Analysis of Unsteady Pressure Loads on the Themis T3 Vehicle in Unpropelled Backward Flight

The commercial success of the SpaceX Falcon 9 rocket has proven that reusable space launch systems are viable concepts for increasing the efficiency and reducing the cost to access space. Even though this system has been operational since 2015, no other company from the sector of space transportation has yet managed to successfully land the first stage of a rocket, neither in Europe, Asia or the US. In order to facilitate the development of a reusable launcher in Europe, the EU has financed several programs and initiatives to develop Vertical Take-off Vertical Landing (VTVL) capabilities. One of these is the Horizon Europe project SALTO (reusable Strategic Launcher Technologies & Operations) with support from ESAs Themis program. The goal is to design a rocket prototype to deliver up to 2000 kg into low Earth orbit. Themis uses the Prometheus Methane-LOx engine and is designed for reusability with a landing accuracy of 20 x 20 meters. During the project SALTO, several fly/recover/re-fly cycles are anticipated, starting with hop tests and later test flights up to 100 km altitude using the so-called T3 configuration featuring three Prometheus engines. This current study focusses on the aspect of unsteady pressure loads on different parts of the T3 vehicle, which hasn't been addresses in much detail in past studies of reusable launchers. In this work we will use the DLR TAU code together with a scale-resolving Detached-Eddy simulation (DES) turbulence model to resolve vortex shedding and the impingement of vortices on different parts of the vehicle at two flight conditions. The results show the spatial distribution and amplitude of pressure fluctuations on the vehicle surface and help to identify the regions of highest loads. By using methods from signal analysis, the study also provides frequency spectra of the unsteady pressure loads as an input variable for structural analysis. This study will also investigate grid resolution requirements for the Detached-Eddy simulation and also outline differences in predicted unsteady pressure loads for different mesh sizes. Also, a comparison between time-averaged pressure levels from the DES and steady RANS simulations for the same flight conditions will be shown.