PSI - Issue 14

G Vamsi Krishna et al. / Procedia Structural Integrity 14 (2019) 820–829 G. Vamsi Krishna / Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Avionics are mounted in airframe sections of the rockets by using mounting interface brackets. The design of these brackets are based on functional and operational requirement of the avionic packages mounted on it. Following are the consideration that decides the configuration design of the brackets: 1) size of the package 2) weight and CG of the package 3) space and contour available in airframe 4) Orientation requirement of the package 5) mounting accessibility of bracket and the package 6) availability of sufficient static margin of safety for very high deceleration flight loads 7) Limiting the overall vibration response to the specified vibration inputs on packages mounted on brackets within specified limits 8) Weight of the bracket etc. Static margins available for high deceleration loads and random vibration response (maximum grms experienced by package) of bracket for specified vibration input dominates the design criteria of these brackets. Paik J K et al. (1999) has established that sandwich-type honeycomb panel which is a typical laminated anisotropic composite is an important alternative aeronautical material. Kim B J et al. (2010) and Bianchi G et al. (2010) also has brought out that the use of honeycomb panels is particularly attractive in various aeronautical structures due to their high strength-to-weight ratio and stiffness-to-weight ratio. Ermolaeva et al. (2004) presented the application of a structural optimization system to the optimal choice of foams as a core material for sandwiches with aluminium alloy faces. Ahmad Partovi Meran et al. (2018) has brought out the effect of solidity ratio on the natural frequency mechanical behaviour of sandwich panel of hexagonal honeycomb structure. Caiqi Zhao et al. (2017) have explained the shear strength of different bolt connectors on large panel honeycomb structure. Boudjemai A. et al. (2012) have carried out modal analysis and testing of hexagonal honeycomb panel used for satellite structures. Naihui Yu et al. (2015) has carried out a detailed comparative study of Al-Li alloy and Aluminium honeycomb panel for the application as aerospace mounting interfaces. In this study, we have carried out comparative study of aluminium alloy and aluminum honeycomb hybrid mounting brackets, both from structural as well as vibration response point of view especially for mounting of lower mass of functional devices of around 13 kg. Shape and mounting features are configured similar for both the configurations. Hybrid brackets are configured and designed to withstand local buckling loads at mounting locations because of bolt pre torquing using aluminium sleeves. For case under study, honeycomb bracket is configured with 25mm aluminium honey comb with 2mm thick face sheet at top, bottom and all around the periphery. It is employed for mounting of two sensor packages – subsystem 1 and subsystem 2- of weight 10.5 kg and 1.5 Kg respectively. Both numerical and experimental comparative study with Aluminium and hybrid brackets is carried out. FEM analysis is carried out to confirm the availability of sufficient margin of safety for high deceleration flight loads of 60g associated with re entry. Random vibration response of both the bracket configurations is carried out along all the 3 axes, employing vibration input which is MIL standard equivalent of in-flight vibration loads. Comparable maximum vibration response is obtained for both cases on the packages mounted on bracket. The hybrid bracket is meeting all the functional requirements with weight of 1.7 kg compared to aluminium bracket weighing 15 kg, which is a tremendous weight saving especially when employed in upper stages of launch vehicle.

2. Geometry and Material Model 2.1. Geometrical configuration

The configuration details of honeycomb bracket are depicted in Fig. 1, 2 & 3. A typical aerospace vehicle is an assembly of number of sections. Each section (shell structure with end annular rings) houses number of subsystems with specific functional requirements. Subsystem 1 and Subsystem 2 are electronic sensor packages. These electronic packages are to be assembled to one of the end Bulk Heads (annular rings) of a section. For mounting of these packages to the end Bulk Head, bracket (mechanical interface) is used. This bracket is configured as a sector plate, which is fastened to BH using the mounting holes around its periphery as depicted in Fig. 2 using M6 bolts and nuts.