Modification of directivity curves for a rocket noise model

Abstract

The spatial extent and downstream origin of rocket noise sources can significantly impact the physical interpretation of directivity index measurements. Valuable updates to historical rocket noise directivity indices, based on recent measurements of Space Shuttle reusable solid rocket motor (RSRM) boosters have been published by Haynes and Kenny (AIAA paper 2009-3160). However, measurements at a radial distance of 80 nozzle diameters from the RSRM nozzle exit plane are insufficient to be called the far field at low frequencies and thus require modification to the apparent source origin prior to their use in the empirical sound pressure level prediction methodologies, such as described in NASA SP-8072 (1971). In this analysis, estimates of plume source sound power level as a function of distance along the plume axis are combined with frequency-dependent, far field directivity indices to predict sound pressure level as a function of angle and range. With geometric modifications in place, the predicted overall sound directivity more closely matches that estimated by convective Mach number alone. These improved, more physically-based directivity indices will aid in the accurate prediction of full-scale launch vehicle noise, which is a key factor in estimating both vibroacoustic loading and environmental noise concerns.

Acoustical measurements during a static firing of the Space Launch System solid rocket motor

“…Aside from the 70° measurement location, which is well below the expected level due to the obstructing hill, the QM-1 measurements are roughly 3 dB higher than could be expected from the RSRM measurements. Similar comparisons with other rocket measurements [11,21] show that the OASPL at the 90° and 120° measurement locations are within 3 dB of expected behavior.…”

“…At 120°, the overall spectral shape is best approximated by a combination of the LSS and FSS spectrum, but we also point out that only FSS phenomena have been previously reported in the jet noise literature in the forward direction. The measured spectral shape may be due to other causes, but then again, the relatively high convective Mach number for this motor will push the peak directivity angle (dominated by LSS radiation) to be somewhere between 65 and 70° [11]. Thus, it is possible that LSS contributions may appear in the forward direction, although further study is needed.…”

Acoustic waveforms produced by a laboratory scale supersonic jet

“…In Figure 3.6a the source is assumed to reside at the nozzle exit plane at x/D j = 0, while in Figure 3.6b the source is moved downstream to x/D j = 17.5. The findings are remarkable and expose the kinds of gross errors that will occur when one chooses to propagate jet noise data along rays emanating from the jet exit plane; nearly identical observations have been made in the rocket noise community [18,25] .…”

Analysis of the Effects of Finite Impedance Ground and Atmospheric Turbulence on Launch Vehicle Noise Measurements

“…First, the extended, partially correlated nature of the rocket plume complicates the total interaction between the direct and reflected paths. The noise sources in launch vehicle exhaust cover a large spatial extent (tens of nozzle diameters) [2][3][4] and are directional because the turbulent structures in the plume are partially correlated.…”