8,032 research outputs found
Integral throat entrance development, qualification and production for the Antares 3 nozzle
Although design analyses of a G-90 graphite integral throat entrance for the Antares 3 solid rocket motor nozzle indicated acceptable margins of safety, the nozzle throat insert suffered a thermostructural failure during the first development firing. Subsequent re-analysis using properties measured on material from the same billet as the nozzle throat insert showed negative margins. Carbon-carbon was investigated and found to result in large positive margins of safety. The G-90 graphite was replaced by SAI fast processed 4-D material which uses Hercules HM 10000 fiber as the reinforcement. Its construction allows powder filling of the interstices after preform fabrication which accelerates the densification process. Allied 15V coal tar pitch is then used to complete densification. The properties were extensively characterized on this material and six nozzles were subjected to demonstration, development and qualification firings
An expert system interfaced with a database system to perform troubleshooting of aircraft carrier piping systems
Maintaining and troubleshooting aircraft carrier through tank piping systems is a labor intensive, operational fleet problem. There is a clear need for a useful database and expert system to aid in fault isolation and repair planning for these systems. The multiple extensive piping systems of an aircraft carrier create an intimidating modeling problem for implementation in a database. The interface of an expert system to a large database to obtain improved execution speed, exploit a useful data model, reduce memory requirements, and enhance total system capability is examined and implemented, A flexible model for representing a large ship's piping systems in a database is presented.http://archive.org/details/expertsysteminte00clayCommander, United States NavyLieutenant, United States NavyApproved for public release; distribution is unlimited
Matching Conditions in Atomistic-Continuum Modeling of Materials
A new class of matching condition between the atomistic and continuum regions
is presented for the multi-scale modeling of crystals. They ensure the accurate
passage of large scale information between the atomistic and continuum regions
and at the same time minimize the reflection of phonons at the interface. These
matching conditions can be made adaptive if we choose appropriate weight
functions. Applications to dislocation dynamics and friction between
two-dimensional atomically flat crystal surfaces are described.Comment: 6 pages, 4 figure
Variable-Speed-of-Light Cosmology and Second Law of Thermodynamics
We examine whether the cosmologies with varying speed of light (VSL) are
compatible with the second law of thermodynamics. We find that the VSL
cosmology with varying fundamental constant is severely constrained by the
second law of thermodynamics, whereas the bimetric cosmological models are less
constrained.Comment: 15 pages, LaTeX, Revised version with minor corrections to appear in
Phys. Rev.
Alpha Element Abundances in a Large Sample of Galactic Planetary Nebulae
We present emission line strengths, abundances, and element ratios (X/O for
Ne, S, Cl, and Ar) for a sample of 38 Galactic disk planetary nebulae (PNe)
consisting primarily of Peimbert classification Type I. Spectrophotometry for
these PNe incorporates an extended optical/near-IR range of 3600-9600 angstroms
including the [S III] lines at 9069 and 9532. We have utilized Emission Line
Spectrum Analyzer, a five-level atom abundance routine, to determine T_e, N_e,
ionization correction factors, and total element abundances. With a compilation
of data from >120 Milky Way PNe, we present results from our most recent
analysis of abundance patterns in Galactic disk PNe. We have examined the alpha
elements against H II regions and blue compact galaxies (H2BCG) to discern
signatures of depletion or enhancement in PNe progenitor stars, particularly
the destruction or production of O and Ne. We present evidence that many PNe
have higher Ne/O and lower Ar/Ne ratios compared to H2BCGs within the range of
8.5-9.0 for 12 + log(O/H). This suggests that Ne is being synthesized in the
low- and intermediate-mass progenitors. Sulfur abundances in PNe continue to
show great scatter and are systematically lower than those found in H2BCG at a
given metallicity. Although we find that PNe do show some distinction in alpha
elements when compared to H2BCG, within the Peimbert classification types
studied, PNe do not show significant differences in alpha elements amongst
themselves, at least to an extent that would distinguish in situ
nucleosynthesis from the observed dispersion in abundance ratios.Comment: 12 pages, 18 figures, 7 tables (note: tables 2-5 are available online
only in machine-readable form
CHEMICALLY MODIFIED PHOTOSYNTHETIC BACTERIAL REACTION CENTERS: CIRCULAR DICHROISM, RAMAN RESONANCE, LOW TEMPERATURE ABSORPTION, FLUORESCENCE AND ODMR SPECTRA AND POLYPEPTIDE COMPOSITION OF BOROHYDRIDE TREATED REACTION CENTERS FROM Rhodobacter sphaeroides R26
Reaction centers from Rhodobacter sphaeroides have been modified by treatment with sodium borohydride similar to the original procedure [Ditson et al., Biochim. Biophys. Acta 766, 623 (1984)], and investigated spectroscopically and by gel electrophoresis.
(1) Low temperature (1.2 K) absorption, fluorescence, absorption- and fluorescence-detected ODMR, and microwave-induced singlet-triplet absorption difference spectra (MIA) suggest that the treatment produces a spectroscopically homogeneous preparation with one of the âadditionalâ bacteriochlorophylls being removed. The modification does not alter the zero field splitting parameters of the primary donor triplet (TP870).
(2) From the circular dichroism and Raman resonance spectra in the1500â1800 cm-1 region, the removed pigment is assigned to BchlM, e.g. the "extra" Bchl on the "inactive" M-branch.
(3) A strong coupling among all pigment molecules is deduced from the circular dichroism spectra, because pronounced band-shifts and/or intensity changes occur in the spectral components assigned to all pigments. This is supported by distinct differences among the MIA spectra of untreated and modified reaction centers, as well as by Raman resonance.
(4) The modification is accompanied by partial proteolytic cleavage of the M-subunit. The preparation is thus spectroscopically homogeneous, but biochemically heterogenous
Are Ti44-Producing Supernovae Exceptional?
According to standard models supernovae produce radioactive Ti, which
should be visible in gamma-rays following decay to Ca for a few
centuries. ^{44}^{44}\tau \simeq\simeq\simeq^{44}Ti gamma-ray sources, but only
one is clearly seen, the 340-year-old Cas A SNR. Furthermore, supernovae which
produce much ^{44}Ti yields, their spatial
distributions, and statistical arguments can be stretched so that this apparent
disagreement may be accommodated within reasonable expectations, or if we have
to revise some or all of the above aspects to bring expectations in agreement
with the observations. We conclude that either core collapse supernovae have
been improbably rare in the Galaxy during the past few centuries, or
^{44}^{40}^{44}$Ca.Comment: 23 pages, 14 figures, accepted for publication in Astronomy and
Astrophysics 2006. Correcting the SN type of Tycho in Table B.1. and add its
associated reference
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