Space vehicles undergo harsh acoustic fields during launch. In order to efficiently reduce the load transmitted to the pad and the launcher, it is necessary to understand and characterize the strong pressure waves that are emitted at Solid propellant Rocket Motor (SRM) start-up. This is one of the main concerns of the CNES/Onera AEID program. Computational studies of the waves observed during the firing of a model SRM are presented in these papers. The experimental set-ups are scaled down models (1/35) for Ariane 5 P230 SRM, named LP10, horizontally fired in free jet configuration for part I or vertically with a flame trench in part II. Pressure signals in the acoustic near and far fields are recorded with microphones. Part I focuses on the transient start-up phase when the Ignition Over Pressure (IOP) is emitted. Previous computations held in 2D axisymmetric configuration yielded deceptive results regarding amplitude and directivity. Discrepancies are thought to be due to 3D nature of turbulence. This paper presents 3D LES computations of a hot supersonic jet expansion carried out with the Onera CEDRE CFD code. As combustion products may react with ambient air, reactive computations are performed using a reduced kinetic scheme. Even if first 3D simulation leads to better results than 2D computations regarding amplitude and directivity, the simulated IOP is still not fully satisfying. This is mainly due to an unsufficiently refined grid where aerodynamic jet noise sources are located. Therefore, jet discretization influence is studied. Finally, the study reveals that the sources discretization quality is impacting the solution at first order, and that efforts must be directed in achieving the most precise noise sources. Good agreements were found between experimental data and 3D simulations.
