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341.
342.
20多年来,火箭工程设计人员一直在研究耐高温、抗氧化陶瓷材料的应用可能性,但是这些材料的主要缺点是易碎和抗热壅塞性差.目前工业用碳陶瓷和陶瓷-陶瓷结构复合材料具有很好的弹性和很强的抗热壅塞性能,这重新引起了人们制造高温火箭发动机复合材料部件的兴趣.这类新材料目前正由欧洲动力装备公司(SEP)生产,其名为SEPCARB和CERASEP,它们具有独特的热和机械性能,可以取代难熔金属而应用于可贮存二元推进剂火箭发动机的整体式喷管和推力室.装有这些部件的工艺发动机已经试验成功,从而为下一代火箭发动机的生产铺平了道路. 相似文献
343.
航天计划中将增加原子频标的使用,以获得精确的时间和稳定的频率。GPS系统所具有的卓越的导航功能主要是由系统中采用的原子频标带来的,新一代的军事通信卫星(Mistar)载有我路复合铷原子频标以满足各种通讯才操作上的需要,第一颗军用通讯卫星,DFS-1使用了事和石英晶体振荡器,第二颗军用通讯卫星,1995年发射的DFS-2,是第一颗使用铷原子频标的军用通讯卫星,这以后发射的卫星都作用了铷原子频标。 相似文献
344.
本文介绍一种新的恒虚警率(CFAR)处理器。Ll-CFAR通过对参考集排列样本线性滤波来形成其噪声功率估值;但是,这一组合的权值不仅仅取决于排列,而且还取决于样本与测试单元的相对接近程度。这类Ll-CFAR中有些能有效检查假目标;有些可以在有杂波边缘时对虚警进行令人信服的控制;而另一些则对这两种不均匀性均具有鲁棒性。当设计中含有这些方案时,它们执行起来不比普通序列统计CFAR(OS-CFAR)累赘 相似文献
345.
The purpose of this work is to analyse the neural mechanisms of human motor perturbations induced by dynamic changes in gravity. A unilateral selective hypergravity stimulation (USHS) was produced by stretching an elastic band between the right shoulder and foot. The consequences of the extensor muscle tone change due to the positioning (increased muscular loading) and to its removal (decreased muscular loading) by the elastic band were observed on motor gait skill. Gait spatio-temporal parameters (horizontal displacement of both feet) and lower limb functional length variations (efficiency of flexion and extension movements of the lower limbs) were measured. The latter measure was performed using a device specially designed for that purpose. The main results were: (1) during and after USHS, gait perturbations appeared on the left--the body side not directly stimulated, (2) just after the end of USHS, perturbations were present on the right (homolateral) side evidencing a post treatment effect which caused a decrease in functional shortening of the lower limb during extension and an increase of functional shortening of the lower limb during stance (opposite in sense to the modification observed during swing). Such results afford evidence that, in addition to vestibular receptors, the mechanoreceptors of extensor muscles are involved in determining the changes in motor skills observed at the beginning and at the end of space flights. 相似文献
346.
The electrical activity of the brain (EEG) has been recorded during parabolic flights in trained astronauts and non trained volunteers as well. The Fast Fourier analysis of the EEG activity evidenced more asymmetry between the two brain hemispheres in the subjects who suffered from motion sickness than in the others. However, such a FFT classification does not lead to a discrimination between deterministic and stochastic events. Therefore, a first attempt was made to calculate the dimensionality of "chaotic attractors" in the EEG patterns as a function of the different g-epochs of one parabola. Very preliminary results are given here. 相似文献
347.
