全文获取类型
收费全文 | 501篇 |
免费 | 3篇 |
国内免费 | 7篇 |
专业分类
航空 | 146篇 |
航天技术 | 68篇 |
综合类 | 55篇 |
航天 | 242篇 |
出版年
2021年 | 1篇 |
2020年 | 1篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2016年 | 3篇 |
2015年 | 2篇 |
2014年 | 2篇 |
2013年 | 1篇 |
2012年 | 16篇 |
2011年 | 18篇 |
2010年 | 5篇 |
2009年 | 3篇 |
2008年 | 11篇 |
2007年 | 14篇 |
2006年 | 8篇 |
2005年 | 19篇 |
2004年 | 6篇 |
2003年 | 10篇 |
2002年 | 8篇 |
2001年 | 25篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 13篇 |
1996年 | 14篇 |
1995年 | 23篇 |
1994年 | 52篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 14篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1988年 | 23篇 |
1987年 | 13篇 |
1986年 | 5篇 |
1985年 | 31篇 |
1984年 | 26篇 |
1983年 | 29篇 |
1982年 | 31篇 |
1981年 | 12篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有511条查询结果,搜索用时 15 毫秒
271.
272.
本报告记述了应用技术卫星(ATS)远地点发动机的设计、研制、鉴定和飞行试验。这一远地点发动机用来把 NASA 哥达特航天中心(GSFC)的 ATS 卫星从椭园轨道送入近同步轨道。总体对于燃烧过程中的推力向量和对燃烧前后的动平衡的要求都是很高、很严格的。所以研制阶段中主要问题是制造高精度部件。研制阶段试验中,安排了一系列试车,以进一步确定制造工艺,部件设计以及在模拟发射和宇宙空间环境条件下的发动机生存能力,并确定发动机的初步性能特性。鉴定试验由八发高空模拟试车组成,自旋速率100周/分,药温保持在.5℃或38℃,用来确定发动机准确的性能参数。 相似文献
273.
Interplanetary transfer of photosynthesis: an experimental demonstration of a selective dispersal filter in planetary island biogeography 总被引:2,自引:0,他引:2
Cockell CS Brack A Wynn-Williams DD Baglioni P Brandstätter F Demets R Edwards HG Gronstal AL Kurat G Lee P Osinski GR Pearce DA Pillinger JM Roten CA Sancisi-Frey S 《Astrobiology》2007,7(1):1-9
We launched a cryptoendolithic habitat, made of a gneissic impactite inoculated with Chroococcidiopsis sp., into Earth orbit. After orbiting the Earth for 16 days, the rock entered the Earth's atmosphere and was recovered in Kazakhstan. The heat of entry ablated and heated the rock to a temperature well above the upper temperature limit for life to below the depth at which light levels are insufficient for photosynthetic organisms ( approximately 5 mm), thus killing all of its photosynthetic inhabitants. This experiment shows that atmospheric transit acts as a strong biogeographical dispersal filter to the interplanetary transfer of photosynthesis. Following atmospheric entry we found that a transparent, glassy fusion crust had formed on the outside of the rock. Re-inoculated Chroococcidiopsis grew preferentially under the fusion crust in the relatively unaltered gneiss beneath. Organisms under the fusion grew approximately twice as fast as the organisms on the control rock. Thus, the biologically destructive effects of atmospheric transit can generate entirely novel and improved endolithic habitats for organisms on the destination planetary body that survive the dispersal filter. The experiment advances our understanding of how island biogeography works on the interplanetary scale. 相似文献
274.
