全文获取类型
收费全文 | 8240篇 |
免费 | 37篇 |
国内免费 | 16篇 |
专业分类
航空 | 3636篇 |
航天技术 | 2853篇 |
综合类 | 31篇 |
航天 | 1773篇 |
出版年
2021年 | 89篇 |
2019年 | 50篇 |
2018年 | 209篇 |
2017年 | 141篇 |
2016年 | 150篇 |
2015年 | 61篇 |
2014年 | 218篇 |
2013年 | 271篇 |
2012年 | 255篇 |
2011年 | 377篇 |
2010年 | 277篇 |
2009年 | 409篇 |
2008年 | 433篇 |
2007年 | 274篇 |
2006年 | 188篇 |
2005年 | 233篇 |
2004年 | 218篇 |
2003年 | 258篇 |
2002年 | 185篇 |
2001年 | 268篇 |
2000年 | 144篇 |
1999年 | 181篇 |
1998年 | 220篇 |
1997年 | 136篇 |
1996年 | 188篇 |
1995年 | 243篇 |
1994年 | 222篇 |
1993年 | 127篇 |
1992年 | 169篇 |
1991年 | 54篇 |
1990年 | 61篇 |
1989年 | 159篇 |
1988年 | 65篇 |
1987年 | 66篇 |
1986年 | 75篇 |
1985年 | 221篇 |
1984年 | 177篇 |
1983年 | 135篇 |
1982年 | 141篇 |
1981年 | 247篇 |
1980年 | 65篇 |
1979年 | 55篇 |
1978年 | 58篇 |
1977年 | 51篇 |
1975年 | 58篇 |
1974年 | 46篇 |
1973年 | 37篇 |
1972年 | 46篇 |
1971年 | 44篇 |
1970年 | 42篇 |
排序方式: 共有8293条查询结果,搜索用时 15 毫秒
371.
The concept of designing a high-speed, permanent magnet, brushless DC motor aircraft fuel pump drive using a cycloconverter link is examined. A combination of sinusoidal and DC steady-state analysis is used to produce a simple model of the system. A closed-loop control system with an outer loop based on speed and an inner loop based on current is postulated wherein a proportional-plus-integral controller is placed in the forward path to assure minimum speed error. Gains are then set to assure that the eigenvalues of the linearized control system lie within the left half s -plane over the entire full range 相似文献
372.
Two energetic events in the Earth’s magnetotail detected by Geotail are examined with detailed analysis of three-dimensional
velocity phase space density. It is found that the occurrence of multiple ion components is high during these dynamic episodes.
Different populations evolve independently of each other, suggesting particles from multiple activity sites contributing to
the observed phase space density. The transport properties with consideration of multiple components are evaluated, with the
result showing significant differences from those based on a single fluid approach. This comparison indicates that precise
evaluation of the energy and magnetic flux transport of energetic events in the magnetotail requires resolving individual
populations in the phase space density. 相似文献
373.
S. Mereghetti A. Tiengo P. Esposito G. Vianello A. De Luca D. Götz G. Weidenspointner A. von Kienlin G.L. Israel L. Stella N. Rea R. Turolla S. Zane 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2011
We describe the results obtained with Target of Opportunity observations of the galactic sources SGR 1627–41 and 1E 1547–5408. These two transients show several similarities supporting the interpretation of Anomalous X-ray Pulsars and Soft Gamma-ray Repeaters as a single class of strongly magnetized neutron stars. 相似文献
374.
Basic equations of hydrodynamics in an elastic pipeline that are found using the vectorial notation of Newton’s law for a viscous fluid are presented. An analog of Poiseuille’s formula for the fluid flowrate in the elastic pipeline and the results of the comparative analysis of a viscous flow in elastic and rigid pipelines are given. It is shown that the ratio of the maximal fluid velocity in the pipeline cross section and the section-average velocity is similar for elastic and rigid pipelines. 相似文献
375.
