全文获取类型
收费全文 | 5718篇 |
免费 | 32篇 |
国内免费 | 18篇 |
专业分类
航空 | 2543篇 |
航天技术 | 1896篇 |
综合类 | 22篇 |
航天 | 1307篇 |
出版年
2021年 | 54篇 |
2019年 | 34篇 |
2018年 | 164篇 |
2017年 | 129篇 |
2016年 | 138篇 |
2015年 | 59篇 |
2014年 | 153篇 |
2013年 | 189篇 |
2012年 | 177篇 |
2011年 | 238篇 |
2010年 | 186篇 |
2009年 | 284篇 |
2008年 | 290篇 |
2007年 | 202篇 |
2006年 | 129篇 |
2005年 | 164篇 |
2004年 | 154篇 |
2003年 | 185篇 |
2002年 | 136篇 |
2001年 | 199篇 |
2000年 | 90篇 |
1999年 | 114篇 |
1998年 | 133篇 |
1997年 | 92篇 |
1996年 | 122篇 |
1995年 | 172篇 |
1994年 | 155篇 |
1993年 | 70篇 |
1992年 | 107篇 |
1991年 | 42篇 |
1990年 | 47篇 |
1989年 | 107篇 |
1988年 | 37篇 |
1987年 | 37篇 |
1986年 | 44篇 |
1985年 | 137篇 |
1984年 | 136篇 |
1983年 | 104篇 |
1982年 | 98篇 |
1981年 | 162篇 |
1980年 | 42篇 |
1979年 | 38篇 |
1978年 | 41篇 |
1977年 | 36篇 |
1976年 | 27篇 |
1975年 | 42篇 |
1974年 | 33篇 |
1973年 | 26篇 |
1972年 | 31篇 |
1971年 | 27篇 |
排序方式: 共有5768条查询结果,搜索用时 15 毫秒
91.
Yu. F. Gortyshov V. M. Gureev R. Sh. Misbakhov I. F. Gumerov A. P. Shaikin 《Russian Aeronautics (Iz VUZ)》2009,52(4):488-490
We present the results of experimental studies of the fuel hydrogen additive influence on the characteristics of a gas-piston
engine converted for operation by natural gas under changes of an ignition advance angle (IAA). The results of investigations
were used to determine the influence of the hydrogen additive on the effective engine efficiency and fuel consumption under
IAA changes. 相似文献
92.
藜蒿中绿原酸溶剂提取方法研究 总被引:1,自引:0,他引:1
应用醇溶剂回流法提取藜蒿中绿原酸,通过正交试验,对藜蒿中绿原酸进行溶剂提取条件的优化研究,并分剐讨论了各因素对绿原酸提取率的影响。实验表明,乙醇浓度70%、12倍的提取剂用量、温度85℃、提取时间4h、提取两次为乙醇溶剂提取藜蒿中绿原酸的最佳条件,总绿原酸提取率为1.24%。 相似文献
93.
介绍一种新型的、具有最小喉道面积的三维高超声速进气道 (称之为收敛形进气道 )的数值和实验研究结果。表明使用这种形式的进气道 ,在整个飞行速度范围内可以降低阻力和高超声速发动机表面的热防护要求 ,通过降低外压缩表面的倾斜度和减少进气道及燃烧室壁的面积就可以做到这一点。在采用低维次流动的气体动力设计方法的基础上设计成这种形式的进气道。计算是在无粘气体模型构架内用有限体积法进行的。同时用边界层方程计算出计及粘性的气流特性和进气道特性。数值算法是通过收敛形进气道的有限宽楔形外压缩表面的计算和实验数据来验证的。进行实验研究的马赫数M=2~ 1 0 7,基于模型进气道高度的雷诺数Re=( 1~ 5) × 1 0 6。数值计算与实验结果一致性很好。这些结果也和通常的二维进气道的数据作了比较。 相似文献
94.
95.
ROSETTA — the Comet Nucleus Sample Return mission — is one of the four Cornerstone missions to which ESA has committed itself in its approved Long-Term Programme Horizon 2000. The mission is currently being studied in collaboration with NASA. The comet-nucleus samples that ROSETTA is to provide will allow us to study some of the most primitive material in the solar system and the physical and chemical processes that marked the beginning of the system 4.6 billion years ago. For ESA, ROSETTA is a new type of mission: one which will return a sample at cryogenic temperature, and where as much effort has to be spent on preparing the laboratory analysis on-ground as has to be invested in preparing the space segment with the sample acquisition and in situ documentation. As part of the preparation for this mission, ESA is now starting to consider Planetary Protection issues. 相似文献
96.
Drumheller D.M. Titlebaum E.L. 《IEEE transactions on aerospace and electronic systems》1991,27(1):2-10
The problem of determining the cross-correlation properties of signals based on algebraically constructed Costas arrays is addressed by examining the discrete cross-correlation of the algebraically constructed Costas arrays for a given construction and dimension. Finding two arrays that minimally correlate implies that the signals based on these arrays also minimally correlate. The properties of finite fields are reviewed, and the major algebraic constructions for Costas arrays are presented, i.e. the Welch construction and the Golomb construction. The discrete cross-correlation properties of the Costas arrays are derived for arrays of the same dimension derived from the same construction. The use of Costas arrays in the signal design problem is discussed, and examples are given to show the cross-correlation of the signals based on the algebraically constructed arrays 相似文献
97.
The proposed KAAD (knowledge-based automated air defense) system demonstrates a man-machine environment for airspace defense systems. When the unknown aircraft is hostile, a threat rating and response methods are generated by the system. It serves as a double-check decision-making system for a war control center. In addition to this application, the KAAD system can also be a useful tool as a training program for the war controller. The capabilities of the system are limited due to the shortage of knowledge resources. It requires communications among war controllers and air fighter pilots to organize a practical knowledge base. It is shown that the KAAD system can be combined with an automated ATC (air traffic control) system to become a practical system for air defense applications 相似文献
98.
It is shown that the volume under the wideband ambiguity function is close to the square of the signal energy. The ambiguity volume is asymptotically conserved as the signal approaches the narrowband case. The narrowband ambiguity volume is a lower bound for the volume of the wideband ambiguity function. 相似文献
99.
Cantrell B.H. Gordon W.B. Trunk G.V. 《IEEE transactions on aerospace and electronic systems》1981,(2):213-221
The maximum likelihood estimates of the elevation angles of two closely spaced targets within the beamwidth is considered. For an array divided into three subapertures, a simple, closed form solution is found whose accuracy compares favorably to the maximum likelihood estimate which uses all the individual elements. Simulation results are presented for the case of a radar target located over a smooth reflecting surface. 相似文献
100.
Powers E.J. Hong J.Y. Kim Y.C. 《IEEE transactions on aerospace and electronic systems》1981,(4):602-605
The nonlinear system concepts of nonlinear transfer functions are extended to nonlinear cross sections which may be used to quantitatively describe the characteristics of a nonlinear scatter. The concept of nonlinear cross sections is used to generalize the radar equation for nonlinear scattering objects. 相似文献