全文获取类型
收费全文 | 13058篇 |
免费 | 9516篇 |
国内免费 | 2090篇 |
专业分类
航空 | 19061篇 |
航天技术 | 1907篇 |
综合类 | 781篇 |
航天 | 2915篇 |
出版年
2024年 | 73篇 |
2023年 | 225篇 |
2022年 | 389篇 |
2021年 | 476篇 |
2020年 | 571篇 |
2019年 | 1220篇 |
2018年 | 1422篇 |
2017年 | 1353篇 |
2016年 | 1380篇 |
2015年 | 1351篇 |
2014年 | 1233篇 |
2013年 | 1222篇 |
2012年 | 1196篇 |
2011年 | 1153篇 |
2010年 | 1213篇 |
2009年 | 1248篇 |
2008年 | 1062篇 |
2007年 | 887篇 |
2006年 | 691篇 |
2005年 | 506篇 |
2004年 | 424篇 |
2003年 | 418篇 |
2002年 | 472篇 |
2001年 | 387篇 |
2000年 | 358篇 |
1999年 | 421篇 |
1998年 | 349篇 |
1997年 | 313篇 |
1996年 | 271篇 |
1995年 | 226篇 |
1994年 | 235篇 |
1993年 | 264篇 |
1992年 | 204篇 |
1991年 | 354篇 |
1990年 | 297篇 |
1989年 | 324篇 |
1988年 | 232篇 |
1987年 | 101篇 |
1986年 | 54篇 |
1985年 | 19篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 22篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1972年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
This article studies the efficiency of ejecting waste generated by the life support system (LSS) of a manned spacecraft to reduce initial mass on low earth orbit. The spacecraft is used for a long-duration interplanetary mission and is equipped with either a chemical or a nuclear-thermal propulsion system. For this study we simulate an optimal control problem for a given spacecraft maneuver. An impulsive approximation of the optimal interplanetary spacecraft trajectory is assumed, which allows us to reduce the general optimal control problem to hierarchic structure of 'outer' and 'inner' subproblems. This structure is analyzed using the Pontryagin's Maximum principle. Numerical results, illustrating the efficiency of waste ejection are shown for typical Earth-Mars transfer trajectories. This results confirm in theory that using a waste ejection system makes an early manned Mars mission possible without having to design and build new, advanced biological LSS. 相似文献
82.
比较两种盲多用户检测算法在动态环境中的收敛性能,并通过计算机进行模拟仿真。其中一种是能随输入信号矢量变化而变化的变步长最小输出能量盲算法;另一种是既保持最小输出能量检测器的全局收敛性,又具有最小均方误差检测器高输出信干比优点的判决反馈变步长盲算法。利用前者在动态强干扰下收敛快及后者在稳态时输出信干比高的优势,将二者结合起来应用到动态环境中,提高系统性能,同时抑制多址干扰和窄带干扰。 相似文献
83.
In 1994–1995 Lavochkin Association (Russia) together with the other enterprises in accordance with technical requirements of the Russian Space agency, developed a new Russian communication satellite of a small class that will operate in both the geostationary (GSO) and high-elliptical (HEO) orbits. This satellite may be injected into operational orbits using a SOYUZ-2 launch vehicle (LV) and a FREGAT upper stage (US) from Plesetsk and Baykonur space launch sites (SLS).The main reason for creating such a satellite was to decrease the cost of the support and development of the Russian communication geostationary satellites group.Russian satellites Horizont, Express, Ekran and Gals, which operate in GSO, are the basis of the space segment for communications, radio and TV broadcasting. All of these satellites are injected into GSO by the PROTON LV. PROTON is a launch vehicle of a heavy class. The use of a middle class LV instead of a heavy class will allow to reduce considerably the launch cost. The change of a heavy class LV to a LV of middle class determined one economic reason for this project. Besides, the opportunity to launch S/C into GSO from Russian Plesetsk SLS increases the independence of Russia in the domain of space communications, despite the presence of the contract with Kazachstan about the rent of Baykonur SLS. Finally, use of small satellites with a rather small number of transponders is more effective than the use of big satellites. It will allow also to increase a satellite group (by the launch of additional satellites) precisely in accordance to the development of the ground segment. 相似文献
84.
较详细地对复合材料壳体与喷管卡环连接结构进行了有限元应力,应变分析,对卡环,接头及倒锥等多体接触问题进行研究;对“I”型及“L”型卡环结构分别进行了计算和比较,并给出计算结果及结果分析。 相似文献
85.
86.
In 1994-1995 Lavochkin Association (Russia) together with the other enterprises in accordance with technical requirements of the Russian Space agency, developed a new Russian communication satellite of a small class that will operate in both the geostationary (GSO) and high-elliptical (HEO) orbits. This satellite may be injected into operational orbits using a SOYUZ-2 launch vehicle (LV) and a FREGAT upper stage (US) from Plesetsk and Baykonur space launch sites (SLS).The main reason for creating such a satellite was to decrease the cost of the support and development of the Russian communication geostationary satellites group.Russian satellites Horizont, Express, Ekran and Gals, which operate in GSO, are the basis of the space segment for communications, radio and TV broadcasting. All of these satellites are injected into GSO by the PROTON LV. PROTON is a launch vehicle of a heavy class. The use of a middle class LV instead of a heavy class will allow to reduce considerably the launch cost. The change of a heavy class LV to a LV of middle class determined one economic reason for this project. Besides, the opportunity to launch S/C into GSO from Russian Plesetsk SLS increases the independence of Russia in the domain of space communications, despite the presence of the contract with Kazachstan about the rent of Baykonur SLS. Finally, use of small satellites with a rather small number of transponders is more effective than the use of big satellites. It will allow also to increase a satellite group (by the launch of additional satellites) precisely in accordance to the development of the ground segment. 相似文献
87.
本文首先从国内外的发展现状,概述了印刷电路板缺陷焊点的几种检测手段。并着重介绍了作者与其同事们正在研制的激光全息印刷电路板焊点缺陷自动检测系统。 相似文献
88.
89.
90.