Measurements of Unsteady Pressures near the Plume of a Solid Rocket Motor

In support of efforts to improve predictions of the acoustic environment of the Crew Exploration Vehicle during pad abort scenarios, measurements of near-plume fluctuating pressures were obtained during the ground firing of five Improved Orion solid rocket motors. These measurements were obtained with eleven water-cooled sensors in two placements to provide a survey range of from 7.4 to 25.5 nozzle exit diameters downstream of the nozzle exit, and from 4.4 to 8.9 diameters radius from the plume centerline. A microphone at 102 nozzle exit diameters and 90 deg. to the plume centerline also recorded the far-field signature. The flow during the initial high-thrust phase of the burn was nearly ideally expanded, and the primary noise source was Mach wave radiation. This was followed by a low thrust interval with highly overexpanded flow, and shock associated noise was also significant during this phase. This paper presents and discusses time histories, spectra and near-field distributions of 1/3-octave fluctuating pressure levels for the two thrust conditions. The test method was used later to measure the near-plume unsteady pressures of a full-size solid motor for the new Orion CEV launch abort system.