Microphone Phased Array to Identify Liftoff Noise Sources in Model-Scale Tests

Abstract

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

Acoustic sources identified using a microphone phased array during a rocket engine test

“…The benefits of phased array technology were demonstrated earlier in static-burn tests, 2 static engine tests, as well as the first launch of the Northrop Grumman’s Antares vehicle. 3 More recently, a phased array proved to be a valuable tool in the redesign of launch pads for Ariane 4 and Vega 5 vehicles.…”

Validation and Verification of a Numerical Prediction Method for Lift-off Acoustics of Launch Vehicles

“…Then, acoustic measurements using 2.5-10%scale models are conducted for the detailed design and to predict the full-scale acoustic environments. [3][4][5][6][7] However, it is hard to determine the mechanism of acoustic generation using the empirical method and subscale measurements. The lack of understanding generation of the acoustics results in difficulty to devise an acoustic reduction technique.…”

Prediction of Near-Field Jet Cross Spectra

“…They examined two-point statistics of the various fluctuating quantities and compared them with far-field radiating noise in the downstream direction. Panda and Mosher [24] examined rocket exhaust with a large 70-microphone array and derived contours of the sound pressure level that illustrated the highly distributed source in a launch system environment. These particular studies are useful in the present work to help define the two-point cross correlation of the Lighthill stress tensor.…”