Solid Propellant Rocket Motors as Acoustic Oscillators

Abstract : A solid propellant rocket motor can be regarded as an acoustic cavity containing two media, the solid and the gas. The thin burning zone is rather generally capable of amplifying pressure disturbances at the surface, and self sustaining oscillations occur when this gain balances the cavity losses. The tendency to oscillate in a given cavity mode is thus greatest when the web thickness is such that the mode has an acoustic pressure maximum at the surface, while stability is assured when the surface is at a pressure node. The frequencies of the modes and their regions of instability are sensitive to the propellant case boundary condition. Changes in mean burning rate arise from the 'erosive burning' associated with tangential surface velocities and are positive or negative according to whether the propellant is characterized by positive or negative 'erosive burning' under the prevailing conditions. Fracture in brittle propellants and changes in burning properties may occur as results of acoustic 'working' of the solid. (Author)