Preliminary Analysis of Quasi-Steady State Pressure Oscillations of an Overloaded VEGA First Stage by means of Q1D Modelling

The operative life of many large solid rocket motors (e.g. US Space Shuttle SRM, Ariane 5 P230 SRM, Titan SRM, P80 SRM, the five-segment test motor ETM-3) is characterized by the presence of sustained longitudinal pressure and thrust oscillations during their quasi-steady state, that may induce severe impact on the payload comfort and on the dynamic operational environment of the launch vehicle. This paper is aimed at providing a first assessment of the pressure oscillations during the quasi-steady state of the new large monolithic SRM, foreseen in the future configuration of VECEP upgraded first stage, which concept design is based on a stretched and over-loaded version of the current VEGA first stage P80. The SRM configuration selected is inspired to the P105, VECEP first stage, designed as a stretched version of the P80 SRM, with an aft-finocyl configuration and the nozzle configuration of the P80. The numerical simulation of the SRM pressure oscillations is performed by means of a Q1D model, named AGAR (Aerodynamically Generated Acoustic Resonance), which set-up is based on the data of pressure oscillations provided by all the P80 firings (static firing tests: P80 DM and QM and VEGA flights: VV01 and VV02). To this aim, an analysis of the P80 firings is performed in order to identify, from the experimental data, the occurrence of the pressure oscillations phenomena and their repeatability among the firings. This analysis shows clearly that the P80 is characterized by the presence of four time windows of sustained pressure oscillations during the firing (B0, B1, B2 and B3), which are due to the excitement of the first longitudinal acoustic mode of the combustion chamber. The results of the numerical simulations for the P80, performed considering a calibration of the semi-empirical model of the AGAR code, are in good agreement with the experimental data (especially for the B0 and B1). The results for the P80 XL stretched tentative configuration show that also this tentative configuration is characterized, during its quasi-steady state, by the presence and the occurrence of pressure blows, in three time windows during the firing (B0, B1 and B2). Moreover, this first assessment indicates that the level of the pressure oscillations is higher with respect to the one shown by the P80 and involves only the excitement of the first longitudinal acoustic mode of the SRM chamber.