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High-Frequency Instability (HFI) in liquid rocket engine (LRE) combustion chambers remains one of the most challenging problems in engine design. The classical framework of non-equilibrium thermodynamics and acoustics-including the Rayleigh criterion and modal analysis of the chamber+nozzle system-successfully describes the conditions for instability onset (phasing of pressure p′ and heat release q′ oscillations) but does not reveal the primary cause of the phase-based energy supply to modes: why does the system tend to maintain the required phase lag, and where is the universality of this mechanism encoded? This creates a methodological limit: suppression reduces to dampers, injector reconfiguration, tuning of empirical parameters, and "detuning" phases, while fundamentally new strategies remain weakly justified. |