On Destructive Liquid Rocket Resonant Combustion

*Experience indicates that the most destructive form of liquid rocket combustion instability is characterized by a steep-fronted, high-amplitude pressure wave sweeping through the combustion chamber in the form of a spinning tangential mode resonance. This type of pressure disturbance is observed to travel circumferentially about the chamber at supersonic speed with a period of rotation approximating the fundamental transverse acoustical frequency of the chamber cavity. It is generally recognized, however, that the fully developed wave’s detonation-like structural features cannot be adequately interpreted in terms of classical acoustic resonance theories, only. On the other hand, neither can it be interpreted solely in terms of detonation theory. Moreover, it has proven difficult to explain the severe destructive potential of the fully developed phenomenon, namely the extremely highpressure amplitudes and dramatically increased heat transfer rates near the injector face. Here, an alternative generating and sustaining mechanism is examined based on the supposition that low volatility propellant is transported back toward the injector where it may wet the surface and burn in a transpiring boundary with local reactivity. It is demonstrated via computational methods that this action can lead to the continuous production of vorticity at the injector face and that a spontaneous injection driven mechanism for driving a spinning mode instability arises through a localized coupling between the emerging vortical field and the irrotational acoustic field within the transpiring reactive boundary. The fully developed instability takes the form of a high-amplitude, steep-fronted pressure wave accompanied by an intense patch of vorticity near the injector face, which travel together around the chamber at a speed closely corresponding to the fundamental tangential mode acoustical frequency. The resulting detonation-like pressure wave exhibits many structural features that are consistent with observations from a landmark experimental study carried out at the Jet Propulsion Laboratory during the late 1960’s, and it is suggested that the intense local scrubbing action associated with the accompanying vortical structure could explain the highly destructive heat transfer rates frequently observed during engine development programs. Furthermore, the rotating combustion resonance is found to exhibit the essential characteristics of a solitary wave and may therefore be classified as a soliton.