Abstract: A monopropellant fluid is ignited in a combustion chamber to form a pressurized gas volume which may include the interior of a piston valve opening into the combustion chamber. A first gas path leading out of the combustion chamber conducts gas to surface areas of the piston so as to exert pressure urging the piston valve closed. A second gas path leading out of the combustion chamber conducts gas to other surface areas of the piston so as to exert pressure urging the piston valve to open a plurality of radial exhaust ports which vent the gas out of the combustion chamber and thereby generate a sonic pulse. Periodic opening and closing of the piston valve causes a train of sonic pulses to be generated.