Robust Design Optimization by First Design Method

Launch vehicles are subjected to severe vibratory conditions during takeoff and flight ascent. The loads are not well known at the very beginning of a project, and, even at the end, some uncertainties still prevail. In spite of this, the preliminary design has to be robust with regard to the vibration levels encountered at the payload/launch vehicle interface. This paper presents a new robust design methodology called the First Design Method. This methodology has been applied to a typical launch vehicle excited by a low-frequency transient load. The First Design Method optimizes the natural frequencies of the structural configuration of interest and, subsequently, designs the structural configuration with the modes provided by the optimization procedure. Furthermore, the method, which is based on energy conservation for integrating uncertainties about excitation, is compliant with the laws of physics. In the industrial case investigated in this paper, the First Design Method has been used to design a typical launch vehicle with a very satisfactory behavior as far as the mass and vibration levels at the payload/launch vehicle interface are concerned.