Research on combustion instability in liquid propellant rockets

Abstract

Progress in rocket development has resulted in a shift of interest from spontaneous to triggered instability. Hence, the primary objective of research has moved into the field of nonlinear phenomena. Analytical methods of dealing with such phenomena are under development. Results are discussed for particular chamber and nozzle geometries using the heuristic n−τ combustion model, already extensively used in linear instability studies and in practical instability correlations. They consider either longitudinal or pure transverse waves, with or without shock waves. Final wave shapes are calculated in the regions of linear instability. In welldefined portions of the region of linear stability, nonlinear instability is predicted with the corresponding wave shapes and triggering criterion. A physically more-realistic combustion model, based on droplet evaporation, was also treated, and the corresponding results are discussed. Preliminary experimental results are presented obtained from combustion chambers reproducing the conditions of the analytical study, the ultimate objective of these experiments being to check the validity of the theoretical assumptions. A more-direct check on the dynamics of the combustion phenomena is also being carried out through a kind of shock-tube technique applied to the actual combustion zone. Preliminary results from this apparatus are also presented. Another important field of research concerns the ways of improving the dissipation of the oscillation energy so as to favorably affect the energy balance responsible for instability. This problem has recently attracted wide attention in rocket development. The best known ways of dissipating oscillatory energy are through the exhaust nozzle itself, through baffles, and through acoustic resonators. Theoretical and/or experimental results on the three damping devices are discussed. The exhaust nozzle can only provide damping when a longitudinal component is present in the oscillation. Baffles can be quite effective if properly designed, but the control of their damping quality is not easy. Acoustic resonators seem to provide the most effective and easily controllable damper, if properly used. Probably the best design is not in the form of an “acoustic liner” but in more concentrated forms. Combinations of baffles and acoustic resonators may provide the best solutions.