Based on a combustion characteristic that some propellants cannot burn in an intermediate pressure range while they can burn at lower and higher pressure, an active thrust modulation system was developed. The motor changed its thrust in dual combustion mode. It chose alternately high and low thrusts during combustion. The transitions from the low mode to the high mode were attained by a secondary ignition system, and those from the high mode to the low mode were done by a brief gas release system. Up to three times mode transitions were successfully demonstrated. Required time for the mode transitions and the effects of the thrust modulations on the thrust performance were evaluated. On the present motor scale, the transition time from the low mode to the high mode ranged from 0.12 s to 0.23 s, and that from the high mode to the low mode was from 0.19 s to 0.38 s. The thrust variable range was adjustable by the throat area. The ratio of the low-mode thrust to the high mode thrust was variable from 3 to 5. Specific impulse was decreased from 195 s to 175 s, when the number of thrust modulations was increased.

An active thrust modulation system of solid propellant motor was proposed and verified experimentally. Candidate propellants have a self-quenched property at intermediate pressure, while they can deflagrate at lower and higher pressure. Appropriate selection of throat diameter presents two combustion modes in the same motor geometry. Propellant with 73 % oxidizer content had demonstrated successfully two-mode active thrust modulation. The propellant with 75 % oxidizer content was examined. While the specific impulse showed little difference from 73 % AP propellant, the thrust density of 75 % AP propellant motor became high. The higher burning rate and the narrowed self-quenched pressure range made the thrust modulation a little difficult. An effect of a negative catalyst LiF was discussed. Slight addition of the catalyst had an effect to widen the intermediate self-quenched pressure range, but little effect to suppress the pressure amplitude in regulating the low mode pressure. More motor combustion tests are needed to confirm its availability.