Motor scale and propellant geometry effects on pressure oscillations in aft-finocyl solid rocket motors

The characterization of pressure oscillation phenomena in large solid rocket motors is fundamental for the identification, since the earliest phase of SRM design, of their impacts on the launch vehicle, both for the definition of the dynamic environment in the low- frequency domain, and for the possible impacts for multi-boosters configurations, because of the generation of thrust imbalances. Pressure oscillations phenomena in large segmented solid rocket motors have been largely studied (e.g. Space Shuttle boosters and Ariane 5 boosters) in the past, on the contrary to large monolithic aft-finocyl motors which are of recent interest in the present (i.e. VEGA with its first stage P80, showing pressure oscillations phenomena), and the future European launch vehicles (i.e. P120C, as common block for Ariane 6 and VEGA-C, whose design derives from the P80 one). This work has the aim to investigate the motor scale and geometry eects in the class of aft-finocyl motors with a single-phase Q1D unsteady model of pressure oscillations during the steady- state, relying on semi-empirical parameters. The model is validated against P80 SRM experimental data, exploiting the work performed in the recent past by the same authors, and enlarging the perimeter of validation with the numerical reconstruction of a mid-scale solid rocket motor, named POD-Y, from the data published and available in the open literature. Results show that the model is able to provide reasonable good agreement with the experimental data for both the P80 SRM and the POD-Y type SRM, for model set- up close each other and reasonable since the differences of the two motor congurations. For the two motors, at large (P80) and mid-scale (POD-Y type SRM), the numerical simulations point out that the different scenario in terms of amplitude and time windows of the pressure oscillations, shown experimentally, is due as combined effect of both the motor geometrical configuration and the motor scale. A discussion on the open points at the state of the art of pressure oscillations assessment in aft-finocyl solid rocket motors is also provided.