全文获取类型
收费全文 | 1241篇 |
免费 | 102篇 |
国内免费 | 47篇 |
专业分类
航空 | 619篇 |
航天技术 | 411篇 |
综合类 | 55篇 |
航天 | 305篇 |
出版年
2024年 | 4篇 |
2023年 | 37篇 |
2022年 | 22篇 |
2021年 | 21篇 |
2020年 | 30篇 |
2019年 | 27篇 |
2018年 | 19篇 |
2017年 | 15篇 |
2016年 | 17篇 |
2015年 | 20篇 |
2014年 | 63篇 |
2013年 | 34篇 |
2012年 | 52篇 |
2011年 | 87篇 |
2010年 | 31篇 |
2009年 | 43篇 |
2008年 | 81篇 |
2007年 | 92篇 |
2006年 | 64篇 |
2005年 | 86篇 |
2004年 | 45篇 |
2003年 | 62篇 |
2002年 | 37篇 |
2001年 | 76篇 |
2000年 | 34篇 |
1999年 | 39篇 |
1998年 | 38篇 |
1997年 | 22篇 |
1996年 | 28篇 |
1995年 | 26篇 |
1994年 | 73篇 |
1993年 | 23篇 |
1992年 | 8篇 |
1991年 | 11篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1984年 | 4篇 |
排序方式: 共有1390条查询结果,搜索用时 15 毫秒
761.
提出了一种复合材料机翼结构的形状/尺寸综合优化设计方法,对形状/尺寸综合优化问题进行解耦处理,得到了一种双层循环优化策略。采用均匀设计法给出若干样本点,使用准则优化手段进行尺寸优化设计,构造了形状设计变量与尺寸优化结果之间的二次多项式响应面模型,采用复合形法完成了基于响应面的形状优化设计。最后,对某型大展弦比飞翼结构进行了优化设计。优化算例结果表明,所提方法可行且有效,有较高的优化效率。 相似文献
762.
针对虹桥机场通信管网人井的监控问题,本文利用基于模糊综合函数的目标识别融合理论实现多传感器决策级数据融合,提高通信人井井盖监测的准确性,并以B/S模式构建分布式通信人井多传感器集中智能监控系统。这两种理论保证了本套在线实时监控系统成功地应用于虹桥机场,保证了机场的通信管网安全畅通。 相似文献
763.
764.
A lunar L2-Farside exploration and science mission concept with the Orion Multi-Purpose Crew Vehicle and a teleoperated lander/rover 总被引:1,自引:0,他引:1
Jack O. Burns David A. Kring Joshua B. Hopkins Scott Norris T. Joseph W. Lazio Justin Kasper 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
A novel concept is presented in this paper for a human mission to the lunar L2 (Lagrange) point that would be a proving ground for future exploration missions to deep space while also overseeing scientifically important investigations. In an L2 halo orbit above the lunar farside, the astronauts aboard the Orion Crew Vehicle would travel 15% farther from Earth than did the Apollo astronauts and spend almost three times longer in deep space. Such a mission would serve as a first step beyond low Earth orbit and prove out operational spaceflight capabilities such as life support, communication, high speed re-entry, and radiation protection prior to more difficult human exploration missions. On this proposed mission, the crew would teleoperate landers/rovers on the unexplored lunar farside, which would obtain samples from the geologically interesting farside and deploy a low radio frequency telescope. Sampling the South Pole-Aitken basin, one of the oldest impact basins in the solar system, is a key science objective of the 2011 Planetary Science Decadal Survey. Observations at low radio frequencies to track the effects of the Universe’s first stars/galaxies on the intergalactic medium are a priority of the 2010 Astronomy and Astrophysics Decadal Survey. Such telerobotic oversight would also demonstrate capability for human and robotic cooperation on future, more complex deep space missions such as exploring Mars. 相似文献
765.
766.
767.
768.
769.
Marcello Giroletti V. Pavlidou A. Reimer G.B. Taylor G. Tosti G. Giovannini C. Casadio E. Liuzzo S. Tamburri 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2012
Radio and gamma-ray emissions in Active Galactic Nuclei (AGNs) are both related to the presence of relativistic particles in jets. With the advent of the Fermi Large Area Telescope (LAT), and thanks to its large sensitivity up to several GeV, many observational results are changing our understanding of these phenomena. BL Lac objects, which made up only a fraction of the known extragalactic gamma-ray source population before Fermi, have now become the most abundant class. However, since they are relatively weak radio sources, most of them are poorly known as far as their parsec scale structure and multi-wavelength properties are concerned. For this reason, we have selected a complete sample of 42 low redshift BL Lacs (independently of their gamma-ray properties) to study with a multi-wavelength (radio, optical, X-ray, gamma-ray) approach. Here, we present results and images of sources in the sample (most of which have never been observed before), using new VLBA observations at 8 and 15 GHz. Beyond this sample of BL Lacs, the population of gamma-ray AGNs has also dramatically enlarged in the Fermi era, permitting us to discuss the presence of a correlation between radio and gamma-ray properties with improved statistical significance. We explore the radio-gamma relation with several hundreds sources and using both simultaneous and archival radio data, thus tackling the impact of time variability. 相似文献
770.
C. Ferrari F. Govoni S. Schindler A. M. Bykov Y. Rephaeli 《Space Science Reviews》2008,134(1-4):93-118
We review observations of extended regions of radio emission in clusters; these include diffuse emission in ‘relics’, and
the large central regions commonly referred to as ‘halos’. The spectral observations, as well as Faraday rotation measurements
of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant
densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are
reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic
protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive
and detailed radio measurements; we briefly review prospects for future observations. 相似文献