Suppression of Jet Noise by Staged Water Injection during Launch Vehicle Lift-Off

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Jet noise during the lift-off of a launch vehicle is complicated by the simultaneous flow of multiple jets, and their deflection by jet deflectors. Further, the presence of the launch pedestal, the service structure, and the moving vehicle itself, act as reflecting surfaces, which contribute to the noise environment. The present work involves the suppression of noise as measured at different parts of the launch vehicle in a small-scale replica of a full launch pad, for different locations of the vehicle along its vertical lift-off trajectory. The primary source of noise is the two jets emerging from the base of the launch vehicle at a Mach number of 3.38. Noise is suppressed by water injection at different locations in the launch pad such as the upstream and downstream edges of the jet deflector cover-plate, bottom and top of the launch pedestal, and at two different locations on the service structure. The effect of staged injection of water, i.e., without and with injection at different heights relative to the position of the jets, as the vehicle is at different locations in its trajectory, is examined. It is found that sustained suppression of noise is obtained only with injection of water in successive stages closer to the nozzle exit, as the jet position is moved up. The effect of angle of injection of water is investigated to verify previously reported results that an injection angle of 60°, i.e., along the flow, is superior to injection at 90° to the jet. The effect of injection pressure shows an optimum that supports the effective atomization of the injected water jets into droplets due to shear by the gas jet flow. Investigation with hot jets shows increased noise levels by at least ~2 dB relative to cold jets of nearly the same nozzle-exit Mach number. However, an increased reduction of noise by water injection is apparent with hot jets. The reduction is nearly independent of the jet temperature in the 600-900 K range. The present results collectively confirm the qualitative applicability of different previous studies on single free jets to the complicated launch vehicle noise scenario.