全文获取类型
收费全文 | 8976篇 |
免费 | 19篇 |
国内免费 | 23篇 |
专业分类
航空 | 4029篇 |
航天技术 | 3110篇 |
综合类 | 198篇 |
航天 | 1681篇 |
出版年
2021年 | 85篇 |
2019年 | 57篇 |
2018年 | 183篇 |
2017年 | 123篇 |
2016年 | 124篇 |
2015年 | 62篇 |
2014年 | 214篇 |
2013年 | 263篇 |
2012年 | 262篇 |
2011年 | 381篇 |
2010年 | 271篇 |
2009年 | 408篇 |
2008年 | 459篇 |
2007年 | 261篇 |
2006年 | 200篇 |
2005年 | 237篇 |
2004年 | 231篇 |
2003年 | 272篇 |
2002年 | 279篇 |
2001年 | 351篇 |
2000年 | 155篇 |
1999年 | 221篇 |
1998年 | 246篇 |
1997年 | 158篇 |
1996年 | 241篇 |
1995年 | 270篇 |
1994年 | 251篇 |
1993年 | 146篇 |
1992年 | 193篇 |
1991年 | 72篇 |
1990年 | 71篇 |
1989年 | 179篇 |
1988年 | 79篇 |
1987年 | 74篇 |
1986年 | 87篇 |
1985年 | 252篇 |
1984年 | 208篇 |
1983年 | 150篇 |
1982年 | 175篇 |
1981年 | 273篇 |
1980年 | 68篇 |
1979年 | 57篇 |
1978年 | 65篇 |
1977年 | 55篇 |
1975年 | 64篇 |
1974年 | 60篇 |
1972年 | 49篇 |
1971年 | 52篇 |
1970年 | 49篇 |
1969年 | 47篇 |
排序方式: 共有9018条查询结果,搜索用时 296 毫秒
701.
In this paper a linear, closed-form analysis of the buckling behavior of an orthotropic plate with elastic clamping and edge reinforcement under uniform compressive load is presented. This is a typical structural situation found in aerospace engineering for instance as stiffeners in wings or the fuselage. All governing equations are transformed in a dimensionless system using common characteristic quantities to gain good analytical access. The buckling behavior is analyzed and generic buckling diagrams are presented. The solutions show excellent agreement with results from literature and numerical analyses.The minimum bending stiffness of the edge reinforcement needed to withstand buckling is examined and a minimum stiffness criterion is presented. Furthermore an absolute minimum bending stiffness is found which is sufficient to enable the reinforcement to act as a near-rigid support for arbitrarily long plates. These criteria are of interest for optimized lightweight design of stringers and stiffeners. 相似文献
702.
William P. Schonberg 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
Spacecraft that are launched to operate in Earth orbit are susceptible to impacts by meteoroids and pieces of orbital debris (MMOD). The effect of a MMOD particle impact on a spacecraft depends on where the impact occurs, the size, composition, and speed of the impacting object, the function of the impacted system. In order to perform a risk analysis for a particular spacecraft under a specific mission profile, it is important to know whether or not the impacting particle (or its remnants) will exit the rear of an impacted spacecraft wall. A variety of different ballistic limit equations (BLEs) have been developed for many different types of structural wall configurations. BLEs can be used to optimize the design of spacecraft wall parameters so that the resulting configuration is able to withstand the anticipated variety of on-orbit high-speed impact scenarios. While the level of effort exerted in studying the response of metallic multi-wall systems to high speed particle impact is quite substantial, the extent of the effort to study composite material and composite structural systems under similar impact conditions has been much more limited. This paper presents an overview of the activities performed to assess the resiliency of composite structures and materials under high speed projectile impact. The activities reviewed will be those that have been aimed at increasing the level of protection afforded to spacecraft operating in the MMOD environment, and more specifically, on those activities performed to mitigate the mechanical and structural effects of an MMOD impact. 相似文献
703.
A model of the main support of the aircraft landing gear is presented and its dynamics is studied using the equations for the distributed motion of a wheel equipped with a brake. Stability of the support wheel rolling motion is analyzed and the brake parameter values which may cause stability loss are found. The results of numerical experiments are presented. 相似文献
704.
The volume finite element in the form of hexahedron with nodal unknowns as components of the displacement vector and stress
tensor has been developed to analyze the shells of revolution. The displacement vector components for the inner point of the
finite element and the components of its stress tensor are expressed through the nodal unknowns using the method of vector
and tensor fields interpolation by the trilinear shape functions; that provides taking into account the finite element displacement
as a whole solid. The variational principle in a mixed formulation is applied to form the matrix of hexahedron deformation.
