Studies of Rocket Noise Simulation with Substitute Gas Jets and the Effect of Vehicle Motion on Jet Noise

The feasibility of using helium jets as a practical substitute for actual rockets in scale model acoustic tests was investigated by conducting an experimental program with four heated helium models. Sufficient evidence is presented to indicate that the substitute gas modeling concept is valid, i.e., simulation of rocket noise can be achieved if the essential rocket flow parameters are duplicated. It is not possible to duplicate all flow parameters simultaneously and retain the essential feature of simplicity; compromises must be made. The helium model per formed best. This evaluation is based on agree ment in sound pressure levels with a small solid propellant rocket. An investigation was made to determine the effect of flight vehicle motion on propulsion system noise propagated to parts of the vehicle located in the near field. Ex periments were performed using a 0.6-in. diam eter heated air jet operating in a 16-in. diameter acoustically-treated wind tunnel. Experi mental results compare favorably with predic tions based on the hypothesis which explains the effect of vehicle motion by two separate factors: the noise produced by a jet in motion depends on the relative velocity between the jet and the air through which it moves; and a shifting of the noise radiation pattern toward the rear occurs because of combined effects of vehicle motion and the finite velocity of sound.