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文中针对某大口径同轴三反相机次镜承力筒展开研究。该次镜承力筒采用薄壁筒式、支撑三翼、次镜座的组合形式[16-17]。次镜承力筒的外径为Φ1200 mm,高度为405 mm;支撑三翼的壁厚为8 mm。如表1所示,可选的支撑结构材料有钛合金TC4、Invar、C/SiC。
Sample name TC4 Invar C/SiC Density/kg·m-3 4.4×103 8.18×103 2.4×103 Tensile modulus/GPa 114 150 110 Thermal conductivity/
W·(m·K)−18.8 1.47 4.6 Linear expansion coefficient/K−1 8.9×10−6 0.55×10−6 2×10−6 Table 1. Properties of alternative materials
文中对比两种材料方案:方案一采用钛合金TC4铸造成型;方案二采用C/SiC整体成型,对外接口则采用TC4预埋件,以弥补C/SiC机械切削加工工艺性差的缺点[18]。针对两种方案进行不同工况下的仿真,忽略接口影响,建立统一模型,以次镜承力筒上法兰安装孔为固定约束,分析TC4和C/SiC两种材料的结构性能差异。对于连续纤维增韧C/SiC材料而言,其材料特性为各向异性,但考虑到次镜承力筒为薄壁结构,整体性能主要表现为面内拉压,因此仿真时按各向同性作简化计算。
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在仿真软件中建立有限元模型,如图1所示,对不同材料方案下次镜承力筒的质量、模态、热变形进行校核。
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方案一、方案二中次镜承力筒的质量分别为58.7 kg和32 kg。可以看出,采用C/SiC整体成型方案,次镜承力筒质量相比采用钛合金铸造成型方案质量下降了26.7 kg,减轻了45.5%。
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文中研制次镜承力筒的设计要求为一阶固有频率大于100 Hz。分析时次镜承力筒加上次镜部件作为负载,对比两种方案下组件的固有频率和一阶振型。仿真结果如表2和图2所示。
Sample name TC4 C/SiC 1st natural frequency/Hz 183 204 2nd natural frequency/Hz 236 253 3rd natural frequency/Hz 237 254 4th natural frequency/Hz 284 335 Table 2. Natural frequencies of the structure with two schemes
从表2可见,两种方案均具有足够的刚度,能够满足设计要求。但C/SiC复合材料整体成型结构性能更优,其前四阶固有频率分别比采用TC4铸造成型结构提高了11.5%、7.2%、7.2%和18%。
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分析次镜承力筒在均匀温升4 ℃工况下的变形情况,从图3可见,该工况下钛合金结构变形在10 μm级,而C/SiC结构变形要小一个量级。反映到次镜面形上,如表3所示,C/SiC结构支撑下的反射镜面形RMS为4.01 nm,相较前者提升约12%。
Name RMS wavefront Image TC4 4.53 nm C/SiC 4.01 nm Table 3. Influence of uniform temperature rise on the surface shape
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在经过预制体编织、热解碳制备、致密化、粗精加工等一系列流程之后,如图4所示,完成次镜承力筒的实物研制。通过对研制过程中的随炉试片进行检测,主要物理性能见表4。
Name C/SiC Density/kg·m−3 2.31×103 Tensile modulus/GPa 118.6 Thermal conductivity/W·(m·K)−1 4.95 Linear expansion coefficient/K−1 1.72×10−6 Table 4. Physical properties of the sample
在次镜承力筒的预留孔位处利用硅橡胶粘接嵌套,同时备紧螺母,如图5所示,并对嵌套端面进行精加工,以达到设计的尺寸精度。
对次镜承力筒开展三个方向的力学环境试验考核,包括鉴定级正弦(频谱区为5~100 Hz,幅值分别为2、4.5、7.5、15 g)和随机振动试验(频谱区为10~2 000 Hz,总均方根加速度4 grms)。次镜承力筒X向振动基频为223 Hz,Y向振动基频为220 Hz,Z向振动基频为196 Hz,且经过正弦振动和随机振动之后特征级试验曲线复核较好,如图6所示,频漂小于1%,说明C/SiC次镜承力筒结构性能稳定。
Figure 6. Frequency sweep curve before and after vibration. (a) X direction vibration; (b) Y direction vibration; (c) Z direction vibration
为考核振动过程中次镜承力筒的变形情况,用三坐标在振动试验前后对结构进行精密测量。如图7所示,主要考察以下重要参数:
A—下法兰嵌套面,作为基准;
b—上法兰嵌套面的平面度;
c—上法兰嵌套面相对下法兰嵌套面的平行度;
d—次镜座嵌套面的平面度;
e—次镜座嵌套面相对下法兰嵌套面的平行度;
h—次镜座嵌套面与上法兰嵌套面的距离;
H—下法兰嵌套面与上法兰嵌套面的距离。
如表5所示,振动前后测量数据有所变化,表明结构在振动中存在应力释放变形。但鉴于三坐标自身测量精度为微米级,而振动试验引起次镜支撑筒嵌套面的测量偏差(2.6、1.6、6.5、2.9、11.9、9.7 μm)在同一量级,可以认为C/SiC次镜支撑筒具有优异的位置稳定性,能够满足大口径空间望远镜对次镜支撑结构的要求。
Sample name Before vibration
/mmAfter vibration
/mmAmount of change
/mmb 0.0265 0.0291 0.0026 c 0.1283 0.1299 0.0016 d 0.0108 0.0173 0.0065 e 0.1153 0.1182 0.0029 H 405.0046 404.9927 −0.0119 h 178.3791 178.3694 −0.0097 Table 5. Structure measurement before and after vibraton
Application of C/SiC to secondary mirror bearing cylinder in large aperture space telescope
doi: 10.3788/IRLA20210710
- Received Date: 2021-09-26
- Rev Recd Date: 2021-11-15
- Publish Date: 2022-06-08
Abstract: C/SiC composite material was applied to design of secondary mirror bearing cylinder in large aperture space telescope. Firstly, the material characteristics of C/SiC composite material and the applications in space optical remote sensor were introduced. Secondly, taking secondary mirror bearing cylinder of a large aperture space telescope as an example, the weight and structural-thermal performance based on different materials were compared. The analysis results indicate that C/SiC reduces the weight of secondary mirror bearing cylinder to 32 kg by 45.5% compared with titanium alloy cylinder. With natural frequency of 204 Hz, the structure meets the design requirement and can control the influence of thermal deformation on the shape of mirror. Finally, the C/SiC secondary mirror bearing cylinder was developed and the main physical properties were tested, and the mechanical vibration test was carried out, and the three-coordinate measurement data of the structure before and after vibration were compared. The results show that the fundamental frequency is excellent. Moreover, the frequency drift is less than 1% before and after the vibration test, and the micro displacement of the structure is in micron level. This paper provides several reference value for the design of large size integral bearing structure of space remote sensor by using C/SiC.