Investigation on ignition overpressure wave characteristics and its ambience in launch pad

Abstract

Large solid rocket motors are widely used as booster stages as well as in core stages of heavy lift launch vehicles due to its simple operation and high thrust delivery capability compared to other propulsion types. The ignition transient of the solid rocket motors is marked by a very high chamber pressure rise rate compared to the liquid or other type of engines. This distinction causes generation of ignition overpressure wave during the start-up phase of solid rocket motor. Ignition overpressure (IOP) wave has a distinct characteristic of very high positive pressure, sharp rise and a relaxation (or negative pressure) waveform. Typically the overpressure relaxation wave has a very low frequency of < 25 Hz for the large solid rocket motors and is very detrimental for the structures with large surface area. Unlike acoustic pressure fluctuations, overpressure waves are transient and poses high risk to the safety of launch vehicle and its sub-assemblies as it propagates upward towards the base of launch vehicle. This phenomenon happens when the launch vehicle is still sitting on the launch mount (or pedestal) with its nozzle inside the launch mount. Hence, the geometry of the launch mount (with blind and open zones) may affect the overpressure load distribution on the nozzle or its base protective structures. Post lift-off analysis of the developmental launch of a heavy lift launch vehicle revealed the damaging effects of overpressure wave especially on the base portion of the launch vehicle. In order to study this phenomenon, measurements are taken during the subsequent launches by measuring the unsteady pressure distribution inside the launch mount. It is observed that the intensity of IOP wave has a highly asymmetric distribution inside the launch mount, with the amplitude as high as two times at certain regions. Similar measurements were also taken with a thermo-physically scaled down model of actual launch vehicle along with geometrically scaled down launch pad elements. As the chamber pressure rise rate in the scaled models is much higher compared to that of full scale motors, the absolute amplitude of overpressure wave cannot be compared. Nevertheless similar asymmetric distribution of overpressure was also observed in the scaled model experiments. The experimental results of sub-scale and full scale overpressure measurements like amplitude and frequency of trailing wave are compared with the Broadwell-Tsu empirical model predictions and the results are found to be closely matching, which helps in the prediction of IOP intensity for the future developmental launch vehicles from scaled down models.