Peak directivity analysis of far-field acoustical measurements during three GEM 63 static firings

This study investigates far-field rocket noise from three static firings of the Northrop Grumman GEM 63 motor. In addition to determining the peak directivity angle from measurements, this paper considers the overall sound pressure level, spectra, and skewness of the pressure time derivative. Although there is temporal variability in overall level during each firing, linked to variation in thrust and possibly wind, the peak directivity during the maximum thrust period is located around 62-63° for all three firings. Additionally, the analysis shows that nonlinear acoustic propagation effects remain significant around the peak directivity direction at distances around 1.5 km. Overall levels, spectral high-frequency content, and derivative skewness are greater at larger angles when there is wind with an upstream component.