Effect of ambient pressure fluctuations on the dual-bell transition behavior

An experimental investigation was conducted to study the effect of ambient pressure fluctuations on dual-bell transition behavior. Back pressure fluctuations of frequency (f=0.5,1.0,1.5, 2 and 3Hz) were artificially introduced inside the HASC using three magnetic valves. The dual-bell transition behavior is found to be very sensitive to P_b fluctuations of magnitude > 20% which triggers a flip-flop phenomenon that is observed to continue as long as the fluctuations prevail. The tests show that for frequencies >2Hz, although significant fluctuations in P_w in the nozzle extension can be felt, no full transition is achieved. This behavior too is not favorable as it can lead to a delay in transition. These preliminary test results suggest that the dual-bell nozzle can present problems in real-flight operation as the launcher experiences the buffeting phase of flight. The only solution therefore lies in increasing the DB stability corridor to a value greater than the magnitude of P_b fluctuations (of 20%) so as to avoid the NPR range that will set the dual-bell into a flip-flop mode during buffeting.