MHD measurement of acoustic velocities in rocket motor chambers

A linear analysis of longitudinal acoustic waves (either forced or self-induced) at or near the fundamental frequency in solid rocket chambers revealed that the acoustic velocity phase angle relative to the head-end pressure at the chamber midpoint is strongly dependent on the real part of the overall pressure-coupled response function and the imaginary part of the overall velocity-coupled response function. A flowmeter based on Faraday's Law was devised to demonstrate the feasibility of such velocity measurements under rocket motor conditions. The interaction of an externally-excited magnetic field with the unsteady velocity of hightemperature combustion gases results in a corresponding unsteady electrical potential whose magnitude is proportional to the flow velocity. The system is readily calibrated since electrical output does not depend on knowledge of ionization level and electrical conductivity (within certain limits) and measurements are independent of propellant type. Motor firing data has been used to deduce the real part of the pressure-coupled response from the phase angles between head-end pressures and midpoint velocities and the imaginary part of the velocity-coupled response from the phase angles between head- and nozzle-end pressures. The overall response functions measured in the device are compatible to the form used in stability prediction models.