12,982 research outputs found
Demonstration of a sterilizable solid rocket motor system
A solid propellant rocket motor containing 60.9 Kg (134-lb) of propellant was successfully static fired after being subjected to eight heat sterilization cycles (three 54-hour cycles plus five 40-hour cycles) at 125 C (257 F). The test motor, a modified SVM-3 chamber, incorporated a flexible grain retention system of EPR rubber to relieve thermal shrinkage stresses. The propellant used in the motor was ANB-3438, and 84 wt% solids system (18 wt% aluminum) containing 66 wt% stabilized ammonium perchlorate oxidizer and a saturated hydroxylterminated polybutadiene binder. Bonding of the propellant to the EPR insulation (GenGard V-4030) was provided by the use of SD-886, an epoxy urethane restriction
Determination of failure limits for sterilizable solid rocket motor
A structural evaluation to establish probable failure limits and a series of environmental tests involving temperature cycling, sustained acceleration, and vibration were conducted on an 18-inch diameter solid rocket motor. Despite the fact that thermal, acceleration and vibration loads representing a severe overtest of conventional environmental requirements were imposed on the sterilizable motor, no structural failure of the grain or flexible support system was detected. The following significant conclusions are considered justified. It is concluded that: (1) the flexible grain retention system, which permitted heat sterilization at 275 F on the test motor, can readily be adopted to meet the environmental requirements of an operational motor design, and (2) if further substantiation of structural integrity is desired, the motor used is considered acceptable for static firing
An Inherited Eye Defect in the Guinea Pig
The data presented in this paper were obtained from the descendants of one defective-eyed male guinea pig that appeared in our colony several years ago. Notation was made of the defective eye, but as the male was being used in another experiment no matings were made at that time to determine whether the condition was genetic. However, when the same type of defect was later observed in the inbred descendants of this male, some matings were made to determine, if possible, the manner of inheritance of the condition
Rubidium and lead abundances in giant stars of the globular clusters M4 and M5
We present measurements of the neutron-capture elements Rb and Pb for bright
giants in the globular clusters M4 and M5. The clusters are of similar
metallicity ([Fe/H] = -1.2) but M4 is decidedly s-process enriched relative to
M5: [Ba/Fe] = +0.6 for M4 but 0.0 for M5. The Rb and Pb abundances were derived
by comparing synthetic spectra with high-resolution, high signal-to-noise ratio
spectra obtained with MIKE on the Magellan telescope. Abundances of Y, Zr, La,
and Eu were also obtained. In M4, the mean abundances from 12 giants are
[Rb/Fe] = 0.39 +/- 0.02 (sigma = 0.07), [Rb/Zr] = 0.17 +/- 0.03 (sigma = 0.08),
and [Pb/Fe] = 0.30 +/- 0.02 (sigma = 0.07). In M5, the mean abundances from two
giants are [Rb/Fe] = 0.00 +/- 0.05 (sigma = 0.06), [Rb/Zr] = 0.08 +/- 0.08
(sigma = 0.11), and [Pb/Fe] = -0.35 +/- 0.02 (sigma = 0.04). Within the
measurement uncertainties, the abundance ratios [Rb/Fe], [Pb/Fe] and [Rb/X] for
X = Y, Zr, La are constant from star-to-star in each cluster and none of these
ratios are correlated with O or Na abundances. While M4 has a higher Rb
abundance than M5, the ratios [Rb/X] are similar in both clusters indicating
that the nature of the s-products are very similar for each cluster but the gas
from which M4's stars formed had a higher concentration of these products.Comment: Accepted for publication in Ap
Investigation of long term stability in metal hydrides
It is apparent from the literature and the results of this study that cyclic degradation of AB(5) type metal hydrides varies widely according to the details of how the specimens are cycled. The Rapid Cycle Apparatus (RCA) used produced less degradation in 5000 to 10000 cycles than earlier work with a Slow Cycle Apparatus (SCA) produced in 1500 cycles. Evidence is presented that the 453 K (356 F) Thermal Aging (TA) time spent in the saturated condition causes hydride degradation. But increasing the cooling (saturation) period in the RCA did not greatly increase the rate of degradation. It appears that TA type degradation is secondary at low temperatures to another degradation mechanism. If rapid cycles are less damaging than slow cycles when the saturation time is equal, the rate of hydriding/dehydriding may be an important factor. The peak temperatures in the RCA were about 30 C lower than the SCA. The difference in peak cycle temperatures (125 C in the SCA, 95 C in RCA) cannot explain the differences in degradation. TA type degradation is similar to cyclic degradation in that nickel peaks and line broadening are observed in X ray diffraction patterns after either form of degradation
Mathematical properties and numerical approximation of pseudo-parabolic systems
The paper is concerned with the mathematical theory and numerical
