全文获取类型
收费全文 | 206篇 |
免费 | 0篇 |
国内免费 | 2篇 |
专业分类
航空 | 100篇 |
航天技术 | 35篇 |
航天 | 73篇 |
出版年
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 1篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 2篇 |
2014年 | 9篇 |
2013年 | 4篇 |
2012年 | 9篇 |
2011年 | 13篇 |
2010年 | 9篇 |
2009年 | 12篇 |
2008年 | 7篇 |
2007年 | 10篇 |
2006年 | 5篇 |
2005年 | 5篇 |
2004年 | 5篇 |
2003年 | 5篇 |
2002年 | 3篇 |
2001年 | 4篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 7篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1989年 | 1篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 10篇 |
1978年 | 1篇 |
1974年 | 2篇 |
1972年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
1968年 | 9篇 |
1967年 | 4篇 |
1966年 | 5篇 |
1963年 | 1篇 |
排序方式: 共有208条查询结果,搜索用时 15 毫秒
1.
Human space exploration since Apollo appears to lack an overall context. There has been an overall context for the world's space efforts. But it is an unofficial one and it is outmoded, because it was based on a false assumption. This is the space exploration plan articulated by Von Braun in the 1950s and restated as the Integrated Space Program - 1970–1990, whose principal aim is to send humans to explore Mars. The critical underlying assumption of this plan was that Mars is a planet much like Earth, with an active biosphere. This Program has persisted nearly two decades after this underlying assumption has been shown to be false. There is a competing context re-emerging for human space exploration and development which is better fitted to the needs of human society in the post-Cold War era than the Mars program embraced by NASA and, to a large extent, the USSR during the period of US-Russian competition. The original space program uses the resources of free space and provides an economic rationale for human space activity. 相似文献
2.
3.
Richard Hills 《Space Science Reviews》1995,74(1-2):119-123
Some of the considerations in the design of the telescope for FIRST are discussed. It is pointed out that instruments that operate at submillimetre wavelengths need to be analyzed with techniques derived from both the Radio and Optical/IR traditions. The issue of emissivity of reflector surfaces is also discussed. 相似文献
4.
Atmospheric tides 总被引:3,自引:0,他引:3
5.
Luh J. Y. S. Shafran Joel S. Harvey C. Arthur 《IEEE transactions on aerospace and electronic systems》1968,(2):218-233
This paper considers the problem arising from the design of an autopilot for a large booster. The motion-controlling actuators of the booster have both position and rate limits. The problem is formulated as a bounded phase-coordinate problem and analyzed by the ``backing out of the target' procedure. A method of constructing the optimal control is presented. An example of an oscillatory system with two control inputs is given, and the optimal control is expressed as an explicit time function. 相似文献
6.
Direct Broadcast Satellites covering large countries such as Canada, require more than one spacecraft, located at different orbital positions, as well as the use of multiple shaped beams. This would minimize eclipse requirements over several time zones, provide increased capacity by frequency reuse and permit the use of cost effective ground receivers.Two such satellites are envisaged, one covering Eastern Canada, the other Western Canada, using two different sets of three highly shaped beams. This paper is the result of a feasibility study of a satellite antenna designed such that while at either orbital location it can be reconfigured in orbit, by ground command and hence can save the cost of one additional spare spacecraft.An offset parabolic reflector is proposed for the 12 GHz downlink, with a switched “dual feed” structure, consisting of two separate but contiguous sets of pyramidal horns and their associated beam forming networks (BFN). Only one BFN set is used at any one orbital location. Detailed radiation patterns demonstrate good beamshaping capabilities, with coverage efficiencies of the order of 94%. Other considerations such as the effect of orbital locations, gain equalization and TWTA standardization are discussed.It is concluded that a satellite, reconfigurable in orbit with a “dual feed” antenna, is feasible and cost effective, for a DBS spare as well as for the main spacecraft. 相似文献
7.
In the 36 years between June 1965 and February 2001, the US human space flight program has conducted 100 spacewalks, or extravehicular activities (EVAs), as NASA officially calls them. EVA occurs when astronauts wearing spacesuits travel outside their protective spacecraft to perform tasks in the space vacuum environment. US EVA started with pioneering feasibility tests during the Gemini Program. The Apollo Program required sending astronauts to the moon and performing EVA to explore the lunar surface. EVA supported scientific mission objectives of the Skylab program, but may be best remembered for repairing launch damage to the vehicle and thus saving the program. EVA capability on Shuttle was initially planned to be a kit that could be flown at will, and was primarily intended for coping with vehicle return emergencies. The Skylab emergency and the pivotal role of EVA in salvaging that program quickly promoted Shuttle EVA to an essential element for achieving mission objectives, including retrieving satellites and developing techniques to assemble and maintain the International Space Station (ISS). Now, EVA is supporting assembly of ISS. This paper highlights development of US EVA capability within the context of the overarching mission objectives of the US human space flight program. 相似文献
8.
9.
Semenov Vladimir S. Kubyshkin Igor V. Kiendl Michael T. Biernat Helfried K. Rijnbeek Richard P. Besser Bruno P. 《Space Science Reviews》1997,82(3-4):451-461
In this article we address several criticisms of Petschek-type reconnection models which have recently been raised by Heikkila. We discuss features of the time-dependent Petschek-type models in the context of the solar wind-magnetosphere interaction, and point out that such models can incorporate and reproduce observed features at the magnetopause, such as plasma jets and erosion of the current sheet. We argue that some of Heikkila's criticisms can be attributed to weaknesses in the analysis due to incomplete experimental information, rather than to flaws in the concept of reconnection per se; in this category we include the question of which instability leads to the localised breakup of the magnetopause current sheet. Other criticisms are based on an adherence to steady-state models, and cannot be sustained within the extended time-dependent theory. We discuss, for example, how the time-dependent model can provide a consistent picture of how energy from the incoming solar wind is transferred and converted as it enters the magnetosphere. 相似文献
10.