The strain–stress state of the solid propellant rocket engines (SPREs) is simulated under impact. The effect of orientation of elastic and strength properties of orthotropic organoplastic shell material on the strain–stress state of the solid propellant is investigated. Normal and oblique impact of single steel cylinder projectiles, both simultaneous and at different times of multiple, converging steel spheric particles with SPRE are investigated in this study. The investigation is conducted numerically. The numerical modeling was carried out in a three-dimensional formulation by the method of finite elements for the continuous approach of the mechanics of a deformable solid. The destruction of the anisotropic material is described by the tensor-polynomial criterion of the fourth degree, which takes into account the influence of hydrostatic pressure. 相似文献
376.
A set of algorithms is presented for finding the best set of K mutually exclusive paths through a trellis of N nodes, with worst-case computation time bounded by N 3log n for a fixed-precision computation. The algorithms are based on a transformation of the K-path trellis problem into an equivalent minimum-cost network flow (MCNF) problem. The approach allows the application of efficient MCNF algorithms, which can obtain optimal solutions orders of magnitude faster than the algorithm proposed by J.K. Wolf et al. (1989). The resulting algorithms extend the practicality of the trellis formulation (in terms of required computations) to multiobject tracking problems with much larger numbers of targets and false alarms. A response by Wolf et al. is included 相似文献
377.
Ground penetrating radar VIY-2 总被引:2,自引:0,他引:2
Prokhorenko V.P. Ivashchuk V.E. Korsun S.V. 《Aerospace and Electronic Systems Magazine, IEEE》2005,20(7):16-18
VIY-2 ground penetrating radar (GPR) with unique sounding possibilities and use simplicity is presented at this paper. VIY-2 GPR combines all units (synchronizer, transmitting and receiving modules, powering, and antenna system) into single case. The VIY-2 GPR communicates with computer via standard interface RS232 or USB1.0. Technical solutions utilized by the VIY-2 GPR reduce deployment time and simplify surveying process. The VIY-2 GPR design features and its components interaction are considered at this paper. Some field results are also presented here. The VIY-2 GPR design concept allows reducing the data acquisition time,, optimizing the time-varying gain control function, applying depth-stacking dependence, controlling the surveying window position and interference reducing by pulse repetition frequency randomizing. 相似文献
378.
Prateek R. Srivastava Sneha A. Gokani Ajeet K. Maurya Rajesh Singh Sushil Kumar B. Veenadhari R. Selvakumaran Abhay K. Singh Devendraa Siingh Janos Lichtenberger 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
One-to-one relation with its causative lightning discharges and propagation features of night-time whistlers recorded at low-latitude station, Allahabad (geomag. lat. 16.05°N, L = 1.08), India, from continuous observations made during 1–7 April, 2011 have been studied. The whistler observations were made using the Automatic Whistler Detector (AWD) system and AWESOME VLF receiver. The causative lightning strikes of whistlers were checked in data provided by World-Wide Lightning Location Network (WWLLN). A total of 32 whistlers were observed out of which 23 were correlated with their causative lightnings in and around the conjugate location (geom. lat. 9.87°S) of Allahabad. A multi-flash whistler is also observed on 1 April with dispersions 15.3, 17.5 and 13.6 s1/2. About 70% (23 out of 32) whistlers were correlated with the WWLLN detected causative lightnings in the conjugate region which supports the ducted mode of propagation at low latitude. The multi-flash and short whistlers also propagated most likely in the ducted mode to this station. 相似文献
379.
V.?A.?Afanas’ev G.?L.?Degtyarev A.?S.?Meshchanov "mailto:mas@list.ru " title= "mas@list.ru " itemprop= "email " data-track= "click " data-track-action= "Email author " data-track-label= " ">Email author 《Russian Aeronautics (Iz VUZ)》2018,61(1):23-31
Real values of parameters for a space vehicle and its steering devices are specified by using the motion parameters measured in flight based on solving the differential equations of motion. 相似文献
380.
Energetic particle observations in the interplanetary medium provide fundamental information about the origin, development
and structure of coronal mass ejections. This paper reviews the status of our understanding of the ways in which particles
are energised at the Sun in association with CMEs. This understanding will remain incomplete until the relationship between
CMEs and flares is determined and we know the topology of the associated magnetic fields. The paper also discusses the characteristics
of interplanetary CMEs that may be probed using particle observations. 相似文献