Unsteady wall-pressure measurements for underexpanded nozzles exhausting into launch tubes

An experimental program was conducted using transducers capable of measuring the unsteady pressures acting on the launcher wall for a nozzle-exit-location of 8.0h. At this position, the exhaust flow impinges on the launcher wall upstream of the ring but close enough to the ring to generate significant blow-by flow. The most interesting finding of the investigation was the transient characteristics of the flow initiation. As the rocket motor comes up to pressure (i.e., as the stagnation pressure increases toward its 'steady-state' (value), large negative gage pressures exist on the wall. Thus, at the instant of ignition, the system acts as an ejector. Hence, because the wall pressure in this region (which serves as the base pressure) is relatively low, the flow tends to expand through a relatively large angle. This initial expansion angle is larger than the 'steady-state' angle. Thus, the initial impingement shock is probably stronger than the corresponding steady-state condition. The flow then oscillates until reaching the appropriate steady state value. Transient behavior similar to this has been observed in several of the flight-test programs, that have been studied by personnel from the University of Texas at Austin.