The stability of a rocket motor depends on the response of the solid propellant to pressure and velocity fluctuations on the combustion surface, and the determination of this response is at present only possible by experimental means. The technique developed for the study of pressure coupling consists in creating, by modulation of the exhaust throat, forced pressure oscillations in the combustor of a test engine with short residence time. Progress obtained in analyzing this type of engine and in experimental procedure yielded encouraging results. The study of velocity coupling, approached with similar means, is still at the explorating stage.