A hybrid fem-sea approach for the vibro-acoustic analysis of vega-c launcher at lift off

In the framework of the European Space Agency funded program VECEP, aimed at further increasing the performance of the VEGA launcher, a vibro-acoustic analysis, driven by the acoustic field generated at lift-off, is performed on the VEGA Inter-stages. The Inter-stages are examined to derive the acceleration level at the equipment locations and the average vibration response of the connecting interfaces. Due to high modal density of the structure, a random noise and vibration analysis through deterministic methods, e.g. based on Finite Element/Boundary Element techniques, turns out to be unfeasible in a medium to high frequency range. Conversely, the employment of a fully energy based approach in such frequency range is restricted by the need to simulate the equipment as lumped masses, connected to the main structure throughout rigid links. A Hybrid method is thus adopted to combine the equipment local deterministic responses with the mean value of the dynamic response of the launcher mayor sections. The implemented analysis resorts to a FEM solver to extract the modal parameters associated to the deterministic subsystems of the VEGA Inter-stages. The development of the statistical subsystems and the set-up of the global hybrid models are realised within a numerical environment, formerly devoted to SEA methodology only, in which the required dynamic analysis is also performed. The structural and acoustic models of the Inter-stages are validated through comparison with theoretical and numerical estimates. As required for the assessment of the equipment qualification status, the activity is finalized with the delivery of the dynamic response database, numerically generated by applying a stochastic diffuse field to the Inter-stages validated models.