The subject investigation demonstrated that it is feasible to use the microwave technique to measure radial burning rates ,and differential burning rates in solid propellant rocket motors. A simulator, consisting of a spiral rotating in an oil bath, was used to represent the curved burning surface of a tubular grain of propellant with the outer surface and ends restricted. The radial movement of the spiral, simulating radial burning, was detected by recording the phasor difference of the reflected microwaves from the reflecting surface of the spiral and the reflected microwaves from a stationary reference surface. The reflected microwaves changed in phase relation producing successive minimum values in the detected signal for each one-half of a microwave wavelength in oil displacement of the spiral. The displacement rates were calculated as average rates for a displacement of one-half of a microwave wavelength in oil. The curved reflective surface did not present a measurement problem. The differential displacement rates were detected by recording the phasor difference of the reflected signals from two spirals. The reflected signals changed in phase relation, if the reflecting surfaces were moving at different rates, producing a beat frequency in the detected signal. The differential displacement rate was determined from the number of beat frequency cycles, the one-half microwave wavelength in oil, and the time. The addition of the aluminum powder to the oil simulating aluminized propellants did not prevent detection of the moving surface. The results indicated that the microwave technique can be applied to aluminized propellants. / M.S.