全文获取类型
收费全文 | 857篇 |
免费 | 151篇 |
国内免费 | 205篇 |
专业分类
航空 | 570篇 |
航天技术 | 182篇 |
综合类 | 38篇 |
航天 | 423篇 |
出版年
2024年 | 2篇 |
2023年 | 55篇 |
2022年 | 84篇 |
2021年 | 103篇 |
2020年 | 75篇 |
2019年 | 65篇 |
2018年 | 47篇 |
2017年 | 10篇 |
2016年 | 53篇 |
2015年 | 45篇 |
2014年 | 82篇 |
2013年 | 101篇 |
2012年 | 83篇 |
2011年 | 99篇 |
2010年 | 38篇 |
2009年 | 52篇 |
2008年 | 26篇 |
2007年 | 55篇 |
2006年 | 33篇 |
2005年 | 9篇 |
2004年 | 2篇 |
2003年 | 4篇 |
2002年 | 7篇 |
2001年 | 36篇 |
2000年 | 11篇 |
1999年 | 20篇 |
1991年 | 1篇 |
1988年 | 2篇 |
1987年 | 13篇 |
排序方式: 共有1213条查询结果,搜索用时 15 毫秒
571.
《Aerospace Science and Technology》2001,5(4):283-291
The paper discusses possibilities to make use of the finite element model (FE model) to prepare and analyse ground vibration tests (GVT) on large aircraft. It reports about experiences gained in the preparation stage and of research GVT on an Airbus A340-300.The observability and controllability of the expected structural modes can be investigated on the basis of FE calculations during the preparation of the GVT. By this means, the engineering skill strictly necessary to set up a GVT can be backed up with analytical findings potentially resulting in improvements of the pick-up and excitation plan. During and after the test the performance of orthogonality checks as well as the calculation of generalized masses on the basis of the measured mode shapes and the analytical mass matrix provide useful additional information for the evaluation and interpretation of test results. The detection of deviations of the measurements from the analytical model might influence the test procedure drawing attention towards the least precisely predicted modes which can prove to be very useful for the updating of the FE model.A prerequisite for any activity related to the FE model is a proper association of the test data with the analytical calculations. For this task the use of a generalized static expansion technique is recommended which is a suitable tool during both the preparation and analysis stage of a GVT. 相似文献
572.
《Air & Space Europe》2001,3(3-4):143-147
Conventional methods of repair for metallic aircraft structures usually include the use of bolted or riveted metallic reinforcement by introducing the cutting of damaged areas, which is not suggested in aging cases, due to the reduction of strength. Although today the bonded repairs are considered more efficient than the mechanically fastened, each repair is looked-upon as tailored made for each job. However, for the industrial engineering and maintenance production line, there is a lack of appropriate technical manuals - standards and specifications to control the implementation of the repair as integrated method - from the design phase, up to application and quality assurance. 相似文献
573.
《Air & Space Europe》2001,3(3-4):129-131
CFRP (carbon fibre reinforced plastics) fuselage structures are expected to be realized with future generations of aircraft. Going into the postbuckling regime with these structures requires improved, fast and reliable procedures for analysis and design of stiffened fibre composite panels. The EC project POSICOSS, which was started in the year 2000, develops such procedures. This article deals with the main objectives of the project, the theoretical and experimental work to be carried out, and with first results. 相似文献
574.
《Air & Space Europe》2001,3(3-4):318-320
The Group for Aeronautical Research and Technology in EURope (GARTEUR) was set up with the main aim of facilitating European aeronautics R&T collaboration, covering both the civil and the military applications. Since its creation more than 25 years ago, GARTEUR has conducted numerous collaborative projects. This paper describes briefly the GARTEUR organisation and provides an overview of the scientific and technical activity performed under this umbrella. 相似文献
575.
