Effect of spike truncation on of the acoustics of an annular aerospike Nozzle Flow

THERE is considerable interest in the flow physics of aerospike nozzles in line with change in launcher stage design from the tandem to parallel configuration, as the main stage engine is now expected to fulfill a wide range of operating conditions during the launchers ascent. Studies show that aerospike nozzles deliver the best performance at all altitudes1 which was first reported on, in the 60s and 70s2-6. The wall of an aerospike (which serves as the inner wall of the conventional bell nozzle) is always exposed to an ambient pressure (which acts like an invisible outer wall) and thus allows the ambient pressure to govern the expansion process. Several types of aerospike nozzles are in use for aerospace vehicle applications. Among them are annular aerospike nozzles (either full length or truncated spike nozzles) and the two-dimensional linear aerospike nozzles. These nozzles typically run overexpanded during takeoff which is thus the condition of interest for such flows. Ruf and McConnaughey7 reported that most of the thrust on the spike is generated over the first quarter so that the remaining three quarters can be truncated without much loss in total thrust. Additionally, the thrust lost by removing some portion of the spike can be made up by nozzle base pressure acting on the base area. Such a truncated plug nozzle configuration has added advantages of reduction in engine size and weight which can then be used to increase the payload capacity of the launch vehicle and hence, an overall gain in system performance. Altitude compensating effects still occur for such a plug configuration as the flow structure downstream of the plug is significantly modified and hence, studies on plug nozzle configuration with and without afterbody flows are needed. Recently Verma8 has carried out investigations on the performance characteristics of both full length and truncated spike nozzles in the absence and presence of freestream flows and documents the decrease in performance from the ideal due to truncation and freestream effects. Since jet noise mechanisms have been shown to be sensitive to nozzle exit conditions, the acoustics of aerospike nozzles, which have very different exit geometry can be expected to be different. It is thus pertinent to investigate the acoustic behavior of such modifications to the nozzle, in order to estimate parameters like the acoustic loading of the structure during launch.