The efficiency of the proposed method for approximating the values being sought as vector and tensor fields in comparison
with the traditional method for approximating the values being sought as scalar fields is confirmed by a numerical example. 相似文献
705.
The paper presents a technique of calculating the life for gas turbine engine parts in a low cycle area based on the influence coefficients taking into account analytical and operational factors. A statistical method is used to determine the influence coefficients. 相似文献
706.
A. Lukaszczyk R. Williamson 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2010
Non-governmental organizations (NGOs) play a unique role in international affairs, providing access to resources, expertise, and assistance to supplement State resources. Sometimes the diplomatic skills and unofficial access of NGOs to policymakers through Track Two diplomacy can move a previously stalled critical issue forward and assist policymakers from different countries to find common ground outside official channels. Because they work outside of official channels, they are not bound by State policy that may inhibit negotiations between States. Some also have a convening power that sometimes makes it possible for State representatives to meet discipline experts and each other for informal discussions on issues of mutual interest. Finally, NGOs can draw attention to issues that may be overlooked or avoided by State organizations. 相似文献
707.
A comparison of temporal profiles of low-energy ion intensity and magnetic field magnitude in different periods of solar activity
in the outer heliosphere is carried out using the data of the Voyager 1 and Voyager 2 spacecraft. It is shown that temporal, spectral, and statistical characteristics of particle fluxes and magnetic field in
the heliospheric regions before and after the terminal shock in 2002–2008 had similar dynamics in different hemispheres. This
similarity allowed one to assume that, in the region of the inner heliospheric boundary, a quasistable spatial structure existed
moving together with the terminal shock in accordance with the solar wind pressure, as well as, probably, under the action
of the interstellar medium. It was revealed that the spatial dimensions of most details of this structure are less on Voyager 2, which, probably, is due to variation of the solar activity level, difference in latitude of spacecraft disposition, and
also the influence of the interstellar magnetic field. 相似文献
708.
I. Antonenko G.R. Osinski M. Battler M. Beauchamp L. Cupelli A. Chanou R. Francis M.M. Mader C. Marion E. McCullough A.E. Pickersgill L.J. Preston B. Shankar T. Unrau D. Veillette 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
Remote robotic data provides different information than that obtained from immersion in the field. This significantly affects the geological situational awareness experienced by members of a mission control science team. In order to optimize science return from planetary robotic missions, these limitations must be understood and their effects mitigated to fully leverage the field experience of scientists at mission control. 相似文献
709.
Kh. Karami S. Ghader A. Raeen 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
In the present work values of peak electron density (NmF2) and height of F2 ionospheric layer (hmF2) over Tehran region at a low solar activity period are compared with the predictions of the International Reference Ionosphere models (IRI-2001 and IRI-2007). Data measured by a digital ionosonde at the ionospheric station of the Institute of Geophysics, University of Tehran from July 2006 to June 2007 are used to perform the calculations. Formulations proposed by and are utilized to calculate the hmF2. The International Union of Radio Science (URSI) and International Radio Consultative Committee (CCIR) options are employed to run the IRI-2001 and IRI-2007 models. Results show that both IRI-2007 and IRI-2001 can successfully predict the NmF2 and hmF2 over Tehran region. In addition, the study shows that predictions of IRI-2007 model with CCIR coefficient has closer values to the observations. Furthermore, it is found that the monthly average of the percentage deviation between the IRI models predictions and the values of hmF2 and NmF2 parameters are less than 10% and 21%, respectively. 相似文献
710.
G.L. Smith K.J. Priestley N.G. Loeb B.A. Wielicki T.P. Charlock P. Minnis D.R. Doelling D.A. Rutan 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
The Clouds and Earth Radiant Energy System (CERES) project’s objectives are to measure the reflected solar radiance (shortwave) and Earth-emitted (longwave) radiances and from these measurements to compute the shortwave and longwave radiation fluxes at the top of the atmosphere (TOA) and the surface and radiation divergence within the atmosphere. The fluxes at TOA are to be retrieved to an accuracy of 2%. Improved bidirectional reflectance distribution functions (BRDFs) have been developed to compute the fluxes at TOA from the measured radiances with errors reduced from ERBE by a factor of two or more. Instruments aboard the Terra and Aqua spacecraft provide sampling at four local times. In order to further reduce temporal sampling errors, data are used from the geostationary meteorological satellites to account for changes of scenes between observations by the CERES radiometers. 相似文献