首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2254篇
  免费   13篇
  国内免费   10篇
航空   1092篇
航天技术   817篇
综合类   10篇
航天   358篇
  2021年   22篇
  2018年   38篇
  2017年   21篇
  2016年   26篇
  2014年   48篇
  2013年   57篇
  2012年   49篇
  2011年   91篇
  2010年   64篇
  2009年   90篇
  2008年   97篇
  2007年   52篇
  2006年   43篇
  2005年   57篇
  2004年   70篇
  2003年   72篇
  2002年   37篇
  2001年   57篇
  2000年   39篇
  1999年   47篇
  1998年   67篇
  1997年   42篇
  1996年   67篇
  1995年   79篇
  1994年   55篇
  1993年   49篇
  1992年   64篇
  1991年   30篇
  1990年   19篇
  1989年   49篇
  1988年   23篇
  1987年   20篇
  1986年   21篇
  1985年   85篇
  1984年   53篇
  1983年   53篇
  1982年   60篇
  1981年   72篇
  1980年   21篇
  1979年   26篇
  1978年   26篇
  1977年   26篇
  1976年   20篇
  1975年   20篇
  1974年   20篇
  1973年   11篇
  1972年   14篇
  1970年   18篇
  1969年   19篇
  1967年   10篇
排序方式: 共有2277条查询结果,搜索用时 0 毫秒
91.
A constant-frequency diode-clamped series resonant converter (CFCSRC) is proposed as a solution to problems associated with frequency-controlled resonant converters. This converter has two resonant frequencies, and control is achieved by varying the relative time spent at each switching frequency. Two zero-current-switching (ZCS) modes are examined and plotted in the output plane. An equation is given for the boundary between the two ZCS modes, as well as an expression for the boundary between ZCS and non-ZCS operation; both are plotted in the output plane. The output equation for the main mode is shown to be hyperbolic. Converter peak voltages limited to the input voltages, and peak currents are less than those of the frequency-controlled clamped series resonant converter over a large operating range. Data from a prototype converter are compared with theoretical data and are shown to be in good agreement with the theoretical model  相似文献   
92.
    
Two bed rest analog studies of space flight were performed; one 14 d and the other 28 d in duration. Exercise response was studied in detail during the 28 d study and following both the 14 d and 28 d studies. This paper relates the results of these studies to physiologic changes noted during and following space flight. The most consistent change noted after both bed rest and space flight is an elevated heart rate during exercise. A second consistent finding is a postflight or postbed rest reduction in cardiac stroke volume. Cardiac output changes were variable. The inability to simulate inflight activity levels and personal exercise makes a direct comparison between bed rest and the results from specific space flights difficult.  相似文献   
93.
    
Space Science Reviews - Time measured by an ideal clock crucially depends on the gravitational potential and velocity of the clock according to general relativity. Technological advances in...  相似文献   
94.
In the presence of unknown disturbances and model parameter uncertainties, this paper develop a nonlinear backstepping sliding-mode controller (BSMC) for trajectory tracking control of a stratospheric airship using a disturbance-observer (DO). Compared with the conventional sliding mode surface (SMS) constructed by a linear combination of the errors, the new SMS manifold is selected as the last back-step error to improve independence of the adjustment of the controller gains. Furthermore, a nonlinear disturbance-observer is designed to process unknown disturbance inputs and improve the BSMC performances. The closed-loop system of trajectory tracking control plant is proved to be globally asymptotically stable by using Lyapunov theory. By comparing with traditional backstepping control and SMC design, the results obtained demonstrate the capacity of the airship to execute a realistic trajectory tracking mission, even in the presence of unknown disturbances, and aerodynamic coefficient uncertainties.  相似文献   
95.
Heading and speed errors are analytically determined for noneumavering targets at the output of an x, y tracking filter which processes range and bearing measurements from a radar sensor in a track-while-scan (TWS) operation. These errors are shown to depend upon target range and speed, the angle between the radius and velocity vectors, sensor accuracies, and tracking filter parameters. eters. Depending upon the tracking filter implementation, these errors may also be a function of target bearing.  相似文献   
96.
97.
The leakage reactance of the machine is expressed as an equation involving slot leakage, end-connection leakage, differential leakage, stator-halves leakage, and tooth-top leakage reactance. This leakage reactance equation, or objective function, is then minimized under suitable constraints. Geometric programming is used to carry through the optimization procedure. A computer program is developed and applied to find the optimum leakage reactance for a 95 kVA, 208 V, wye-connected, 40 800 r/min aerospace alternator, using 8-, 10-, and 12-pole machines.  相似文献   
98.
99.
Shea  M.A.  Smart  D.F. 《Space Science Reviews》2000,93(1-2):229-262
Cosmic radiation has been measured by a variety of techniques since 1933. This paper presents the evolution of data acquisition, processing, and availability of cosmic radiation data from the early years to the present time. Information on the worldwide network of neutron monitor stations and the availability of these cosmic radiation records is included. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   
100.
This paper will review the potential of a relatively new type of testbed platform for environmental education and research because of the unique advantages resulting from their material closure and separation from the outside environment. These facilities which we term “modular biospheres”, have emerged from research centered on space life support research but offer a wider range of application. Examples of this type of facility include the Bios-3 facility in Russia, the Japanese CEEF (Closed Ecological Experiment Facility), the NASA Kennedy Space Center Breadboard facility, the Biosphere 2 Test Module and the Laboratory Biosphere. Modular biosphere facilities offer unique research and public real-time science education opportunities. Ecosystem behavior can be studied since initial state conditions can be precisely specified and tracked over different ranges of time. With material closure (apart from very small air exchange rate which can be determined), biogeochemical cycles between soil and soil microorganisms, water, plants, and atmosphere can be studied in detail. Such studies offer a major advance from studies conducted with phytotrons which because of their small size, limit the number of organisms to a very small number, and which crucially do not have a high degree of atmospheric, water and overall material closure. Modular biospheres take advantage of the unique properties of closure, as representing a distinct system “metabolism” and therefore are essentially a “mini-world”. Though relatively large in comparison with most phytotrons and ecological microcosms, which are now standard research and educational tools, modular biospheres are small enough that they can be economically reconfigured to reflect a changing research agenda. Some design elements include lighting via electric lights and/or sunlight, hydroponic or soil substrate for plants, opaque or glazed structures, and variable volume chambers or other methods to handle atmospheric pressure differences between the facility and the outside environment.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号