全文获取类型
收费全文 | 6052篇 |
免费 | 30篇 |
国内免费 | 12篇 |
专业分类
航空 | 2727篇 |
航天技术 | 2012篇 |
综合类 | 29篇 |
航天 | 1326篇 |
出版年
2021年 | 59篇 |
2019年 | 39篇 |
2018年 | 161篇 |
2017年 | 127篇 |
2016年 | 133篇 |
2015年 | 57篇 |
2014年 | 152篇 |
2013年 | 189篇 |
2012年 | 179篇 |
2011年 | 244篇 |
2010年 | 199篇 |
2009年 | 290篇 |
2008年 | 321篇 |
2007年 | 185篇 |
2006年 | 135篇 |
2005年 | 175篇 |
2004年 | 162篇 |
2003年 | 199篇 |
2002年 | 139篇 |
2001年 | 217篇 |
2000年 | 103篇 |
1999年 | 120篇 |
1998年 | 152篇 |
1997年 | 105篇 |
1996年 | 131篇 |
1995年 | 195篇 |
1994年 | 155篇 |
1993年 | 84篇 |
1992年 | 122篇 |
1991年 | 42篇 |
1990年 | 57篇 |
1989年 | 121篇 |
1988年 | 50篇 |
1987年 | 48篇 |
1986年 | 48篇 |
1985年 | 159篇 |
1984年 | 141篇 |
1983年 | 123篇 |
1982年 | 117篇 |
1981年 | 158篇 |
1980年 | 47篇 |
1979年 | 43篇 |
1978年 | 41篇 |
1977年 | 40篇 |
1976年 | 30篇 |
1975年 | 47篇 |
1974年 | 33篇 |
1973年 | 23篇 |
1972年 | 43篇 |
1971年 | 28篇 |
排序方式: 共有6094条查询结果,搜索用时 15 毫秒
601.
Zhu Jiang Abimael F D Loula Laboratório Nacional de Computaco Científica MCT Av Getúlio Vargas Petrópolis RJ Brazil 《南京航空航天大学学报(英文版)》2001,18(Z1)
INTRODUCTIONThe stationary incompressible Navier- Stokesequations in the primitive variables( velocity uand pressure p)- vΔu + u . u + p =f . u =0u =g inΩinΩonΓ( 1 )whereΩ is a set of R2 ,Γ its boundary,v thekinematic viscosity of the fluid,f a field of givenbody forces,and g a given field defined onΓ sa-tisfying the global conservation property∫Γg . n =0 ( 2 )n denotes the unit outer normal vector toΓ.The vorticity- velocity system of Eq.( 1 ) ,byintroducing the vorticityω… 相似文献
602.
Heilbronn L Frankel K Holabird K Zeitlin C McMahan MA Rathbun W Cronqvist M Gong W Madey R Htun M Elaasar M Anderson BD Baldwin AR Jiang J Keane D Scott A Shao Y Watson JW Zhang WM Galonsky A Ronningen R Zecher P Kruse J Wang J Cary R 《Acta Astronautica》1998,42(1-8):363-373
In order to help assess the risk to astronauts due to the long-term exposure to the natural radiation environment in space, an understanding of how the primary radiation field is changed when passing through shielding and tissue materials must be obtained. One important aspect of the change in the primary radiation field after passing through shielding materials is the production of secondary particles from the breakup of the primary. Neutrons are an important component of the secondary particle field due to their relatively high biological weighting factors, and due to their relative abundance, especially behind thick shielding scenarios. Because of the complexity of the problem, the estimation of the risk from exposure to the secondary neutron field must be handled using calculational techniques. However, those calculations will need an extensive set of neutron cross section and thicktarget neutron yield data in order to make an accurate assessment of the risk. In this paper we briefly survey the existing neutron-production data sets that are applicable to the space radiation transport problem, and we point out how neutron production from protons is different than neutron production from heavy ions. We also make comparisons of one the heavy-ion data sets with Boltzmann-Uehling-Uhlenbeck (BUU) calculations. 相似文献
603.
