Development of a Near-field Intensity Measurement Capability for Static Rocket Firings

Near-field characterization of the acoustical environment near rockets has often involved extrapolating far-field measurements. However, because far-field amplitude data reveals only limited information about source characteristics, a vector intensity measurement system and analysis package has been developed to examine source features more directly. This paper describes the development of the measurement and analysis capability and its application to a horizontal firing of a GEM-60 solid propellant rocket motor firing conducted at ATK Space Systems near Promontory, Utah. An analysis of near-field intensity data provides insight both into the spatial extent and principal radiation lobe as a function of frequency. For 50 Hz, the far-field spectral peak frequency in the maximum radiation direction, the dominant source region derived from tracing the near-field intensity vectors spans 17-32 nozzle diameters, with peak radiation at ~68. At high frequencies, the radiation results from a more contracted region that occurs farther upstream and is directed at about ~85°. These results point to the potential utility of near-field vector intensity measurements, in part because the near-field environments represented do not agree with historical far-field data-based models.