A 70 microphone, [Formula: see text], microphone phased array was built for use in the harsh environment of rocket launches. The array was set up at NASA Wallops launch pad 0A during a static-test firing of Orbital Sciences’ Antares engines and again during the first launch of the Antares vehicle. It was placed 400 ft away from the pad and was hoisted on a scissor lift 40 ft above ground. The data sets provided unprecedented insight into rocket noise sources. The duct exit was found to be the primary source during the static-test firing; the large amount of water injected beneath the nozzle exit and inside the plume duct quenched all other sources. The maps of the noise sources during launch were found to be time dependent. As the engines came to full power and became louder, the primary source switched from the duct inlet to the duct exit. Further elevation of the vehicle caused spilling of the hot plume, resulting in a distributed noise map covering most of the pad. As the entire plume emerged from the duct, and the on-deck water system came to full power, the plume itself became the loudest noise source. These maps of the noise sources provide vital insight for optimization of sound suppression systems for future Antares launches.

A 70-microphone phased array, protected to withstand the harsh environment of a rocket test facility, was used to identify noise sources in the Ares I Scale Model Acoustics Test. In the unobstructed burn of a single solid rocket motor, the free-flowing plume itself was found to make a long noise source. The scenario changed completely in launch configurations where a 5%-scale model of the Ares I vehicle was tested in static firings. It was found that the impingement by the plume on various regions of the launch pad constituted the primary noise sources. The scenario is very different from current models, which assume that the plume itself is the noise source and do not account for impingement sources. As expected, the addition of water in the trench and the hole for the plume passage attenuated the associated noise sources. Water injection on the top of the pad ("rainbird") was found to attenuate only the peripheral sources around the primary plume impingement zone. The noise maps suggest that the minimization of impingement by reducing vehicle drift, reducing plume spillage via increasing the size of the hole, and covering-up leakage paths for the sound waves from the trench will attenuate the liftoff acoustics level. Nomenclature b = beamformed output d = aperture of the array G = cross-spectral matrix R = radial position of a microphone from array center S = weight applied to individual microphones St = Strouhal frequency U = plume velocity at nozzle exit w = steering vector θ = angular position from the image center λ = wavelength Subscripts D = nozzle exit diameter f = frequency i, j = index for interrogation grid m = microphone index R = Rayleigh resolution

The aerodynamic noise experiment of the ducted tail rotor includes the hysteresis state and the former flight state. In recent years, the most commonly used and reliable test technology of aerodynamic noise prediction and control in aircraft-"Microphone phase array" measurement technology, which is use a large number of calibrated microphone to identify the wave front of the spatial sound source and use the same phase detection space acoustic field section, determining the distribution of source, and then research the noise propagation characteristics and control strategy of full-size model. At present, the aerodynamic experiment of helicopters is mainly focused on the main rotor and tail rotor and the acoustic test of the tail rotor is less. Therefore, this paper has concentrate on the aerodynamic noise test of the ducted tail rotor, which is of great significance to the development of it.

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