全文获取类型
收费全文 | 1536篇 |
免费 | 331篇 |
国内免费 | 330篇 |
专业分类
航空 | 1048篇 |
航天技术 | 380篇 |
综合类 | 238篇 |
航天 | 531篇 |
出版年
2024年 | 11篇 |
2023年 | 21篇 |
2022年 | 60篇 |
2021年 | 57篇 |
2020年 | 68篇 |
2019年 | 70篇 |
2018年 | 66篇 |
2017年 | 66篇 |
2016年 | 57篇 |
2015年 | 79篇 |
2014年 | 87篇 |
2013年 | 89篇 |
2012年 | 115篇 |
2011年 | 108篇 |
2010年 | 116篇 |
2009年 | 130篇 |
2008年 | 102篇 |
2007年 | 103篇 |
2006年 | 79篇 |
2005年 | 71篇 |
2004年 | 53篇 |
2003年 | 62篇 |
2002年 | 91篇 |
2001年 | 62篇 |
2000年 | 46篇 |
1999年 | 47篇 |
1998年 | 54篇 |
1997年 | 42篇 |
1996年 | 31篇 |
1995年 | 28篇 |
1994年 | 26篇 |
1993年 | 19篇 |
1992年 | 12篇 |
1991年 | 17篇 |
1990年 | 20篇 |
1989年 | 8篇 |
1988年 | 14篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1984年 | 1篇 |
1965年 | 1篇 |
1900年 | 1篇 |
排序方式: 共有2197条查询结果,搜索用时 156 毫秒
951.
Zhong Lingwei Liu Yu Li Juan Gu Zuo Jiang Haocheng Wang Haixing Tang Haibin 《中国航空学报》2010,23(1):15-21
Charge exchange(CEX) ions could inflict severe damages on the ion thruster optical system. This article is aimed at investigating the characteristics of the CEX ions and their influences upon the optical system by means of particle-in- cell(PIC) ion simulation and Monte Carlo collision(MCC) methods. The results from numerical simulation indicate that despite the fact that CEX ions appear in the entire beamlet region near the ion optical system, the ones that present themselves downstream of the accelerator grid have good reason for attracting more attention. As their trajectories are significantly affected by the local electric field, a great number of CEX ions are accelerated toward grids resulting in sputtering erosion. When the influences of the CEX ions are considered in the numerical simulation, there could hardly be observed augments in the screen grid current, but the accelerator grid current increases from zero to 1. 4% of the beamlet current. It can be understood from the numerical simulation that the CEX ions formed in the region far downstream of the accelerator grid should be blamed for the erosion on the downstream surface of the accelerator grid. 相似文献
952.
953.
954.
截止频率以下时矩形波导可以看作是一个负介电常数环境,如果在矩形波导的截止频率以上实现负介电常数的环境就可以使电磁波无法传播。左手材料是一种新型材料,是通过结构来实现负磁导率和负介电常数的;文章通过对波导加载左手材料,实现了在矩形波导的截止频率以上出现阻带,可以用做带阻滤波器。 相似文献
955.
Optimal guidance based on receding horizon control for low-thrust transfer to libration point orbits
Haijun Peng Qiang Gao Zhigang Wu Wanxie Zhong 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
This paper addresses the design and computation of a guidance law for a transfer mission from an orbit near the Earth to a halo orbit around the libration point L2 in the Sun–Earth system. The guidance law, which is designed based on receding horizon control and compensates for launch velocity errors that are introduced by inaccuracies of the launch vehicle, is solved using the generating function method. During the design of the closed-loop guidance law, the entire transfer mission, which is considered a nonlinear optimal control problem, is evaluated to obtain a nominal reference trajectory. Using the launch velocity errors and the uncertainty of the model, a spacecraft controlled by the proposed guidance law tracks the reference trajectory. Furthermore, the original Riccati differential equation in the receding horizon control algorithm is replaced by an equivalent convenient form of the Riccati differential equation that is based on the generating function. The high-efficiency solution of the equivalent equation avoids the online direct integration of the original Riccati differential equation, which significantly increases the computational efficiency for the receding horizon control problem. Numerical simulations using a nonlinear bicircular four-body model demonstrate the capabilities of the proposed receding horizon guidance law for the transfer mission. In addition, the generating function method improves the computational efficiency by at least one order of magnitude over the backward sweep method in solving the receding horizon control problem. 相似文献
956.
Long term dynamics and optimal control of nano-satellite deorbit using a short electrodynamic tether 总被引:1,自引:0,他引:1
R. Zhong Z.H. Zhu 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2013
This paper studies the long term dynamics and optimal control of a nano-satellite deorbit by a short electrodynamic tether. The long term deorbit process is discretized into intervals and within each interval a two-phase optimal control law is proposed to achieve libration stability and fast deorbit simultaneously. The first-phase formulates an open-loop fast-deorbit control trajectory by a simplified model that assumes the slow-varying orbital elements of electrodynamic tethered system as constant and ignores perturbation forces other than the electrodynamic force. The second phase tracks the optimal trajectory derived in the first phase by a finite receding horizon control method while considering a full dynamic model of electrodynamic tether system. Both optimal control problems are solved by direct collocation method base on the Hermite–Simpson discretization schemes with coincident nodes. The resulting piecewise nonlinear programing problems in the sequential intervals reduces the problem size and improve the computational efficiency, which enable an on-orbit control application. Numerical results for deorbit control of a short electrodynamic tethered nano-satellite system in both equatorial and highly inclined orbits demonstrate the efficiency of the proposed control method. An optimal balance between the libration stability and a fast deorbit of satellite with minimum control efforts is achieved. 相似文献
957.
针对因影响定位误差因素较多而不易对机器人定位误差进行准确标定的问题,首先建立了基于雅克比矩阵的机器人全闭环定位误差数学模型;然后给出了用于分析各因素单点敏感度的机器人单因素微分定位误差及相对单因素微分定位误差公式;接着采用正交实验法设计了能全面反映机器人定位误差分布的姿态样本空间并基于区间概率密度给出了单因素综合微分定位误差及相对单因素综合微分定位误差公式;最后通过仿真分别分析了机器人各因素对其定位误差影响的单点敏感度及多点综合敏感度,为进一步进行机器人定位误差的标定打下了基础. 相似文献
958.
便携式相控阵探伤仪在复合材料检测中的应用 总被引:1,自引:0,他引:1
相控阵探伤仪能够通过图像的形式直观地显示缺陷,并通过线性B扫描图或扇形图显示一定区域范围内的缺陷,有利于对缺陷的评判. 相似文献
959.
960.
疲劳裂纹扩展是结构健康监测的主要问题之一,为了保证金属结构的可靠和安全运行,实时监测结构的疲劳裂纹扩展过程十分必要。针对结构裂纹扩展的问题,采用压电传感器(PZT)和电阻应变片两种传感器,提出结合能够连续监测结构损伤的被动监测方法以及对微小损伤敏感的主动监测方法对裂纹扩展进行综合监测,以提高裂纹扩展的监测水平。采用随机森林算法对裂纹长度进行识别,并使用D-S证据理论对两种传感器的识别结果进行数据融合,得到比单一传感器更准确、可靠的裂纹扩展识别结果。进行了基于应变和主动Lamb波的裂纹扩展监测实验研究,验证了该方法对提高裂纹扩展监测识别准确率的有效性和实用性。 相似文献