Indian Journal of Science and Technology

Abstract

Several transient events occur during the startup of the Solid Rocket Motor (SRM) during the launch vehicle lift-off. Each event produces a complex transient signal and requires systematic assessment. The event discussed in this paper is Ignition Over Pressure (IOP). This Ignition Over Pressure is resulting from fluid dynamic compression of the accelerating plume gas, subsequent rarefaction and propagation during the pressure rise rate period in the rocket combustion chamber. These high-amplitude unsteady fluid-dynamic perturbations can adversely affect the vehicle and its surrounding structure. This wave behaves as a blast or shock wave characterized by a positive triangular shaped first pulse and negative half sine wave second pulse. The pulse travels upwards towards the propulsion system and has the potential to overload the individual elements or exciting overall vehicle dynamics. The later effect results from the phase difference of the wave from one side of the vehicle to the other due to the skew in the ignition of the strap-on boosters. In the case of the launchers, the mechanical stress due to the ignition over pressure wave comes, in addition to the acoustic constraint due to the jet noise. The over pressure phasing or ∆P environment, because of its spectral content as well as amplitudes becomes a crucial input for the design of sub-assemblies viz., thermal shields, and pay loads etc. In this paper, an attempt is made to numerically visualize the propagation of the blast wave causing the unsteady pressure oscillations during the transient pressure rise in the combustion chamber. To understand further, pressure measurements have been made at different heights along the umbilical tower during the solid rocket motor lift-off with the presence of jet deflectors, in order to capture the shock propagation phenomena. From the results, it appears that a sudden shock front that is generated during flow development within the rocket nozzle leaves a clear signature in the form of a well defined peak at typical time intervals. This overpressure amplitude seems to be related to the slope of the combustion chamber pressure rise rate during the transient period. It is also observed that the shock front initially propagates at supersonic speed but decays with time later. Similarly, the magnitude of the IOP peaks are also seen to decay with distance. 

Keywords: Ignition Over Pressure, Shock Wave Propagation, Ignition Transient, Solid Rocket Motor, Numerical Flow Visualization.