全文获取类型
收费全文 | 352篇 |
免费 | 521篇 |
国内免费 | 17篇 |
专业分类
航空 | 601篇 |
航天技术 | 14篇 |
综合类 | 11篇 |
航天 | 264篇 |
出版年
2023年 | 5篇 |
2022年 | 31篇 |
2021年 | 9篇 |
2020年 | 15篇 |
2019年 | 12篇 |
2018年 | 8篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 4篇 |
2014年 | 11篇 |
2013年 | 17篇 |
2012年 | 9篇 |
2011年 | 17篇 |
2010年 | 14篇 |
2009年 | 12篇 |
2008年 | 14篇 |
2007年 | 27篇 |
2006年 | 19篇 |
2005年 | 25篇 |
2004年 | 17篇 |
2003年 | 20篇 |
2002年 | 17篇 |
2001年 | 16篇 |
2000年 | 22篇 |
1999年 | 42篇 |
1998年 | 51篇 |
1997年 | 59篇 |
1996年 | 64篇 |
1995年 | 44篇 |
1994年 | 51篇 |
1993年 | 61篇 |
1992年 | 58篇 |
1991年 | 22篇 |
1990年 | 28篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 13篇 |
排序方式: 共有890条查询结果,搜索用时 15 毫秒
521.
522.
523.
复合固体推进剂燃速标准物质均匀性研究 总被引:1,自引:0,他引:1
采用声发射燃速测试方法研究了复合固体推进剂方坯燃速的均匀性。叙述了这种复合体系固有的非均匀特征,用改进工艺解决了这个问题,研制出了均匀性合格的复合固体推进剂燃速标准物质。 相似文献
524.
The aging behavior of softening composite solid propellant was investigated by measuring its mechanical and ballistic prosperities during prolonged storage at elevated and room temperatures. Accelerated aging was conducted at 65 °C for 231 days while the normal aging was performed at 25 ± 3 °C and relative humidity less than 50% for 8 years. The mechanical properties were obtained from uniaxial tensile tests for the aged propellant specimens while the ballistic properties were determined from static firing tests of subscale motors aged for 112 days at 65 °C. The mechanical results show that the maximum tensile strength and Young's modulus initially increase and subsequently decrease with increasing aging time, while the maximum tensile strain generally increases with increasing aging time. The ballistic properties like burning rate show a small change which cannot affect the ballistic performance. The experimental results show that the changes in the mechanical properties are significant during the aging period, but the burning rate does not undergo significant changes. From this study, it is observed that the propellant ages through a combination of reactions like post-cure, oxidative cross-linking, chain scission, and hydrolysis. The chain scission and the hydrolysis effect are the most significant process, which makes the propellant soft and extendible. The observed aging mechanism has been modeled using an exponential function with two terms which can describe the complex behavior of the aging. By applying Arrhenius equation,the activation energy values were obtained based on the propellant mechanical properties. The shelf life of this propellant formulation at 25 °C is predicted to be 13 years using the modulus as failure criteria and control parameter. 相似文献
525.
526.
本文基于曲面相交理论,依据辅助圆锥面和辅助平面与已回转面相交的三面共点原理,给出了图解回转体表面上等倾曲线的原理与方法,同时还给出了适合于微机求解等倾曲线上点的坐标的数值计算公式。 相似文献
527.
《Acta Astronautica》2014,93(2):463-475
The influences of miscellaneous combustor structures for solid fuel scramjet combustion on the performance are investigated, including a detailed interaction analysis between shocks/waves and combustion. Hydroxyl-terminated polybutadiene is chosen as the solid fuel with the non-premixed equilibrium probability density function combustion model. The results show combustion enhancement when structure of combustor is modified. The radical emphasis is to examine the sensitivity of the properties due to variations on the length-to-depth ratio of cavity, aft wall angle, and offset ratio. It is noted that there is an appropriate structure of cavity (L/D=4, θ=45°, and Dd/Du=1.25–1.5) regarding the combustion efficiency, total pressure loss and specific impulse. The observation of function for combustor components provides instructional insight into the design considerations for a combustor of a solid-fuel scramjet. 相似文献
528.
The infrared radiation signature of the plume from solid propellants with different energy characteristics is not the same. Three kinds of double-base propellants of different energy characteristics are chosen to measure the infrared spectral radiance from 1000 cm 1 to 4500 cm 1 of their plumes. The radiative spectrum is obtained in the tests. The experimental results indicate that the infrared radiation of the plume is determined by the energy characteristics of the propellant. The radiative transfer calculation models of the exhaust plume for the solid propellants are established. By including the chemical reaction source term and the radiation source term into the energy equation, the plume field and the radiative transfer are solved in a coupled way. The calculated results are consistent with the experimental data, so the reliability of the models is confirmed. The temperature distribution and the extent of the afterburning of the plume are distinct for the propellants of different energy characteristics, therefore the plume radiation varies for different propellants. The temperature of the fluid cell in the plume will increase or decrease to some extent by the influence of the radiation term. 相似文献
529.
An overview of the material developments currently being pursued by the European Space Agency (ESA) with respect to X-ray, γ-ray and neutron detection media is given. The benefits underlying the selection of specific materials are discussed. In the X-ray regime the basic microphysics behind degraded spectral performance caused by the polarization effect and Te inclusions, have been investigated. We have found that both effects degrade the spectra by producing a localised distortion of the electric field. The underlying cause for the distortion does however differ. For the γ-ray regime, a low-noise equivalent scintillator to LaBr3 has been developed in the form of CeBr3. Three inch crystals with a FWHM energy resolution of 4.4% at 662 keV have successfully been produced. For neutron detection, a boron compound based solid state neutron detector has been developed that is sensitive to neutrons, alpha particles and 60 keV X-rays. The measured detection efficiency in detection for thermal neutrons was found to be in the order of a few per cent. 相似文献
530.
立式捏合机搅拌桨的设计 总被引:4,自引:1,他引:3
介绍了立式捏合机搅拌桨的设计方法和几何参数的计算方法;推导了两搅拌桨螺旋升角的关系式,根据捏合原理,建立了搅拌桨水平截面曲线的数学模型。 相似文献