Optimization of a novel stiffening scheme using tension rods for a multi-lobe passenger cabin

Abstract The goal of this research is to enhance the structural performance of a passenger cabin designed for hypersonic aircraft. These new prototypes are based on the multi-lobe concept to benefit from wider fuselages that improve the lift-to-drag ratio. The vehicle is subjected to very demanding thermal and pressure loading conditions due to the high temperatures of the propulsion system and the low temperatures inside the cryogenic tanks. As a result, the cabin cannot be over-expanded to provide enough insulation between the passenger compartment and the remaining subsystems. Tension rod elements were conceived as an effective structural scheme to brace the upper and lower parts of the cabin. Nevertheless, it is challenging to determine the optimum number, arrangement, and areas of these cables because it gives rise to a computationally expensive mixed-integer nonlinear programming problem. Thus, a novel optimization strategy is proposed to convert the discrete nature of cable existence into continuous variables, enabling the use of gradient-based optimization techniques. To achieve this, a sigmoid function is assigned to each cable, yielding a value close to 0 or 1 depending on whether its area makes a significant structural contribution. By combining all these functions, the total number of cables can be determined and used as the objective function. The optimal cable arrangement should be selected from the Pareto front, aiming to balance a lightweight design with the minimal number of components.