A Model of the Hypergolic Propellant Pop Phenomena

Abstract

Random high amplitude short duration (less than a millisecond) pressure and accelerometer disturbances were observed with the hypergolically fueled Apollo Spacecraft engines during their development phase. These disturbances, called pops, were and are undesirable because they may trigger damaging combustion instability. In all instances the pops were either eliminated or reduced to acceptable levels through trial and error testing since there were no design criteria by which logical element or pattern changes could be made. Generally, the pops were attributed to random accumulation and mono-propellant explosion of fuel pockets or zones caused by such things as poor element or pattern design, orifice flow instabilities (hydraulic flip), plugged oxidizer orifices, or fuel leakage through weld cracks. Although these postulated causes are certainly possible, recent investigations (Refs. 1-5) suggest that most of the observed pops are related to combustion phenomena associated with unlike hypergolic propellant stream impingement. For instance it has been shown (Ref. 1) that small local explosions of mixed fuel and oxidizer, rather than monopropellant fuel explosions, are most likely the source of pop triggers. On the basis of these investigations a semiempirical model has been developed which relates these small local hypergolic stream explosions to the occurrence of pops. This model was used to correlate development testing pop data obtained from the Apollo Lunar Module (LEM) ascent engine injectors, the Apollo Service Module (SPS) engine injectors, an advanced research SPS injector (SPS-IOS), and a NASA Jet Propulsion Lab. combustion research engine injector. The resultant correlations were used to predict injector pattern changes which successfully eliminated the popping observed previously with the advanced research SPS-IOS injector. The pop model is based on the supposition that the small local explosions which occur within the impingement region of unlike hypergolic propellant streams act to trigger unburned spray detonations which produce transient pressure and accelerometer spikes that are identified as pops. The basis for this hypothesis lies in two basic pieces of experimental work reported in Refs. 1 and 2. The supposed relationship between the small local explosion and the observed pop is illustrated in Fig. 1. It is postulated that the small local explosion emits a spherical blastwave which can initiate detonation of the adjacent