Modeling Launch Vehicle Liftoff Acoustics with Ray Tracing

Abstract. Spacecraft vehicles are subject to important external acoustic load during liftoff and when they are operated in the atmosphere. During liftoff, this acoustic load is caused by the engines that generate a fluctuating pressure over the vehicle surface. This Wall Pressure Fluctuation (WPF) is one of the most important sources of structural vibration and internal noise during the liftoff or the staticfiring operation. Moreover, surviving this acoustic load is critical to the proper functioning of the vehicle and payloads. Therefore, the prediction of the acoustic load is essential to provide a necessary input to determine the structural vibration and the sound transmission through the vehicle structure. This is necessary for the development of the acoustic and vibration test specifications and the associated dynamic design requirement which are necessary to ensure the vehicle performance. The modelling of such vehicles with discretization methods might be accurate but is computationally expensive with a cost growing rapidly as a function of both the vehicle size and launch pad geometry. A ray-based technique, including the effect of diffraction, is employed in this paper. This provides a mesh and frequency independent method and subsequently an efficient alternative for quick and accurate solution. The engines which constitute the main source of noise are replaced by Compact Acoustic Sources. The predicted Wall Pressure Fluctuation is then used to model the noise transmission into the inside of the spacecraft. The validity of the presented approach is demonstrated by comparison versus empirical methods provided in the literature and good agreements are found.