全文获取类型
收费全文 | 6503篇 |
免费 | 620篇 |
国内免费 | 479篇 |
专业分类
航空 | 3675篇 |
航天技术 | 1728篇 |
综合类 | 432篇 |
航天 | 1767篇 |
出版年
2023年 | 42篇 |
2022年 | 108篇 |
2021年 | 155篇 |
2020年 | 130篇 |
2019年 | 116篇 |
2018年 | 272篇 |
2017年 | 262篇 |
2016年 | 172篇 |
2015年 | 185篇 |
2014年 | 231篇 |
2013年 | 235篇 |
2012年 | 280篇 |
2011年 | 403篇 |
2010年 | 428篇 |
2009年 | 470篇 |
2008年 | 419篇 |
2007年 | 359篇 |
2006年 | 198篇 |
2005年 | 249篇 |
2004年 | 170篇 |
2003年 | 201篇 |
2002年 | 200篇 |
2001年 | 207篇 |
2000年 | 149篇 |
1999年 | 185篇 |
1998年 | 152篇 |
1997年 | 139篇 |
1996年 | 126篇 |
1995年 | 159篇 |
1994年 | 150篇 |
1993年 | 78篇 |
1992年 | 106篇 |
1991年 | 56篇 |
1990年 | 54篇 |
1989年 | 73篇 |
1988年 | 38篇 |
1987年 | 43篇 |
1986年 | 29篇 |
1985年 | 88篇 |
1984年 | 48篇 |
1983年 | 49篇 |
1982年 | 63篇 |
1981年 | 92篇 |
1980年 | 28篇 |
1979年 | 26篇 |
1978年 | 25篇 |
1977年 | 19篇 |
1976年 | 16篇 |
1975年 | 18篇 |
1972年 | 18篇 |
排序方式: 共有7602条查询结果,搜索用时 390 毫秒
101.
通过分析返回舱在着陆时所具有的水平速度也可能对着陆安全构成威胁的事实,论述了为返回舱增加横向缓冲装置的必要性,由此提出亟需解决的问题——在返回舱着陆下降过程中测量其水平运动速度。提出了全新概念的以视觉图像为唯一信息来源的返回舱水平速度测量方法,仅以双目摄像机系统作为传感器,避免了对返回舱原有传感器系统的改造。通过对着陆区域地面图像的采集和在线处理,利用双目交会测量原理得到返回舱当前的大地坐标和姿态。仿真实验证明,方法简单易行,测量结果可靠。 相似文献
102.
L. M. Zelenyi M. S. Dolgonosov A. A. Bykov V. Yu. Popov Kh. V. Malova 《Cosmic Research》2002,40(4):357-366
Using both analytical and numerical models of the collisionless anisotropic current sheet generated by the impinging flows of transient ions, we have studied the self-consistent solutions taking the plasma trapped in the sheet into account. It is demonstrated that there exists a limited window in the space of system parameters where self-consistent solutions can exist. When the density of the quasi-trapped plasma is sufficiently large, a redistribution of the total current can be a cause of the sheet decay, when the local current of the trapped particles compensate (totally or in part) the main current in the center and at the edges of the sheet, while the total current generated by ions on the trapped trajectories vanishes. 相似文献
103.
104.
The US RLV program aims to stimulate commercial development of a next-generation heavy-lift launcher and lower launch costs by one order of magnitude from the Space Shuttle. This paper discusses the incentives needed to encourage private investment — income tax relief, investment mitigation, financing assistance — in the venture and uses a specifically developed case study model to evaluate their effectiveness. It finds that an R&D tax credit would be the most practical incentive. Directions for future work are provided. 相似文献
105.
Green AR Andrews HR Bennett LG Clifford ET Ing H Jonkmans G Lewis BJ Noulty RA Ough EA 《Acta Astronautica》2005,56(9-12):949-960
In light of the importance of the neutron contribution to the dose equivalent received by space workers in the near-Earth radiation environment, there is an increasing need for a personal dosimeter that is passive in nature and able to respond to this neutron field in real time. Recent Canadian technology has led to the development of a bubble detector, which is sensitive to neutrons, but insensitive to low linear energy transfer (LET) radiation. By changing the composition of the bubble detector fluid (or “superheat”), the detectors can be fabricated to respond to different types of radiation. This paper describes a preliminary ground-based research effort to better characterize the bubble detectors of different compositions at various charged-particle accelerator facilities, which are capable of simulating the space radiation field. 相似文献
106.
本文针对目前战术导弹广泛使用捷联系统这个现状,以及对大机动飞行器采用姿态角误差反馈进行控制有时出现的奇异问题,提出并研究了基于四元数进行大机动战术导弹姿态控制系统的设计问题.首先,给出了大机动飞行情况下基于四元数描述的弹体动力学模型,然后,基于非线性控制系统综合方法设计了姿态控制系统,并验证了系统的稳定性.研究结果表明,基于四元数进行姿态控制系统的设计,既解决了姿态角表示法存在的奇异性问题,又可减少捷联计算机的计算量,对战术导弹具有重要实际意义.最后,通过数值仿真验证了本文提出方法的有效性. 相似文献
107.
108.
The dynamics of the proton energy spectrum during the solar cycle is studied. The spectra were determined by 1–100 MeV particle fluxes measured by different instruments mounted aboard the Earth's IMP-8 satellite for more than one hundred quiet-time intervals in the period between 1974 and 1991. The galactic branch of the spectra (E
p > 10 MeV) constructed for every quiet interval was fitted by a power law function, J =CE
. The theory predicts that in the 1–100 MeV energy range, where the adiabatic cooling of particles is dominant, = 1, while we have derived a double-peak distribution. The main maximum has the mean value = 1.35. The mean value of the second, much weaker maximum, is = 0.95. Within the main maximum, values are distributed in accordance with the Gaussian law with a standard deviation D/ = 0.12. The substantial difference of from unity requires the elaboration of a new model of modulation processes in the inner heliosphere. The values corresponding to the second maximum show that modulation processes correspond sometimes to theoretical conceptions. It is shown that correlates weakly with parameters A and describing the solar branch of the spectrum (J(E) = AE
–). At the same time, a more significant correlation is observed between and the solar activity index, R
z, the counting rate of the Deep River neutron monitor, and the energy value in the minimum of the energy spectrum flux, E
min. 相似文献
109.
The detection of substellar companions of nearby stars is very difficult, because these objects are very faint and are located close to their bright parent stars. One way to attack this problem is to look for small periodic motion of the bright star, induced by the gravitational attraction exerted by the unseen small companion. Over the past decade stellar spectroscopy has been used by a few teams to monitor the radial velocities of several samples of stars, with a threshold for detection of companions well below the substellar limit of about 0.08 M. So far, only a few possible spectroscopic binaries have been identified where the unseen companions might be substellar.Recently, radio observers discovered that the millisecond pulsar PSR B1257+12 displays periodic variations in the pulse arrival times, indicating the existence of at least three unseen companions in circular orbits around the pulsar. This is the first convincing case for an extra-solar planetary system. This discovery suggests that planetary systems can form in very different situations, and therefore may prove to be common. 相似文献
110.