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 pre¬sented to indicate that the substitute gas modeling concept is valid, i.e., simu¬lation of rocket noise can be achieved if the essential rocket flow parameters are duplicated* Since it is not possible to duplicate all flow parameters simultan¬eously and still retain the essential feature of simplicity, some compromises must be made. The helium model which provided rocket exit values of Mach number, veloc¬ity, and static pressure, and near duplication of density performed best. This evaluation is based on agreement in sound pressure levels with a small solid propellant rocket.nAn investigation was made to determine the effect of flight vehicle motion on propulsion system noise which is propagated to parts of the vehicle located in the near field. Following the selection of a working hypothesis, experiments were performed using a 0.6-inch diameter heated air jet operating in a l6-inch dia¬meter acoustically-treated wind tunnel. Experimental results compare favorably with predictions based on the hypothesis which explains the effect of vehicle motion by two separate factors: (1) the noise produced by a jet in motion is dependent upon the relative velocity between the jet and the air through which it moves;and (2) a shifting of the noise radiation pattern toward the rear occurs because of the combined effects of vehicle motion and the finite velocity of sound. A prediction method is developed which makes use of noise data from a stationary jet operated at a velocity corresponding to the relative jet velocity of a moving vehicle. These measured data are then translated by a calculated amount to account for the rear¬ward shifting of the noise field due to motion.
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