Cognitive performance aboard the life and microgravity spacelab 总被引:2,自引:0,他引:2
The impact of microgravity and other stressors on cognitive performance need to be quantified before long duration space flights are planned or attempted since countermeasures may be required. Four astronauts completed 38 sessions of a 20-minute battery of six cognitive performance tests on a laptop computer. Twenty-four sessions were preflight, 9 sessions were in-orbit, and 5 sessions were postflight. Mathematical models of learning were fit to each subject's preflight data for each of 14 dependent variables. Assuming continued improvement, expected values were generated from the models for in-orbit comparison. Using single subject designs, two subjects showed statistically significant in-orbit effects. One subject was degraded in two tests, the other was degraded in one test and exceeded performance expectations in another. Other subjects showed no statistically significant effects on the tests. The factors causing the deterioration in the two subjects can not be determined without appropriate ground-based control groups. 相似文献
604.
Steven D. Howe 《Space Policy》2001,17(4):275-283
The potential benefits to humankind of space exploration are tremendous. Space is not only the final frontier but is also the next marketplace. The orbital space above Earth offers tremendous opportunities for both strategic assets and commercial development. The critical obstacle retarding the use of the space around the Earth is the lack of low cost access to orbit. Further out, the next giant leap for mankind will be the human exploration of Mars. Almost certainly within the next 30 years, a human crew will brave the isolation, the radiation, and the lack of gravity to walk on and explore the Red planet. Both of these missions will change the outlook and perspective of every human being on the planet. However, these missions are expensive and extremely difficult. Chemical propulsion has demonstrated an inability to achieve orbit cheaply and is a very high-risk option to accomplish the Mars mission. An alternative solution is to develop a high performance propulsion system. Nuclear propulsion has the potential to be such a system. The question will be whether humanity is willing to take on the challenge. 相似文献
605.
606.
The transport characteristics of macroparticles, charged by the solar radiation under microgravity conditions, were investigated by analyzing the videorecords of experiments carried out onboard the Mirorbital station. The temperature, distributions of velocities, charge, friction coefficient, and diffusion coefficient were obtained for bronze particles. 相似文献
607.
The first of a series of problems of the optimization of correction of satellite systems, moving over near-circular orbits, is considered. The correction is accomplished by means of low-thrust engines and is supposed to be flexible, where only the parameters of the relative motion of satellites must be corrected. The problem has a large dimension, but is invariant with respect to renumbering of satellites. This allows us to decompose the problem, i.e., to find new variables, linearly dependent on old ones, in which the problem breaks down into a series of independent subproblems of low dimension. The decomposition is accomplished by means of the technique [1] based on the theory of linear representations of groups. 相似文献
608.
V. I. Badin 《Cosmic Research》2001,39(1):88-91
To describe the generation of the electric field by a discontinuity of the Hall current, an equation of the third order was obtained by using the electric charge conservation and Ohm's laws. Solutions to this equation are used to model the electric impulses detected in experiments aimed to verify the Alfven hypothesis on the critical ionization velocity in the process of the collision of neutral gas with magnetized plasma. A quantitative agreement with the experiment was attained, and the basic features of the measured signals were modeled under an assumption of strong anomalous resistance behind the discontinuity. Apparently, such an anomalous resistance occurs due to trapping of the current carriers by a small-scale modulation of the electric field. 相似文献
609.
The data of measuring the plasma density in the topside ionosphere for the South-Atlantic geomagnetic anomaly region are presented. It is shown that irregular plasma structures with a wide spectrum of irregularity scale (including large-scale structures with a dimension of order of some hundred kilometers) can be generated in the fields of electrostatic turbulence in inhomogeneous plasma. 相似文献
610.
Using a novel space platform-based manipulator with slewing and deployable links, the paper addresses two issues of considerable importance: (a) How important is it to model flexibility of the system? (b) How many modes are needed to adequately represent the elastic character? Results suggest that the fundamental mode is able to capture physics of the response quite accurately. Due to its massive character, the platform dynamics is virtually unaffected, even by severe maneuvers of the manipulator. Hence, treating the platform as rigid would save the computational cost without affecting the accuracy. Although the link flexibility does affect the manipulator's tip vibration, the joint and platform vibrations remain negligible. The revolute joint flexibility appears to be an important parameter affecting both the joint as well as tip responses. The information should prove useful in the design of this new class of manipulators. 相似文献