approximation of systems of partial differential equations (pde) of hyperbolic,
pseudo-parabolic type. Some mathematical properties of the
initial-boundary-value problem (ibvp) with Dirichlet boundary conditions are
first studied. They include the weak formulation, well-posedness and existence
of traveling wave solutions connecting two states, when the equations are
considered as a variant of a conservation law. Then, the numerical
approximation consists of a spectral approximation in space based on Legendre
polynomials along with a temporal discretization with strong stability
preserving (SSP) property. The convergence of the semidiscrete approximation is
proved under suitable regularity conditions on the data. The choice of the
temporal discretization is justified in order to guarantee the stability of the
full discretization when dealing with nonsmooth initial conditions. A
computational study explores the performance of the fully discrete scheme with
regular and nonregular data
Can bars be destroyed by a central mass concentration? I. Simulations
We study the effect of a central mass concentration (CMC) on the secular
evolution of a barred disc galaxy. Unlike previous studies, we use fully
self-consistent 3D -body simulations with live haloes, which are known to be
important for bar evolution. The CMC is introduced gradually, to avoid
transients. In all cases where the mass of the CMC is of the order of, or more
than, a few per cent of the mass of the disc, the strength of the bar decreases
noticeably. The amount of this decrease depends strongly on the bar type. For
the same CMC, bars with exponential surface-density profile, which formed in a
disk-dominated galaxy (MD-type bars), can be totally destroyed, while strong
bars with a flat surface-density profile, whose evolution is largely due to the
halo (MH-type bars), witness only a decrease of their strength. This decrease
occurs simultaneously from both the innermost and outermost parts of the bar.
The CMC has a stronger effect on the Fourier components of higher azimuthal
wave number , leading to fatter and/or less rectangular bars. Furthermore,
the CMC changes the side-on outline from peanut-shaped to boxy or, for massive
CMCs, to elliptical. Similarly, side-on initially boxy outlines can be
destroyed. The CMC also influences the velocity dispersion profiles. Most of
the decrease of the bar strength occurs while the mass of the CMC increases and
it is accompanied by an increase of the pattern speed. In all our simulations,
the mass of the CMC necessary in order to destroy the bar is at least several
per cent of the mass of the disc. This argues that observed super-massive black
holes are not likely to destroy pre-existing bars.Comment: 13 pages, 8 figures, accepted for publication in MNRA
One step multiderivative methods for first order ordinary differential equations
A family of one-step multiderivative methods based on Padé approximants to the exponential function is developed.
The methods are extrapolated and analysed for use in PECE mode.
Error constants and stability intervals are calculated and the combinations compared with well known linear multi-step combinations and combinations using high accuracy Newton-Cotes quadrature formulas as correctors.
w926020
Rubidium in Metal-Deficient Disk and Halo Stars
We report the first extensive study of stellar Rb abundances. High-resolution
spectra have been used to determine, or set upper limits on, the abundances of
this heavy element and the associated elements Y, Zr, and Ba in 44 dwarfs and
giants with metallicities spanning the range -2.0 <[Fe/H] < 0.0. In
metal-deficient stars Rb is systematically overabundant relative to Fe; we find
an average [Rb/Fe] of +0.21 for the 32 stars with [Fe/H] < -0.5 and measured
Rb. This behavior contrasts with that of Y, Zr, and Ba, which, with the
exception of three new CH stars (HD 23439A and B and BD +5 3640), are
consistently slightly deficient relative to Fe in the same stars; excluding the
three CH stars, we find the stars with [Fe/H] < -0.5 have average [Y/Fe],
[Zr/Fe], and [Ba/Fe] of --0.19 (24 stars), --0.12 (28 stars), and --0.06 (29
stars), respectively. The different behavior of Rb on the one hand and Y, Zr,
and Ba on the other can be attributed in part to the fact that in the Sun and
in these stars Rb has a large r-process component while Y, Zr, and Ba are
mostly s-process elements with only small r-process components. In addition,
the Rb s-process abundance is dependent on the neutron density at the
s-processing site. Published observations of Rb in s-process enriched red
giants indicate a higher neutron density in the metal-poor giants. These
observations imply a higher s-process abundance for Rb in metal-poor stars. The
calculated combination of the Rb r-process abundance, as estimated for the
stellar Eu abundances, and the s-process abundance as estimated for red giants
accounts satisfactorily for the observed run of [Rb/Fe] with [Fe/H].Comment: 23 pages, 5 tables, 7 figure
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