全文获取类型
收费全文 | 7589篇 |
免费 | 30篇 |
国内免费 | 23篇 |
专业分类
航空 | 3889篇 |
航天技术 | 2652篇 |
综合类 | 33篇 |
航天 | 1068篇 |
出版年
2021年 | 52篇 |
2019年 | 53篇 |
2018年 | 99篇 |
2017年 | 64篇 |
2016年 | 59篇 |
2014年 | 139篇 |
2013年 | 181篇 |
2012年 | 163篇 |
2011年 | 228篇 |
2010年 | 166篇 |
2009年 | 259篇 |
2008年 | 343篇 |
2007年 | 188篇 |
2006年 | 184篇 |
2005年 | 187篇 |
2004年 | 171篇 |
2003年 | 248篇 |
2002年 | 147篇 |
2001年 | 255篇 |
2000年 | 152篇 |
1999年 | 189篇 |
1998年 | 233篇 |
1997年 | 160篇 |
1996年 | 209篇 |
1995年 | 273篇 |
1994年 | 251篇 |
1993年 | 150篇 |
1992年 | 183篇 |
1991年 | 99篇 |
1990年 | 92篇 |
1989年 | 198篇 |
1988年 | 88篇 |
1987年 | 88篇 |
1986年 | 88篇 |
1985年 | 253篇 |
1984年 | 206篇 |
1983年 | 181篇 |
1982年 | 193篇 |
1981年 | 231篇 |
1980年 | 81篇 |
1979年 | 61篇 |
1978年 | 68篇 |
1977年 | 69篇 |
1976年 | 49篇 |
1975年 | 87篇 |
1974年 | 53篇 |
1973年 | 53篇 |
1972年 | 75篇 |
1971年 | 56篇 |
1970年 | 54篇 |
排序方式: 共有7642条查询结果,搜索用时 15 毫秒
541.
Drag reducing and increasing mechanism on riblet surface has been studied through computational fluid dynamics(CFD).Drag reduction is achieved through the optimization of riblet geometry which would affect flow structure inside riblet grooves.Force and flow structure on riblet surface are analyzed and compared with those of smooth surface based on the k-εturbulence model.Drag reducing and increasing mechanism is proved to be related to microvortexes induced inside riblets which lead to Reynolds shear stress reduction significantly and is considered to be the dominant factor resulting in wall friction reduction.Simulation results also show that the pressure drag generating from the deviation of static pressure on the front and rear ends of riblets occurs and grows exponentially with Mach number,which can cause drag increasing.Furthermore,near-wall vortical structures,Reynolds shear stress and static pressure on riblet surfaces are also analyzed in detail. 相似文献
542.
The high inclination orbit for the International Space Station poses a risk to astronauts on EVA during occasional periods of enhanced high energy particle flux from the sun known as Solar Particle Events. We are currently unable to predict these events within the few-hour lead time required for evasive action. Compounding the threat is the fact that station construction occurs during increasing solar activity and through the peak of the solar cycle. In this paper we present an overview of the risk, the current methods to provide forecasts of SPEs, and potential risk mitigation options. 相似文献
543.
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
544.
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
545.
546.
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
547.
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
548.
A review of achievements in the investigation of the planar periodic oscillations and rotations of a satellite around its center of mass, which moves along an elliptic orbit in the central gravitational field, is presented. These oscillations and rotations of the satellite are described by an ordinary differential equation (Beletsky equation) of the second order with periodic coefficients and two parameters. The equation is equivalent to a periodic Hamiltonian system with a single degree of freedom and has a singularity. It turned out that two-parameter families of the generalized periodic solutions to this simple equation form complicated structures of a new type. A comparison of numerous separate results made it possible to outline a sufficiently unified and complete picture of the location and structure of the families of generalized 2-periodic solutions with an integer number of rotation. These families are compared with known data related to the resonance rotations of the natural satellites in the solar system. 相似文献
549.
The fine structure of the Auroral Kilometric Radiation (AKR) is studied using multicomponent measurements of the electric component of the electromagnetic field in the frequency band 4 kHz–1 MHz (the POLRAD experiment onboard the INTERBALL-2 satellite). Special attention is paid to the measurements near the source of the AKR: under conditions when the lower boundary of the emission range descended sufficiently low, down to the local gyrofrequency of electrons. From the analysis of the electric field structure the conclusion is drawn that the bulk of the AKR power is carried by the signal component fast variable in time and frequency (flickering component). The power of a constant component (continuum) is lower by at least an order of magnitude. During strong bursts of the AKR, the relative contribution of the flickering component increases. The spatial structure of the zone of generation has at least three characteristic scales along and across the magnetic field. 相似文献
550.
The results of comparison of the model profiles of density, obtained by means of the CDPDM model, with the experimental data of the ISEE-1 satellite for the years 1977–1983 are presented. The hypothesis on the validity of the mirror mapping of the convection boundary relative to the dawn–dusk direction is verified. An attempt to improve the CDPDM model for the dayside is made. 相似文献