Grenfell JL Griessmeier JM Patzer B Rauer H Segura A Stadelmann A Stracke B Titz R Von Paris P 《Astrobiology》2007,7(1):208-221
Planets orbiting in the habitable zone of M dwarf stars are subject to high levels of galactic cosmic rays (GCRs), which produce nitrogen oxides (NOx) in Earth-like atmospheres. We investigate to what extent these NO(Mx) species may modify biomarker compounds such as ozone (O3) and nitrous oxide (N2O), as well as related compounds such as water (H2O) (essential for life) and methane (CH4) (which has both abiotic and biotic sources). Our model results suggest that such signals are robust, changing in the M star world atmospheric column due to GCR NOx effects by up to 20% compared to an M star run without GCR effects, and can therefore survive at least the effects of GCRs. We have not, however, investigated stellar cosmic rays here. CH4 levels are about 10 times higher on M star worlds than on Earth because of a lowering in hydroxyl (OH) in response to changes in the ultraviolet. The higher levels of CH4 are less than reported in previous studies. This difference arose partly because we used different biogenic input. For example, we employed 23% lower CH4 fluxes compared to those studies. Unlike on Earth, relatively modest changes in these fluxes can lead to larger changes in the concentrations of biomarker and related species on the M star world. We calculate a CH4 greenhouse heating effect of up to 4K. O3 photochemistry in terms of the smog mechanism and the catalytic loss cycles on the M star world differs considerably compared with that of Earth. 相似文献
275.
An in-depth analysis of the effects exerted on the DNA backbone by 25 crystal phosphate minerals is reported. Degradation of DNA oligomers was performed with two different reactions: Hydrolysis following Nucleophilic Degradation (HND), initiated by the nucleophilic addition of formamide on both purine and pyrimidine nucleobases, and Hydrolysis following Nucleophilic Substitution (HNS) carried on by water and starting with the removal of a nondegraded base. A complete panel of effects on the phosphoester bonds, from protection to enhanced instability to absence of interference, is described. These effects differ in the different degradation pathways and in different physical-chemical conditions. The relationship between the hardness of the mineral and its protective ability is discussed. In addition to its interest per se, this study was prompted by the observed catalytic abilities of soluble and mineral phosphates (Saladino et al., 2006c) on the synthetic reactions by formamide. The relevance of these observations in the search for nonterran life is discussed. 相似文献
276.
要使碳氢燃油在常规燃烧循环中高效燃烧,需要非常高的温度,化学循环燃烧可以为大型动力装置提供一个可行的方法。在化学循环燃烧中,使用含氧介质分子来传输两个氧化还原反应之间的氧气,含氧介质在一个反应中被氧化,在另一个与燃油的反应中被减少。从燃烧残余物中分离出含氧介质可以借助于不同的物态,如果介质用钠、钾或锌,这是液态化学循环燃烧的显著优点。液态化学循环燃烧的原理是能量和熵的循环,在考虑用于分离氮和二氧化碳零部件效率、结合水燃气交换、以及用空气作为氧气源的情况下,高的热效率(35Pa下约75%)在理论上可以达到。 相似文献
277.
THREEASPECTSOFISARSIGNALPROCESSINGSheZhishun;ZhuZhaoda(DepartmentofElectronicEngineering,NUAA29YudaoStreet,Nanjing210016P.R.C... 相似文献
278.
ANOPTICALFIBRESENSORBASEDCUTTINGFORCEMEASURINGDEVICEJinWenling;WangXin(DepartmentofMdchanicalEngineering,NUAA29YudaoStreet,Na... 相似文献
279.
Yang Xintie 《南京航空航天大学学报(英文版)》2001,18(Z1)
1 LORENZ TRANSFORMATIONEXISTED IN AERODYNAMICANALYSISIn earlier paper a relation between magnetand fluid is deduced.Naturally we can think thedeep meaning of it.And man asks:does theaerodynamic system also has the character ofmagnet field?To answer this question,relativityspace- time wave equation are analyzed.It de-scribes the same phenomena of the wave equationof compress fluid in normal space and time.It isamazement.In normal compressible fluid thenon- dimensional simp… 相似文献
280.
本文提出了机载高分辨率二维逆合成孔径雷达(ISAR)对舰船目标的侧视成像问题。通过采用舰船运动的简单模型,避免了采用高级的联合-时间频率(JTF)变换。研究了包括运动估计与校正、最佳处理时间与持续时间估计以及目标形状提取的鲁棒处理方案。强调了这样一个事实,即该处理的鲁棒性造就了单个ISAR成像分析而不必由一组雷达图像进行数据融合。 相似文献