Stability Index for Injection-Coupled Instability in Full-Scale Firing Tests

To construct a stability estimation tool for injection-coupled instability in rocket engines, a linear stability analysis was applied. The amplifying factor related to injector-element acoustics and the damping factor related to chamber acoustics were defined. A complex vector, which is a function of these factors, was introduced. The stability for gain and that for phase were defined by the vector, and the bulk stability was defined by these stabilities. Because of time-lag findings obtained from subscale firing tests, the instability index, which was defined as inverse of the bulk stability, was defined only by the stability of gain. To validate the tool, the stability of cryogenic oxygen/hydrogen firing tests with a full-scale chamber was rated. The average acceleration was applied as an instability index for the firing tests. By setting the instability index by the tool at 1×103 as the threshold that divides “unstable” and “stable,” the stability obtained by the tool and that of the firing test showed...