全文获取类型
收费全文 | 14763篇 |
免费 | 9234篇 |
国内免费 | 1504篇 |
专业分类
航空 | 19994篇 |
航天技术 | 2900篇 |
综合类 | 195篇 |
航天 | 2412篇 |
出版年
2024年 | 96篇 |
2023年 | 184篇 |
2022年 | 282篇 |
2021年 | 344篇 |
2020年 | 435篇 |
2019年 | 1154篇 |
2018年 | 1387篇 |
2017年 | 1295篇 |
2016年 | 1320篇 |
2015年 | 1235篇 |
2014年 | 1139篇 |
2013年 | 1170篇 |
2012年 | 1093篇 |
2011年 | 1115篇 |
2010年 | 1065篇 |
2009年 | 1147篇 |
2008年 | 1125篇 |
2007年 | 827篇 |
2006年 | 653篇 |
2005年 | 492篇 |
2004年 | 454篇 |
2003年 | 459篇 |
2002年 | 440篇 |
2001年 | 436篇 |
2000年 | 355篇 |
1999年 | 428篇 |
1998年 | 404篇 |
1997年 | 350篇 |
1996年 | 310篇 |
1995年 | 327篇 |
1994年 | 317篇 |
1993年 | 300篇 |
1992年 | 282篇 |
1991年 | 386篇 |
1990年 | 328篇 |
1989年 | 426篇 |
1988年 | 265篇 |
1987年 | 134篇 |
1986年 | 90篇 |
1985年 | 187篇 |
1984年 | 157篇 |
1983年 | 120篇 |
1982年 | 127篇 |
1981年 | 175篇 |
1980年 | 57篇 |
1979年 | 65篇 |
1978年 | 59篇 |
1975年 | 66篇 |
1972年 | 57篇 |
1967年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
481.
A. Kiraga Z. K
os V. N. Oraevsky V. C. Dokukin S. A. Pulinets 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,15(12):21-24
There were several cases when modulated electron beam had been injected from APEX satellite into an otherwise unmodified ionospheric plasma. The beams were formed from 2 μs pulses repeated at a rate of 40kHz. Injections took place in the altitude range 400–1100km over Europe. The onboard receiver connected to the dipole antenna swept the frequency interval 1–10 MHz in 1 sec. Due to a relatively narrow receiver bandwidth (15 kHz), moderate frequency step (50 kHz), slow changes of fn and fc (local electron plasma and gyro frequencies) fine structure of excited emissions was detected. In many cases, a very prominent doublet could be convincingly identified as a (fn,fu=sqrt(fn*fn+fc*fc)) band, with fn practically equal to the local plasma frequency in the unperturbed ionosphere. Determination of plasma frequency is of prime importance in analysis of complex structures composed from various harmonics fn,fc,fu. Complexity is manifested as coalescing of various harmonics with maxima shifted from nominal frequencies or splitting them into components due to mixing of local and propagation effects. Despite the existence of strong emissions on frequencies characteristic for the unperturbed ionosphere, very strong broadcasting transmissions are frequently cut off, even when the beam is directed upward. Most typical spectra are discussed. 相似文献
482.
483.
Ample research evidence from space analogs points to the crucial role that teamwork plays in the performance of small groups in isolation and confinement. This paper surveys findings about the impacts of group behavior and social interaction on crew morale, coordination, and productivity. Implications for the organization, selection, and training of crews for extended spaceflight are discussed. 相似文献
484.
Palinkas LA 《Journal of spacecraft and rockets》1990,27(5):471-477
This paper examines the utility of remote, isolated Antarctic research stations as analogs for long-duration spaceflights from the perspective of psychosocial processes of adaptation and adjustment. Certain features of the physical and man-made environments found in Antarctica are similar to those that will be encountered in outer space. In both settings, men and women are likely to experience a number of physiological and psychological changes in response to the extreme environmental conditions and the prolonged isolation and confinement. Biomedical research in Antarctica provides an opportunity to study the causes of these changes and to develop strategies for reducing the risks to health and well-being before they pose a serious threat to crew safety and mission success. A number of lessons for long-duration spaceflight are examined, including screening and selection of personnel; training programs designed to facilitate individual adjustment and group adaptation and minimize group conflict; identification of optimal leadership characteristics for small, isolated groups; an understanding of social dynamics and group "microcultures" necessary for the organization and management of small but heterogeneous groups; organization of work activities; facility design; and support infrastructure. 相似文献
485.
