Abstract
Apollo Service Propulsion System Block II firings exhibited an average spontaneous pressure spike (pop) rate a factor of 5 or more greater than Block I. The incidence of combustion instability also increased. The present study was undertaken to determine if the source of the pops was related to the hydraulic flow characteristics of the injector, or to the gasdynamic environment within the chamber. A mechanism for the occurrence of pops was hypothesized: zones of unreacted propellants may exist as a result of the injector pattern design; if these zones are suddenly mixed, a pop may result. To verify this theory, experimental studies were conducted to evaluate the hydraulic characteristics of the injector elements, and the existence of unreacted zones of propellant. By these studies, the existence of the potential for pops was verified. The injector was modified by counterboring certain orifices to prevent hydraulic flip. These modifications resulted in reduction in the pop rate by two orders of magnitude.
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