VEGA launch vehicle dynamic loads due to solid propulsion ignition transients and pressure oscillations

This paper provides the analysis of the VEGA solid rocket motors loads induced by the ignition transients (P80 lift-off phase, Zefiro 23 and Zefiro 9 ignition phases) and their effects in terms of longitudinal acceleration induced at the payload interface. The forcing functions due to the ignition transient phases are characterized through an ignition transient model (SPIT), widely adopted/validated for the studies of VEGA motors ignition transient during the development and exploitation phase of VEGA launch vehicle, for the internal ballistics analyses. A comparison between the frequency content of the numerical model and the one of the experimental data is carried out showing good agreement with different spectral domain operators (FFT and Hilbert-Huang transform), for the first two flights of VEGA during P80 first phase of the ignition transient, where pressure experimental measurements quality allows for the evaluation of the spectral content of the pressure in the combustion chamber. Then, the analysis of the correlation between spectral content of the pressure signal and the one of the longitudinal acceleration at the payload interface is carried out, showing that the dynamic structural response of both P80 SRM (first stage), and Zefiro 23 SRM (second stage) is characterized by a first phase directly driven by the load signature, and a second phase dominated by the structural response. Instead, Zefiro 9 (third stage) ignition transient, because of its peculiar characteristics, geometrical configuration and rapid ignition, misses this first phase, being the dynamic structural response all driven by the launch vehicle and payload assembly modal response.