An investigation of the instability mechanism present in a laboratory rocket combustor is performed using computational fluid dynamics (CFD) simulations. Three cases are considered which show different levels of instability experimentally. Computations reveal three main aspects to the instability mechanism, the timing of the pressure pulses, increased mixing due to the baroclinic torque, and the presence of unsteady tribrachial flame. The stable configuration shows that fuel is able to flow into the combustor continuously allowing continuous heat release. The unstable configuration shows that a disruption in the fuel flow into the combustor allows the heat release to move downstream and new fuel to accumulate in the combustor without immediately burning. Once the large amounts of fuel in the combustor burn there is rapid rise in pressure which coincides with the timing of the acoustic wave in the combustor. The two unstable cases show different levels of instability and different reignition mechanisms.
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