJO (Multi-Objective JOurneying robot). We perform three locomotion tasks with MOJO: vertical climbing, horizontal climbing, and turning, and find that, due to changes in the orientation of the robot relative to the gravity vector, the success rate of vertical and horizontal climbing is significantly different.

Aeronautics↗
NASA NTRS2008

Recent Developments in the Alignment and Test Plans for the James Webb Space Telescope Integrated Science Instrument Module

The James Webb Space Telescope (JWST) is a 6.6m diameter, segmented, deployable telescope for cryogenic IR space astronomy (approximately 40K). The JWST Observatory architecture includes the Optical Telescope Element (OTE) and the Integrated Science Instrument Module (ISIM) element that contains four science instruments (SI) including a Guider. The SIs and Guider are mounted to a composite metering structure with outer dimensions of 2.1 x 2.2 x 1.9m. The SI and Guider units are integrated to the ISIM structure and optically tested at NASA/Goddard Space Flight Center as an instrument suite using an OTE SIMulator (OSIM). OSIM is a high-fidelity, cryogenic JWST telescope simulator that features a approximately 1.5m diameter powered mirror. The SIs are aligned to the structure's coordinate system under ambient, clean room conditions using laser tracker and theodolite metrology. Temperature-induced mechanical SI alignment and structural changes are measured using a photogrammetric measurement system at ambient and cryogenic temperatures. OSIM is aligned to the ISIM mechanical coordinate system at the cryogenic operating temperature via internal mechanisms and feedback from alignment sensors in six degrees of freedom. SI performance, including focus, pupil shear and wavefront error, is evaluated at the operating temperature using OSIM. We present an updated plan for the assembly and ambient and cryogenic optical alignment, test and verification of the ISIM element.

Ohl, Raymond↗
NASA NTRS1999

A new version of the RDP (Ribosomal Database Project)

The Ribosomal Database Project (RDP-II), previously described by Maidak et al. [ Nucleic Acids Res. (1997), 25, 109-111], is now hosted by the Center for Microbial Ecology at Michigan State University. RDP-II is a curated database that offers ribosomal RNA (rRNA) nucleotide sequence data in aligned and unaligned forms, analysis services, and associated computer programs. During the past two years, data alignments have been updated and now include >9700 small subunit rRNA sequences. The recent development of an ObjectStore database will provide more rapid updating of data, better data accuracy and increased user access. RDP-II includes phylogenetically ordered alignments of rRNA sequences, derived phylogenetic trees, rRNA secondary structure diagrams, and various software programs for handling, analyzing and displaying alignments and trees. The data are available via anonymous ftp (ftp.cme.msu. edu) and WWW (http://www.cme.msu.edu/RDP). The WWW server provides ribosomal probe checking, approximate phylogenetic placement of user-submitted sequences, screening for possible chimeric rRNA sequences, automated alignment, and a suggested placement of an unknown sequence on an existing phylogenetic tree. Additional utilities also exist at RDP-II, including distance matrix, T-RFLP, and a Java-based viewer of the phylogenetic trees that can be used to create subtrees.

Non-NASA Center↗