Combustion of reactants in a confined volume favors excitation of unsteady motions over a broad range of
frequencies. A relatively small conversion of the energy released will produce both random fluctuations or noise,
and, under many circumstances, organized oscillations generically called combustion instabilities. Owing to the
high energy densities and low losses in combustion chambers designed for propulsion systems, the likelihood of
combustion instabilities is high. The accompanying heat transfer to exposed surfaces, and structural vibrations
are often unacceptable, causing failure in extreme cases. This paper is a brief review of combustion instabilities in
liquid-fueled propulsion systems-rockets, ramjets, and thrust augmentors-with emphasis on work accomplished
during the past decade. To provide a common framework for discussing the wide range of works, a theory of
two-phase flow is reviewed as the basis for an approximate analysis of combustion instabilities. The analysis is
directed primarily to treatment of linear stability; it is sufficiently general to accommodate all processes occurring
in actual systems. A new result has been obtained for an extended form of Rayleigh's criterion and its relation
to the growth constant for unstable waves. The chief mechanisms for combustion instabilities in liquid-fueled
systems are reviewed, followed by a summary of the common methods of analysis and applications to the three
classes of propulsion systems. Control of instabilities by passive and active means is examined briefly
