Historical perspective of combustion instability in motors - Case studies

Over the last 40 years, many solid rocket motors have experienced combustion instability. Combustion instability is the interaction with a motors inherent oscillatory modes with the combustion and/or fluid dynamic processes inside the motor combustion chamber. If only a small amount of available energy, less than one percent, is diverted to an acoustic mode, combustion instability can be generated. Standard Missile, Sidewinder, Harm, Trident, Hellfire and Minuteman just to name a few have experienced pressure oscillations sometime during their development and life. Combustion instability can lead to ballistic pressure changes, can couple with other motor components such as guidance or thrust vector control, and in the worst case, cause motor structural failure. Combustion instability in a motor can cost motor developers millions of dollars to eliminate. Sometimes, combustion instability, although present, does not cause system problems and can be tolerated in a motor. In these cases, it still must be characterized and understood. The purpose of this paper will be to review 28 of these motors and what solutions were used. It is hoped that some of the lessons learned in the past can be applied to future motors. Introduction And Background Solid rocket instability is the interaction among the internal combustion and flow processes with a motor's natural acoustic modes. The most common acoustic modes excited are longitudinal and tangential modes. See Fig. 1. Combustion instabilities can be classified into two groups, linear and non-linear. Linear instabilities are those that are usually characterized by low amplitude sinusoidal oscillations that have very simple frequency content. Most motors have low-level linear instabilities that often cause no problems. When linear instabilities become stronger in nature and cause DC pressure shifts (changes in the mean ballistic pressure trace), have relatively high limiting amplitudes, and more complicated frequency content, then they become non-linear in nature. Other forms of non-linear instability behavior include pulsed instability. This is when a motor is pulsed by something passing through the nozzle or sudden increases in burning surface area due to propellant voids. These oscillations are characterized by steep fronted high amplitude pressure waves. Fig. 2 shows both types of instabilities. In Fig. 2, linear tangential oscillations occur at around one second and later, at two seconds, the motor was pulsed into high-level non-linear longitudinal oscillations. No oscillations resulted from Pulse 1. Details of these oscillations are shown in Figs. 3 and 4, respectively. Motor problems from combustion instability can be manifested in a Fig. 1. Example of Tangential Mode (Left) and Longitudinal Mode (Right) variety of ways including thrust oscillations, ballistic pressure changes, adverse coupling with guidance and thrust vector control systems, and in some cases catastrophic motor failure. 1.0 1.5 2.0 Time seconds Fig. 2. Example of Motor Experience Combustion Instability Acoustic stability is determined by the various acoustic gains and losses in the system. Acoustic gains include pressure coupling (coupling between acoustic pressure oscillations and combustion processes, very common), velocity coupling (coupling between acoustic velocity and combustion), distributed combustion (coupling with burning metal particles away from the propellant surface), and flow field interactions (phenomenon like vortex coupling with the motor acoustics). Acoustic loss mechanisms include nozzle damping (acoustic energy lost out through the nozzle), particle damping (for systems with particulates the particles remove acoustic energy), acoustic flow losses (sometimes called flow turning or boundary layer * This effort was sponsored by Independent Research Funding of the Naval Air Warfare Center. # Research Scientist, Research and Technology Division, Senior Member AIAA Approved for Public Release; Distribution is Unlimited. (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. losses), structural damping, and wall losses. Nearly all research in the area of combustion instability is involved in understanding these gains and losses. 1.0 1.2 1.4 1.6 Time seconds Fig. 3. Close-up of Linear Tangential Oscillations