Combustion instability coupling with feed system acoustics

High frequency combustion instability has recently been observed by Rocketdyne in a 40K thrust methane/LOX combustion chamber. The oscillations had frequencies as high as 14,000 Hz with pressure amplitudes in the LOX dome of 500 psi at a chamber pressures of 2,000 psi. At this frequency the wave length associated with a period of oscillation is 2.3 inches in LOX and 1.4 inches in methane. These distances are comparable to the lengths of the injector elements which requires that acoustic waves be considered in the feed systems rather than using lumped parameters as is normally considered for feed system coupled oscillations. To expand the capability of existing models, the Feiler and Heidmann feed system coupled instability model was modified to include acoustic oscillations in the feed system. Similarly the vaporization controlled instability model of Heidmann and Wieber was modified to include flow oscillations that would be produced by feed system coupling. The major elements that control oscillations in a rocket combustion chamber are shown and discussed.