Optimum Design Of Large Coplanar Tank Structures

The propellant tank is one of the most important parts of the launch vehicle,and its size and performance determine the carrying capacity of the launch vehicle.The structure weight is reduced and the efficiency is improved,because the interstages of launch vehicles are removed by utilizing coplanar tanks,which has been a significant future research direction.In this paper,a systematic research is made on the coplanar tank.Firstly,the mechanical properties of monolayer coplanar tanks were analyzed to determine the type of common bulkhead.Then,the failure modes and bolt connections of the tank structure were analyzed based on the detailed model.Afterwards,the influence of imperfections on load-carrying capacity of the tank under the external pressure was discussed in detail.Finally,the optimization model of the monolayer coplanar tank was constructed,and four typical common bulkhead reinforcement methods were put forward.The reinforcement design was optimized on the inner and outer shells of the bilayer coplanar tank.The specific research contents are shown as follows:1.The global and local strength failure modes of coplanar tanks were investigated based on finite element analysis.For monolayer coplanar tanks,the strength and stability analyses were carried out under the condition of internal and external pressure to select the appropriate tank bottom type.Then,the stress distributions of the bottom were compared under two cases considering the mask box was subjected to the pressure or not.Based on this result,detailed finite element calculations were carried out for the whole coplanar tank.It showed that the pressure in the upper tank had a greater influence on the common bulkhead and the common bulkhead instability was the main failure mode under the competition between upper and lower tanks.In the end,the strength analyses of three types of bolt connection in the bilayer coplanar tank were carried out,which showed the advantages of the axial bolt connection such as large radial bending stiffness.2.The effects of typical geometrical imperfections on common bulkhead structure under the external pressure were analyzed.The external pressure buckling mode imperfection,the single dimple imperfection and the ellipticity imperfection were introduced into the common bulkhead respectively.The influence of perturbation load directions and single dimple positions on the load-carrying capacity of the common bulkhead was further studied.Furthermore,the imperfections sensitivities were also investigated.As a result,the resistance of the common bulkhead under the external pressure to the normal deformation in the plane was weaker than to the tangential deformation.The outward dimple stiffness of the common bulkhead was larger than the inward dimple stiffness.3.The reinforcement design optimization based on surrogate model and multi-island genetic algorithm was carried out for the common bulkhead.At first,four types of reinforcement design to increase the load-carrying capacity of thin ellipsoidal shell structures under the external pressure were put forward,and the efficiencies of the load-carrying capacity of each optimum design were compared.The result showed that the methods of thickening the top cover of the common bulkhead and longitudinal reinforcement achieved better performances.Then,the lightweight design with the load-carrying capacity constraints was optimized on the inner and outer shells of the bilayer coplanar tank,and finally the optimization result satisfying the design requirements was obtained.