Roll torque and normal force generation in acoustically unstable rocket motors

Abstract

Roll torque of considerable magnitude is generated in some solid-propellaiit rocket motors during periods of high-intensit y acoustic combustion instability. Forces normal to the motor axis sometimes accompany the roll perturbations. These effects appear to result from nonlinear viscous phenomena (acoustic streaming) that occur in the presence of intense acoustic wave motion in the combustion gases. Direct measurements of the moments and associated vortex flow disturbances originating within the burning cavity verify this theoretical description. Unstable motors using a cylindrical grain perforation geometry are especially subject to roll torque generation. However, similar effects are exhibited in the more complex grain configurations. Star-perforated cavities are capable of producing several levels of roll torque, depending upon the number of star-points. The secondary force effects are sensitive to initial grain temperature and thus can be minimized in some motors by prerun temperature conditioning.