Two simple rotating systems have been designed for the determination of the specific impulse of rocket propellants. The pin wheel consists of a bicycle wheel mounted horizontally in a steel frame. A small rocket motor containing the propellant which is to be tested is mounted on the wheel so that the thrust delivered by the rocket accelerates the speed of rotation of the wheel. A photographic record of the rotational speed is then used to calculate the impulse delivered as a function of the time. The rotating bar is operated in a manner similar to that of the pin wheel.The rotating systems have been used for the determination of relative effective gas velocities of a number of double‐base rocket powders. Impulse measurements of miniature rockets of the order of 1 pound‐second were made. An investigation of the effect of expansion ratio of rocket nozzles on the reduced specific impulse showed good agreement with theoretical calculations.