Vega In-Flight Modal Identification with the Operational Modal Analysis Technique

Operational modal analysis is an interesting procedure that allows identification of the modal characteristic parameters of structures excited randomly by environmental loads. In this paper, an existing operational modal technique, the natural excitation technique is studied and implemented to obtain the modal parameters of the Vega launcher excited by in-flight loads during its maiden flight. The modal parameters of the Vega launcher, obtained with the operational modal technique, are compared with those from a finite element model designed for the same flight event. Because the dynamic system is not stationary due to propellant mass loss, modal identification is only possible using a series of linearized models over short periods of time. In this work, the extraction of modal parameters is performed in the time window that shows the highest excitation of mode shapes. The time domain modal analysis method, Least-Squares Complex Exponential, is used to extract the modal parameters from the correlation functions generated by the natural excitation technique. Results show good agreement with the corresponding ones obtained from the finite element model; this demonstrates that the natural excitation technique can be successfully employed for the extraction of modal parameters of launch vehicles during the flight phase.