Inverse Method for Identification of Acoustic Sources at Launch Vehicle Liftoff

Abstract

At liftoff, launch vehicles are subjected to a very severe overpressure and to an acoustic environment that can induce loads acting on payloads in the low-frequency domain (frequencies lower than 40 Hz). The overpressure starts at ignition of solid-rocket motors. This overpressure phase is followed by the acoustic phase. The liftoff acoustic environment is generated by the rocket exhausts and by their impingement on the launch pad. For a numerical prediction of the acoustic environment, the European Aeronautic Defence and Space Company has developed an inverse method via a time domain boundary integral equations approach that uses an optimal control method, with direct and adjoint equations. The corresponding discrete schemes are highly accurate and unconditionally stable. As an industrial application, the identification of acoustic sources is shown, on the liftoff acoustic environment of ARIANE 5. With the sources from acoustic pressure measurements on the upper part of the ARIANE 5 vehicle and on the pylons around the launch pad having been localized and characterized in the time domain, the complete environment is recovered. By integration of the resulting acoustic pressures over all surfaces of the launchers, the loads created by the liftoff acoustic field can be estimated.