An Experimental Investigation of Unstable Combustion in Solid Propellant Rocket Motors

Unstable combustion in solid propellant rocket motors is characterized by high frequency chamber pressure oscillations, often accompanied by changes in the mean burning rate. Experiments with case-bonded, cylindrically perforated motors were reproducible as a result of careful manufacturing control and extended propellant curing time. In these motors the oscillations were in the fundamental pseudo-standing tangential mode and were accompanied by increases in the average burning rate. At sufficiently high pressure levels all firings were stable. Reduction of the operating level led to mild instability. A sufficient further reduction produced a sudden change to maximum instability. Continued reduction in pressure level from this point resulted in a gradual decrease in degree of instability but it could not be experimentally verified that a low pressure stable region existed. The levels at which these events took place were frequency dependent and generally increased as the tangential frequency was reduced. At given operating level, the instability became less severe when the grain length was reduced below a critical value. Increasing the length above the critical value moved the point of maximum instability to somewhat higher levels but did not affect the level at which the motors became stable. The pressure levels for stability and for maximum instability moved to lower values with decreases in the propellant grain temperature in a manner not entirely accounted for by the effect of grain temperature on burning rate. Stable, mildly unstable and severely unstable operation was observed throughout the range -80°F to 180°F. The maximum instability decreased with grain temperature. Slab motors with opposed-plane grain surfaces exhibited oscillations in the transverse sloshing mode but no accompanying changes in the burning rate. Tangential oscillation of equivalent amplitude strongly affected the burning rate in the cylindric motors; hence it appears that increases in the burning rate are associated with tangential velocities rather than pressure fluctuations