On the Effects of Supersonic Igniter Jets during Pre-Ignition Transient of SRMs

This paper presents and discusses the early phase of the ignition transient of the VEGA Launcher's third stage solid rocket motor, Zefiro 9A. Relevant experimental data coming from two different static ring tests (QM2 - VT1 and VT2) will be compared with numerical simulations performed with a Quasi-1D code, named Spit, as well as a 3D code named Xaer, both developed by DMA. During the last two ring tests of the Z9A SRM (QM2 and VT1) a peculiar unexpected negative force peak as well as a significant pressure unbalance in the motor chamber were measured during the very rst phase of the ignition transient. The Q-1D numerical simulation obtained by the Spit code for the prediction of the SRM behavior underestimates this anomalous pressurization. Hence, 3D simulations were performed to reconstruct the flowfield features affecting the SRM behavior by the Xaer code, showing a much better agreement with the experimental measurements. 3D simulation, indeed, can provide a good insight on the origin of this peculiar pressure distribution, clearly identifying a supersonic region spreading across the entire motor chamber section that temporally "blocks" the motor head section delaying its pressurization. In order to x the unexpected negative force, out of the system specifications, a modification of the igniter design was decided for the Z9A SRM and a third static ring test was carried out (VT2), for which both the Q1D and the 3D models provided numerical predictions of the first phase of the SRM ignition transient. Hence, the Q1D and the 3D models simulation will be compared with the experimental data and used for the reconstruction of the Z9A behavior for the QM2 and VT1 SFTs, for the investigation of the main cause of the negative force peak experienced. Whereas, the 3D tool will be used for the prediction of the flowfield pressure unbalance for the Z9-A VT2 and compared with the SFT experimental data.