Hot-Flow Simulation of Aeroacoustics and Suppression by Water Injection During Rocket Liftoff

A 1:100 scaled-down model experimental setup that exactly replicates a launch vehicle and launch pad for simulating the vehicle liftoff is tested for its aeroacoustic environment. The rocket exhaust consists of twin supersonic hot-air jets of Mach number 3.38. The role of shock noise is investigated in separate single-nozzle freejet and impinging-jet experiments with varying levels of overexpansion. The launch jet noise is suppressed by water jet injection into the exhaust plume for selected locations on the launch pad. Sound reduction up to 9dB is achieved, and noise suppression continues to be effective up to 8D(e) of vehicle altitude (D-e being the nozzle-exit diameter). The sound spectra continuously varies from that of impinging jets at the deflector duct and launch table to that of freejets, as the vehicle is lifting off. The noise sources in heated jets respond similar to those in the cold jet, but comparatively higher sound reduction levels are obtained for hot jets in the impinging flow phase of liftoff (less than 8D(e)). Varying the water injection pressure and mass flow rate show optimal conditions for noise suppression. Injecting water close to the nozzle exit effectively suppresses noise during vehicle liftoff.