《Acta Astronautica》2001,48(5-12):461-468
We have been studying a large membrane space structure named “Furoshiki Satellite,” as a promising candidate of a future space system for those missions requiring large area in space such as solar power generation, a large communication antenna, or a large radiator. This membrane is folded in a very small volume during launch and is deployed and controlled by a set of several satellites at its corners or using centrifugal force generated by rotating the central satellite. It is expected that such a structure will reduce the weight per area of the space structure and, if the control technology is properly applied, it can be efficiently folded during launch and easily deployed after release. This paper shows the concept of Furoshiki Satellite, its applications, and its dynamics on orbit and how to control it. A nano-satellite project on demonstrate the concept of Furoshiki Satellite will also be described briefly. 相似文献
576.
577.
578.
A direct method for rapid generation of combined time-propellant near-optimal trajectories of proximity maneuvers of a chaser spacecraft required to dock a target one, with predetermined thrust history along a master direction, is presented. The predetermined thrust history is generated by applying the Pontryagin maximum principle. The new direct method, already implemented and tested on board real aircraft, is based on three concepts: high-order polynomials as reference functions, preset on–off sequence of a master control, and reduction of the optimization problem to the determination of a small set of parameters. Presetting the master control, the remaining controls act as slaves, guarantying the chaser to move along the desired path. Seeking of the optimum strategy is transformed into a nonlinear programming problem, and then numerically solved through an ad hoc algorithm in accelerated time scale. Examples are reported to prove the rapidness of the approach to generate a sub-optimal docking trajectory. 相似文献
579.
In early 2008, the need arose to predict the orbital decay of the American spacecraft USA-193, whose characteristics, function and orbit were classified information. With no orbit data and independent Italian tracking capability available, we turned our attention on the orbits determined by a worldwide network of about 20 visual satellite observers. The orbits of USA-193 obtained from their visual observations were therefore used as the sole source of orbit information. Contrary to our expectations, this exercise was extremely successful and we learned a lot in the process. The orbits provided by the visual observers were very accurate for such a low satellite (although the minimum and very stable level of solar activity helped considerably); however, data gaps of a few days were sometimes possible, due to unfavorable pass geometry or weather and light conditions. In any case, the orbital period and the semimajor axis were so accurate that it was possible for us to obtain very good decay fits using special perturbation software, including various atmospheric density models together with all the other relevant perturbing accelerations. We were therefore able to estimate accurate values of the ballistic parameter and the resulting decay and reentry predictions were extremely stable. Amateur optical observations and images of USA-193 had also led to a rough estimation of the shape and sizes of the satellite, revealing that the solar arrays had never been deployed. With this information, and taking into account our estimates of the ballistic parameter, we obtained reasonable and consistent values of the spacecraft mass. Based on previous reentry fragmentation analyses, we were then able to guess the expected USA-193 casualty area, casualty expectancy, debris ground footprint and probability of impact in Italy. Lastly, after the decision by the US Government to destroy the satellite, we independently predicted the interception time windows and the post-event ground tracks. Following the successful spacecraft breakup, we analyzed the evolution of the resulting debris cloud and assessed its (very limited) adverse impact on the circumterrestrial environment. 相似文献
580.
This paper deals with the design, development and experimentation of a new test stand for the accurate and precise characterization of small cold-gas nozzles having thrust of the order of 0.1 N and specific impulse of the order of 10 s. As part of the presented research, a new cold-gas supersonic nozzle was designed and developed based on the quasi one-dimensional theory. The test stand is based on the ballistic-pendulum principle: in particular, it consists of a suspended gondola hosting the propulsion system and the sample nozzle. The propulsion system consists of an air tank, pressure regulator, solenoid valve, battery and digital timer to command the valve. The gondola is equipped with a fin, immersed in water, to provide torsional and lateral oscillation damping. A laser sensor measures the displacement of the gondola. The developed test stand was calibrated by using a mathematical model based on the inelastic collision theory. The obtained accuracy was of ~1%. Sample experimental results are reported regarding the comparison of the new supersonic nozzle with a commercially available subsonic nozzle. The obtained measurements of thrust, mass flow rate and specific impulse are precise to a level of ~3%. The broad goal of the presented research was to contribute to an upgraded design of a spacecraft simulator used for laboratory validation of guidance, navigation and control algorithms for autonomous docking manoeuvres. 相似文献