全文获取类型
收费全文 | 6499篇 |
免费 | 624篇 |
国内免费 | 918篇 |
专业分类
航空 | 3625篇 |
航天技术 | 2042篇 |
综合类 | 804篇 |
航天 | 1570篇 |
出版年
2022年 | 43篇 |
2021年 | 59篇 |
2020年 | 111篇 |
2019年 | 41篇 |
2018年 | 65篇 |
2017年 | 60篇 |
2016年 | 84篇 |
2015年 | 138篇 |
2014年 | 292篇 |
2013年 | 304篇 |
2012年 | 315篇 |
2011年 | 340篇 |
2010年 | 465篇 |
2009年 | 386篇 |
2008年 | 457篇 |
2007年 | 352篇 |
2006年 | 267篇 |
2005年 | 254篇 |
2004年 | 204篇 |
2003年 | 212篇 |
2002年 | 139篇 |
2001年 | 242篇 |
2000年 | 234篇 |
1999年 | 243篇 |
1998年 | 282篇 |
1997年 | 223篇 |
1996年 | 220篇 |
1995年 | 190篇 |
1994年 | 193篇 |
1993年 | 171篇 |
1992年 | 159篇 |
1991年 | 121篇 |
1990年 | 101篇 |
1989年 | 146篇 |
1988年 | 71篇 |
1987年 | 58篇 |
1986年 | 66篇 |
1985年 | 96篇 |
1984年 | 57篇 |
1983年 | 64篇 |
1982年 | 62篇 |
1981年 | 71篇 |
1980年 | 34篇 |
1979年 | 27篇 |
1978年 | 25篇 |
1975年 | 26篇 |
1974年 | 25篇 |
1973年 | 24篇 |
1971年 | 31篇 |
1969年 | 26篇 |
排序方式: 共有8041条查询结果,搜索用时 359 毫秒
51.
疲劳裂纹扩展速率试验数据的可靠性分析模型 总被引:2,自引:0,他引:2
以指定ΔK下的da/dN服从对数正态分布为基础,提出3种da/dN试验数据统计处理方法;依据一组40CrNiMo材料的CT试样的试验数据处理结果,对3方法进行了分析对比。 相似文献
52.
The analysis and comments presented in this paper are meant to establish the general communication parameters associated with Martian flyby probes and with lander and manned vehicles. Fundamental data transfer problems are reviewed to define comparisons and trends of tradeoffs for future studies. Selected focal points are based upon the long propagation path length, with inherent time delays, and the high noise produced by the sun. These problems are magnified because large quantities of data must be obtained to satisfy the needs of the scientific community and the curiosity of an interested public. A comparison of two communication systems is provided: the microwave spectrum and the optical spectrum, as represented by the microwaves at 2.3 GHz and the laser at 6328 ?. A method of cost effectiveness or value received from space missions (a criterion of power input for data quantity received) is also presented. 相似文献
53.
On-board/in-flight checkout of future aerospace systems will necessitate a fundamental departure from today's design and checkout activities. The interrelationship of checkout, incorporated into prime vehicular functions, man as a subsystem, and standardization Of hardware/software must be considered as a functional and integral entity if the efforts of today's long-range planning are to become tomorrow's reality. This paper will describe future systems checkout, maintenance and support considerations, and actions and tentative measures necessary for implementation. The realization of the implementation of these concepts into functional elements will depend, accurately and economically, upon the degree of government perceptiveness and the extent of industrial support. 相似文献
54.
The fundamental problem of inertial navigation, double integration of acceleration to obtain position, is defined and discussed. Mechanizations of both space-stable and local-vertical platform systems are exhibited. The synthesis problem for an electrically suspended gyro (ESG) strapdown system is defined and discussed: readout, readout errors due to vehicle motion, synchronization of readout with system computer, alignment, correction and calibration for mass unbalance drift, and digital mechanization. Alignment, calibration, and acceleration measurement are also discussed. Sources of error involved in the electronic gimbaling including those peculiar to strapdown configuration are discussed and compared to mechanically gimbaled systems. Advanced developments required in the component and systems areas are listed, and it is shown that such development will lead to reduced complexity, higher accuracy, and increased reliability and utility for inertial systems. 相似文献
55.
In the first part the state of the art of the most advanced micrometeoroid simulation techniques is reviewed. Then a new accelerator is described, which was developed jointly by the Technische Universität München and the N.A.S.A. George C. Marshall Space Flight Center. Finally the use of this new technique is indicated for basic research in fields other than astronautics. 相似文献
56.
低轨卫星面阵凝视成像技术研究 总被引:2,自引:0,他引:2
文章介绍了面阵凝视成像的工作原理及特点,阐述了低轨卫星对地观测凝视成像面临的像移问题及其解决措施,对像移补偿装置与二维指向机构的结合做了初步探讨。 相似文献
57.
58.
59.
60.
Among of the highlights of the 20th century were flights of spacecraft to other bodies of the Solar System. This paper describes briefly the missions attempted, their goals, and fate. Information is presented in five tables on the missions launched, their goals, mission designations, dates, discoveries when successful, and what happened if they failed. More detailed explanations are given in the accompanying text. It is shown how this enterprise developed and evolved step by step from a politically driven competition to intense scientific investigations and international cooperation. Initially, only the USA and USSR sent missions to the Moon and planets. Europe and Japan joined later. The USSR carried out significant research in Solar System exploration until the end of the 1980s. The Russian Federation no longer supports robotic planetary exploration for economic reasons, and it remains to be seen whether the invaluable Russian experience in planetary space flight will be lost. Collaboration between Russian and other national space agencies may be a solution. 相似文献