Cryogenic upper stages in the Space Shuttle program were prohibited primarily due to a safety risk of a 'return to launch site' abort. An upper stage concept addressed this concern by proposing that the stage be launched empty and filled using shuttle external tank residuals after the atmospheric pressure could no longer sustain an explosion. However, only about 5 minutes was allowed for tank fill. Liquid hydrogen testing was conducted within a near-ambient environment using the multipurpose hydrogen test bed 638.5 ft3 (18m3) cylindrical tank with a spray bar mounted longitudinally inside. Although the tank was filled within 5 minutes, chilldown of the tank structure was incomplete, and excessive tank pressures occurred upon vent valve closure. Elevated tank wall temperatures below the liquid level were clearly characteristic of film boiling. The test results have substantial implications for on-orbit cryogen transfer since the formation of a vapor film would be much less inhibited due to the reduced gravity. However, the heavy tank walls could become an asset in normal gravity testing for on-orbit transfer, i.e., if film boiling in a nonflight weight tank can be inhibited in normal gravity, then analytical modeling anchored with the data could be applied to reduced gravity environments with increased confidence.

关键词：低温液体；流体管；液态氢；膜态沸腾；传热

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Under the Advanced Exploration Systems (AES) Spacecraft Fire Safety Demonstration Project (SFSDP), as a risk mitigation activity in support of the development of a large-scale fire demonstration experiment in microgravity, flame-spread tests were conducted in normal gravity on thin, cellulose-based fuels in a sealed chamber. The primary objective of the tests was to measure pressure rise in a chamber as sample material, burning direction (upward/downward), total heat release, heat release rate, and heat loss mechanisms were varied between tests. A Design of Experiments (DOE) method was imposed to produce an array of tests from a fixed set of constraints and a coupled response model was developed. Supplementary tests were run without experimental design to additionally vary select parameters such as initial chamber pressure. The starting chamber pressure for each test was set below atmospheric to prevent chamber overpressure. Bottom ignition, or upward propagating burns, produced rapid acceleratory turbulent flame spread. Pressure rise in the chamber increases as the amount of fuel burned increases mainly because of the larger amount of heat generation and, to a much smaller extent, due to the increase in gaseous number of moles. Top ignition, or downward propagating burns, produced a steady flame spread with a very small flat flame across the burning edge. Steady-state pressure is achieved during downward flame spread as the pressure rises and plateaus. This indicates that the heat generation by the flame matches the heat loss to surroundings during the longer, slower downward burns. One heat loss mechanism included mounting a heat exchanger directly above the burning sample in the path of the plume to act as a heat sink and more efficiently dissipate the heat due to the combustion event. This proved an effective means for chamber overpressure mitigation for those tests producing the most total heat release and thusly was determined to be a feasible mitigation strategy to incorporate into the microgravity experiment.

关键词：火焰传播;引力效应;燃烧;航空安全;燃烧速率

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Cosmogenic radionuclides, such as ~(10)Be, are commonly used for reconstructing solar activity in the past. The interpretation of ~(10)Be records, most commonly obtained from polar ice cores, is complicated by the mixing of ~(10)Be in the atmosphere, its transport to polar regions and its deposition. Throughout the generations of ~(10)Be studies these complications have been mentioned but never investigated on a physical basis. This manuscript aims to summarize the recent efforts to study the atmospheric transport of ~(10)Be from its production to its deposition into the polar ice using three-dimensional physically based general circulation models (GCMs) of the atmosphere. These models represent our best understanding of the atmospheric processes up to date. The model studies indicate that the most important factor controlling the deposition response of ~(10)Be to production changes is the fact that a major part of it (～65) is produced in the stratosphere where its residence time is long and it undergoes strong mixing. Therefore, in an ideal archive the ~(10)Be concentrations will reflect the global mean production rate and hence changes in the solar activity. An explanation is offered for the partly different deposition responses of ~(10)Be snow concentrations to production changes obtained with two different GCMs.

关键词：大气传输;沉积;气候影响;铍-10

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High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB1919) and the other is a non-DA of spectral type PG1159 (PG1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD246). Aims. The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. Methods. The Chandra spectra are analyzed with chemically homogeneous as well as stratified NLTE model atmospheres that assume equilibrium between gravitational settling and radiative acceleration of chemical elements. Archival EUV and UV spectra obtained with EUVE, FUSE, and HST are utilized to support the analysis. Results. No metals could be identified in LB1919. All observations are compatible with a pure hydrogen atmosphere. This is in stark contrast to the vast majority of hot DA white dwarfs that exhibit light and heavy metals and to the stratified models that predict significant metal abundances in the atmosphere. For GD246 we find that neither stratified nor homogeneous models can fit the Chandra spectrum. The Chandra spectrum of PG1520+525 constrains the effective temperature to T(sub eff) = 150 000 +/- 10 000 K. Therefore, this nonpulsating star together with the pulsating prototype of the GWVir class (PG1159-035) defines the location of the blue edge of the GWVir instability region. The result is in accordance with predictions from nonadiabatic stellar pulsation models.

关键词：白矮星；X射线光学；紫外光谱；X射线天体物理设施；磁悬浮;

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Synthetic biologists must comb through vast amounts of academic literature to design biological systems. The majority of this data is unstructured and difficult to query because they are manually annotated. Existing databases such as PubMed already contain over 20 million citations and are growing at a rate of 500,000 new citations every year. Our solution is to automatically extract chemical reactions from biological text and canonicalize them so that they can be easily indexed and queried. This paper describes a natural language processing system that generates patterns from labeled training data and uses them to extract chemical reactions from PubMed. To train and validate our system, we create a dataset using BRENDA, the BRaunschweig ENzyme DAtabase, with 4387 labeled sentences. Our system achieves a recall of 0.82 and a precision of 0.88 via cross validation. On a selection of 600,000 PubMed abstracts, our system extracts almost 20% of existing reactions in BRENDA as well as many that are novel.

关键词：生物系统;生物文献;化学反应

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关键词：治疗技术；乳腺癌

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Modern storage systems employing quorum replication are often configured to use partial, non-strict quorums. These systems wait only for a subset of their replicas to respond to a request before returning an answer, without guaranteeing that read and write replica sets intersect. While these partial quorum mechanisms provide only basic eventual consistency guarantees, with no limit to the recency of data returned, these configurations are frequently “good enough” for practitioners given their latency benefits. In this work,we discuss why partial quorums are often acceptable in practice by analyzing the staleness of data they return. Extending prior work on strongly consistent probabilistic quorums and using models of Dynamo-style anti-entropy processes, we introduce Probabilistically Bounded Staleness (PBS) consistency, which provides expected bounds on staleness with respect to both versions and wall clock time. We derive a closed-form solution for versioned staleness and model real-time staleness for representative Dynamo-style systems under internet-scale production workloads. We quantitatively demonstrate why, in practice, eventually consistent systems employing partial quorums often serve consistent data.

关键词：现代存储系统；仲裁机制；法定人数

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