Baevsky RM Moser M Nikulina GA Polyakov VV Funtova II Chernikova AG 《Acta Astronautica》1998,42(1-8):159-173
The space flight of physician cosmonaut V.V. Polyakov, the longest to date (438 days), has yielded new data about human adaptation to long-term weightlessness. Autonomic regulation of circulation and cardiac contractility were evaluated in three experiments entitled Pulstrans, Night, and Holter. In the Pulstrans experiment electrocardiographic (ECG), ballistocardiographic (BCG), seismocardiographic (SCG), and some other parameters were recorded. In the Night experiment, only the ballistocardiogram was recorded, but a special feature of this experiment is that the BCG records were obtained with a contactless method. This method has several advantages, the most important of which are the possibility of studying slow-wave variations in physiologic parameters (ultradian rhythms) on the basis of recordings made under standard conditions over a prolonged period. The Holter experiment (24-hour electrocardiographic monitoring) used a portable cardiorecorder (Spacelab, USA). The obtained electrocardiographic data were used to analyze heart rate variability. In the first 6 months of the 14-month flight, the dynamics of cardiovascular parameters in V.V. Polyakov was virtually the same as in the other cosmonauts. The data obtained after the first 6 months of Polyakov's sojourn in space are unique and mention should be made of at least three important aspects: (1) activation of a new, additional adaptive mechanism in the 8th-9th months of flight, as is evidenced by alterations in the periodicity and power of superslow wave oscillations (ultradian rhythms) reflecting the activity of the subcortical cardiovascular centers and of the higher levels of autonomic regulation; (2) growth of cardiac contractility accompanied by a decrease in heart rate during the last few months of flight; (3) a considerable increase in the daily average values of absolute power of heart rate's variability MF component, which reflects the activity of the vasomotor center. Specific mechanisms of adaptation to weightless conditions appear to be associated with activation of higher autonomic centers. The hypothesis that central levels of circulation regulation are activated in a long-term space flight was investigated by analyzing of ultradian rhythms in nighttime. The data, received during the flight of V. V. Polyakov, show, that the process of human adaptation to long influence of weightlessness consists of a number of consecutive stages, during which the activation of more and more high levels of control system of physiological functions occurs. 相似文献
348.
This Note describes the dynamic load sensors (DLS) spaceflight experiment that measured middeck astronaut-induced disturbances during the 14-day STS-62 Space Shuttle mission in March 1994. The DLS experiment was flown in conjunction with the reflight of the Middeck 0-Gravity Dynamics Experiment (MODE). The objective of MODE was to investigate effects of the microgravity environment on large space structures. Where Skylab experiments focused on measuring the forces exerted during vigorous soaring activities, the DLS experiment quantified the reaction forces and moments exerted by the crew going about their normal on-orbit activities. The objective of this Note is to present DLS force data and frequency analysis that characterize astronaut-induced loads during spaceflight. 相似文献
349.
Raulin F Frere C Paillous P De Vanssay E Do L Khlifi M 《Journal of the British Interplanetary Society》1992,45(6):257-271
Titan, the largest satellite of Saturn, has a dense N2-CH4 atmosphere rich in organic compounds, both in gas and in aerosol phases. Its surface is probably covered by oceans of liquid methane-ethane mixtures, with many dissolved organics. This quasi planet appears as a natural laboratory to study chemical evolution toward complex organic systems in a planetary environment over a long time scale. With the Cassini-Huygens mission NASA and ESA will jointly send an orbiter (Cassini) around Saturn and a probe (Huygens) in the atmosphere of Titan. This mission, currently planned to be launched in 1996-1997 for a Saturn - Titan arrival in 2004, offers a unique opportunity to study in detail extra-terrestrial organic processes. Consequently, it has important implications in the field of exobiology and the origins of life. 相似文献
350.
最近正在出现一些东方国家对通信卫星市场构成的威胁。尽管这些来自东方的竞争者无法与美国的休斯、洛·马、洛拉尔公司及欧洲的法国宇航公司和马特拉·马可尼公司相提并论,但它们的存在仍不容忽视。此前在其他领域低估日本、韩国和中国台湾实力的教训比比皆是。当前,亚太地区是通信卫星市场蓬勃发展的主要动力,西方主要卫星制造商在这一市场上的占有率不断提高,其主要原因是这些亚太国家不能自给自足。不过这一情况现已发生变化,预计在下个世纪初,这些刚刚在亚太市场立足的西方卫星制造商会发现他们的订单将逐渐被来自远东的竞争者夺… 相似文献