25,451 research outputs found
Propellant/material compatibility program and results: Ten-year milestones
The analyses and results of a test program to establish the effects of long term (10 years or more) contact of materials with earth-storable propellants for the purpose of designing chemical propulsion system components which are used for current as well as future planetary spacecraft are described. The period from the publication of JPL TM 33-779 IN 1976 through the testing accomplished in 1981 is covered. The following propellants are reported herein: hydrazine, monomethylhydrazine and nitrogen tetroxide. Materials included the following: aluminum alloys, corrosion resistant steels and a titanium alloy. The results of the testing of more than 80 specimens are included. Material ratings relative to the ten year milepost were assigned. Some evidence of propellant decomposition was found. Titanium is rated as acceptable for ten year applications. Aluminum and stainless steel alloys are also rated as acceptable with few restrictions
Analysis of the coupled frequencies and mode shapes of vehicle control systems Final report, 15 Apr. 1965 - 14 Apr. 1966
Coupled frequencies and mode shapes of vehicle control syste
Globular Cluster Formation in the Virgo Cluster
Metal poor globular clusters (MPGCs) are a unique probe of the early
universe, in particular the reionization era. Systems of globular clusters in
galaxy clusters are particularly interesting as it is in the progenitors of
galaxy clusters that the earliest reionizing sources first formed. Although the
exact physical origin of globular clusters is still debated, it is generally
admitted that globular clusters form in early, rare dark matter peaks (Moore et
al. 2006; Boley et al. 2009). We provide a fully numerical analysis of the
Virgo cluster globular cluster system by identifying the present day globular
cluster system with exactly such early, rare dark matter peaks. A popular
hypothesis is that that the observed truncation of blue metal poor globular
cluster formation is due to reionization (Spitler et al. 2012; Boley et al.
2009; Brodie & Strader 2006); adopting this view, constraining the formation
epoch of MPGCs provides a complementary constraint on the epoch of
reionization. By analyzing both the line of sight velocity dispersion and the
surface density distribution of the present day distribution we are able to
constrain the redshift and mass of the dark matter peaks. We find and quantify
a dependence on the chosen line of sight of these quantities, whose strength
varies with redshift, and coupled with star formation efficiency arguments find
a best fitting formation mass and redshift of and . We predict intracluster MPGCs in
the Virgo cluster. Our results confirm the techniques pioneered by Moore et al.
(2006) when applied to the the Virgo cluster and extend and refine the analytic
results of Spitler et al. (2012) numerically.Comment: 13 Pages, 13 Figures, submitted to MNRA
A new magnetic white dwarf : PG2329+267
We have discovered that the white dwarf PG 2329+267 is magnetic, and assuming
a centered dipole structure, has a dipole magnetic field strength of
approximately 2.3MG. This makes it one of only approximately 4% of isolated
white dwarfs with a detectable magnetic field. Linear Zeeman splitting as well
as quadratic Zeeman shifts are evident in the hydrogen Balmer sequence and
circular spectropolarimetry reveals 10% circular polarisation in the two
displaced sigma components of Halpha. We suggest from comparison with spectra
of white dwarfs of known mass that PG 2329+267 is more massive than typical
isolated white dwarfs, in agreement with the hypothesis that magnetic white
dwarfs evolve from magnetic chemically peculiar Ap and Bp type main sequence
stars.Comment: 5 pages, with 2 encapsulated postscipts figures include
Sealed containers in Z
Physical means of securing information, such as sealed envelopes and scratch cards, can be used to achieve cryptographic objectives. Reasoning about this has so far been informal.
We give a model of distinguishable sealed envelopes in Z, exploring design decisions and further analysis and development of such models
Test program to demonstrate the stability of hydrazine in propellant tanks
A 24-month coupon test program to evaluate the decomposition of propellant tanks is reported. The propellant fuel evaluated was monopropellant-grade hydrazine (N2H4), which is normally a colorless, fuming, corrosive, strongly reducing liquid. The degree of hydrazine decomposition was determined by means of chemical analyses of the liquid and evolved gases at the end of the test program. The experimental rates of hydrazine decomposition were determined to be within acceptable limits. The propellant tank materials and material combinations were not degraded by a 2-year exposure to hydrazine propellant. This was verified using change-of-weight determinations and microscopic examination of the specimen surface before and after exposure, and by posttest chemical analyses of hydrazine liquid for residual metal content
Income and the Use of Prescription Drugs by the Elderly: Evidence from the Notch Cohorts
We use exogenous variation in Social Security payments created by the Social Security benefits notch to estimate how retirees' use of prescription medications responds to changes in their incomes. In contrast to estimates obtained using ordinary least squares, instrumental variables estimates based on the notch suggest that lower-income retirees exhibit considerable income sensitivity in their use of prescription drugs. Our estimates are potentially useful for thinking about the health care usage implications of any changes in transfer payments to the elderly that may occur in the future, and for evaluating the benefits of the recently enacted Medicare prescription drug benefit.
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