King CB Butterfield AJ Hypes WD Nealy JE Simonsen LC 《Journal of spacecraft and rockets》1990,27(3):225-226
The space shuttle external tank, which consists of a liquid oxygen tank, an intertank structure, and a liquid hydrogen tank, is an expendable structure used for approximately 8.5 min during each launch. A concept for outfitting the liquid oxygen tank-intertank unit for a 12-person lunar habitat is described. The concept utilizes existing structures and openings for both man and equipment access without compromising the structural integrity of the tank. Living quarters, instrumentation, environmental control and life support, thermal control, and propulsion systems are installed at Space Station Freedom. The unmanned habitat is then transported to low lunar orbit and autonomously soft landed on the lunar surface. Design studies indicate that this concept is feasible by the year 2000 with concurrent development of a space transfer vehicle and manned cargo lander for crew changeover and resupply. 相似文献
486.
487.
<正> 飞机结构所承受的噪声载荷声压级超过130dB时,经较长时间激励,往往在噪声诱导应力较大的区域里产生疲劳破坏。而在各种类型的结构件中声疲劳破坏的相对发生率以蒙皮壁板结构为最高。 本文采用矩形铝合金平板来模拟蒙皮壁板结构,对其进行了声疲劳寿命计算与试验研究. 相似文献
488.
489.
Paddison F.C. Shipley C.A. Maffett A.L. Dawson M.H. 《IEEE transactions on aerospace and electronic systems》1978,(1):27-34
A laboratory method to determine the magnitude and position of radar reflection sources on complex targets is described. In addition the method provides a way to measure the modification of the radar cross section (RCS) due to multipath. The method has application in modeling RCS for radar and electronic countermeasure (ECM) system performance analysis and in the study of the extent to which the signature of the target could be altered. The equipment described, termed MACROSCOPE, was developed for RCS studies by the U.S. Army and is described in limited distribution bution literature. The application to marine targets is new with this paper, as is the technique of measuring the RCS of parts of the target and analytically combining them to represent the whole. An illustration of the need for this type of laboratory equipment was illustrated by the extensive search for full scale data which could be compared to scale model data to validate the technique. 相似文献
490.
Initial ISEE magnetometer results: magnetopause observations 总被引:15,自引:0,他引:15
The magnetic field profiles across the magnetopause obtained by the ISEE-1 and -2 spacecraft separated by only a few hundred kilometers are examined for four passes. During one of these passes the magnetosheath field was northward, during one it was slightly southward, and in two it was strongly southward. The velocity of the magnetopause is found to be highly irregular ranging from 4 to over 40 km s-1 and varying in less time than it takes for a spacecraft to cross the boundary. Thicknesses ranged from 500 to over 1000 km.Clear evidence for reconnection is found in the data when the magnetosheath field is southward. However, this evidence is not in the form of classic rotational discontinuity signatures. Rather, it is in the form of flux transfer events, in which reconnection starts and stops in a matter of minutes or less, resulting in the ripping off of flux tubes from the magnetosphere. Evidence for flux transfer events can be found both in the magnetosheath and the outer magnetosphere due to their alteration of the boundary normal. In particular, their presence at the time of magnetopause crossings invalidates the usual 2-dimensional analysis of magnetopause structure. Not only are these flux transfer events probably the dominant means of reconnection on the magnetopause, but they may also serve as an important source of magnetopause oscillations, and hence of pulsations in the outer magnetosphere. On two days the flux transfer rate was estimated to be of the order of 2 × 1012 Maxwells per second by the flux transfer events detected at ISEE. Events not detectable at ISEE and continued reconnection after passage of an FTE past ISEE could have resulted in an even greater reconnection rate at these times. 相似文献