Effects of Damping Ratio and Critical Coupling Strength on Pogo Instability

Abstract

Limitations of the classical critical damping ratio method are discussed and a new method based on critical coupling strength for pogo stability is proposed in this paper. First, the effects of the damping ratio on pogo stability are investigated in detail. When the structural damping ratio is less than the propulsion system damping ratio, increasing the structural damping ratio will increase pogo stability monotonically. However, when the structural damping ratio is greater than the propulsion system damping ratio, the effect of the structural damping ratio on pogo stability is complex and nonmonotonic. So, applicability of the critical damping ratio method is limited. Then, a critical coupling strength method for pogo stability is proposed, considering the effect of coupling strength on pogo stability is always monotonic. Furthermore, a stability margin is defined based on the critical coupling strength. Compared to the critical damping ratio method, the critical coupling strength method is always applicable. Finally, through analyzing the effects of the physical parameters (such as equivalent stiffness, combustion resistance, and structural damping ratio) on pogo stability of a certain type of Chinese liquid rocket, the limited applicability of the critical damping ratio method and the validity of the critical coupling strength method are demonstrated.