# URANS Analysis of a Launch Vehicle Aero-Acoustic Environment
## 1 Introduction  
The acoustic conditions during spacecraft launch are highly complex. The exhaust gas jet of the rocket engine and its collision with the launchpad produce severe acoustic waves, which are the primary sound sources during lift-off [1, 2]. This acoustic environment causes a severe acoustic and vibration load on the payload, endangering the mission and potentially resulting in financial losses [3]. Hence, predicting and mitigating acoustic levels during lift-off are critical in the launch pad and launch vehicle design since this enhances the reliability of the launcher while also increasing payload comfort. This work is part of a project, conducted under a European Space Agency (ESA) program, to develop a sound mitigation system applicable to the launch facility of the VEGA rocket. The first step towards any mitigation process is the prediction of noise sources. The global study of sound generation and transmission up to the payload entails several problems of different natures, which are summarized in Figure 0(a). First, sound generation is mainly due to the exhaust gases' jet with the blast wave generated during ignition. As shown in Figure 0(b), the noise generated by the jet comprises three distinct sound sources: turbulent mixing noise (TMN), shock-associated noise (SAN), and impinging tones. The TMN is generated by the turbulent mixing of large-scale or small-scale eddies at the mixing layer [2, 4]. Large-scale vortex noise dominates at supersonic speeds, resulting in low-frequency noise with high downstream directivity, as the blue arrows represent in Figure 0(b)[5]. The SAN in impinging jets is produced by the interaction of turbulent structures with shock waves, and it is called broadband shock-associated noise (BBSAN). The BBSAN is broadband and primarily directed upstream, as the green arrows represent, where the launch vehicle is located. Finally, the impinging tones appear when the jet collides and are directed upstream [2]. Second, there is an air transmission of these noise sources, which interact with the launchpad and are redirected to the vehicle. Thirdly, blastwave noise is also transmitted through the launcher structure up to the fairing and payload base. Finally, the structural vibration of the fairing interacts with the airborne noise in a vibroacoustic problem. This work aimed to analyze the sound sources and their propagation during the lift-off stage. The approach here was to use computational fluid dynamics (CFD) to resolve the aero-acoustic problem that includes the study of the engine exhaust jet and its interaction with the launch pad. The simulation of all the physical phenomena that occur during the launch of rockets, such as exhaust gases' combustion, supersonic flow compressibility, and lift of the launch vehicle, is unfeasible. Solving the generated turbulence with a sufficient resolution requires a direct numerical simulation in a vast domain [6]. There is currently not enough computational power to achieve this. This lack of resources forces making simplifications, as done in the literature [7, 8]. Within these simplifications, it is fundamentally under discussion up to what level turbulence should be modeled since it has great importance in flow behavior and noise generation. In the context of supersonic jets, large eddy simulation (LES) is the standard because it is a very accurate model, as demonstrated against experimental data [4, 9, 10]. However, the computational cost of reliably simulating full-scale spacecraft launch environments is still very high. Therefore, we propose the simulation of this environment using the unsteady Reynolds-averaged Navier-Stokes (URANS) model, which provides a level of simplification that is computationally feasible. The URANS model has been previously used and validated to predict shock waves due to impinging supersonic jets as in [11] or for a VEGA fairing in transonic flight in [12]. Furthermore, we attempted to demonstrate that these simplifications are correct in the launchpad environment by validating with experimental data.