Aerodynamic Behaviour of a Descending Launcher First Stage Demonstrator with Grid Fins in the Aerodynamic Phase

In the Horizon Europe Project SALTO, a demonstrator of a vertically landing first stage with three Prometheus engines was investigated, the so-called T3. It shall be recovered with a downrange landing approach. In this paper the aerodynamic phase of the descent trajectory was investigated using wind tunnel experiments of the full launcher configuration in the Trisonic Wind Tunnel Cologne (TMK). The grid fins proved to be efficient to trim and control the vehicle. Increasing the size of the grid fins by 10% does not directly transfer into a larger trimmable angle of attack range. Modelling the engine nozzles as closed surfaces instead of open cavities introduces a systematic offset in the axial force coefficient which vanishes at zero incidence and grows to approximately 0.05 at the extremes of the tested angle of attack range, and it affects the roll moment of the undeflected configuration at the supersonic Mach numbers. This simplification is therefore acceptable for low-fidelity databases, but the nozzles should be modelled as open cavities for high-fidelity databases and for the validation of CFD computations. A Reynolds number variation showed a small but systematic effect on the axial force coefficient and on the roll moment, while no discernible effect was found in the coefficients governing the trim and the static stability. A superposition approach defined in earlier work was validated with the wind tunnel data. During ascent, the increase in length of the configuration influences the pitch moment coefficient considerably more than the deployment of the grid fins, while the deployed fins increase the axial force coefficient by approximately 17% at Mach 3.0.