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High-Frequency (HF) instabilities are observed in many high performance devices like liquid rocket engines. They involve a strong coupling between fluid dynamics, combustion and resonant modes of the chamber. Both longitudinal and transverse (radial and azimuthal) modes are involved in this process. Rocket engine hot fire tests indicate that transverse modes and in many cases rotating modes are the most destructive ones because they are less susceptible to damping by the nozzle and their amplitudes may reach very high levels. The acoustic mode enhances the combustion process, shortening the flame, augmenting the heat flux to the chamber walls and injection plane often leading to a rapid destruction of the system. The experimental description of high-frequency instabilities is generally incomplete. Tests mainly provide pressure records obtained from a limited number of sensors. Most of the HF instability observations correspond to tests carried out during the development of new engines precluding data collection on the unstable combustion process. Systematic experiments were carried out during such developments and extensive efforts were necessary to deal with instabilities and modify the initial design by adding baffles, resonators or cavities to reduce the level of oscillation and check that the system could operate in a stable manner. Much of the available material obtained during the 1960s and early 1970s, an intense period of research on the subject, is gathered in a classical report by Harrje and Reardon [1]. The story of the costly trial and error process followed to suppress instabilities in the F1 engine is synthesized in a …