Incomplete combustion - A possible cause of combustion instability

The performance of and the combustion in solid-propellant rocket motors may be affected by characteristic length (L*). For small characteristic lengths, the chamber pressure and characteristic velocity may be lower than in similar motors with larger L*. Oscillatory combustion may take place if the L* is small enough. In the past, L* oscillations have been explained by an oscillatory heat feedback mechanism and subsequent oscillatory propellant pyrolysis. The same phenomena—i.e., lower chamber pressure, lower characteristic velocity, and oscillatory combustion—can also occur in Hquid-propellant, hybrid, and airbreathing rocket motors. It is shown that these phenomena may be explained by assuming a finite reaction rate in the gas phase in combination with small residence times. Even if a constant burning rate or propellant mass flow rate is assumed, the coupling between the energy release in the gas phase and the residence time is sufficient to cause oscillatory combustion. A stability boundary criterion for solid-propellant rocket motors is derived and is found to be in agreement with experimental results.