During the start of rocket engine nozzle flows, significant combustion or ignition pressure waves are generated that travel down the nozzle and interact with the vehicle and support structure. While this first set of compression and associated rarefaction waves are an important loading set and have been and continue to be studied by many investigators, there can be a secondary transient entrainment suction pressure that is not widely commented on and persists until the terminal shock in the nozzle has been expelled. Test data from the first Antares on-pad static fire with dual AJ-26 liquid rocket engines along with CFD simulations that relate the measured transient pressures to nozzle flow field contours help explain and visualize this phenomena and are presented with-in. In addition simulation results from two different engine combustion chamber rise rates are presented; the older Antares AJ-26 engines and the now upgraded Antares RD-181 engines, which highlight what many investigators have noted, which is that a reasonable sized simplified CFD simulation of 500k-2mil computational cells (in this study) can provide incredible insight and good initial engineering design level predictions.
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