A computer program predicts the noise and ignition over pressure in the vicinity of a rocket during launch. The program has been developed to complement the vibroacoustic-prediction effort for rockets now in use and to provide the capability for prediction of vibroacoustic loads associated with next-generation rockets. Programs like this one are vital parts of the implementation of NASA's "better, faster, cheaper" philosophy; they are needed because full-scale acoustic and vibration testing of launch vehicles or payloads is often difficult, time-consuming, and prohibitively expensive.

This program implements an empirical model based partly on recognition that noise in each frequency band of interest is generated throughout the rocket-engine flow. The empirical model utilizes accumulated data from noise and structural-vibration measurements performed on the space-shuttle launch pad since 1984.

The program is user-friendly. It provides for interactive modification of various parameters that affect the noise environment. Predictions can be made for any position on a launch vehicle and for both near- and far-field positions on the ground. Predictions can be made for both flight-readiness firing and liftoff, for a variety of vehicle and launch-mount configurations (including single or multiple engines and open or closed duct), and for any altitude of the vehicle during ascent.

The program can be represented, in simplified form, by the following set of instructions:

  1. Determine the flow axis relative to the vehicle and stand.
  2. Estimate the overall acoustic power (in watts) from engine thrust, number of nozzles, fully expanded exit velocity, and acoustic-efficiency values.
  3. Convert the overall acoustic power level from watts to decibels.
  4. In the case of a rocket with multiple nozzles, calculate the effective nozzle-exit diameter.
  5. Compute the core length of the plume or the normalized plume core length (normalized to the effective nozzle-exit diameter).
  6. Estimate the number of identical slices of the plume for analysis.
  7. Determine the normalized acoustic power per unit of the plume core length for each identical slice along the plume.
  8. Calculate the overall acoustic power for each of the plume slices.
  9. Convert the normalized spectrum for rockets to a conventional acoustic bandwidth (i.e., the power spectrum per hertz or per 1/3 octave band, as desired) for each slice of the plume.
  10. Compute the sound-pressure level at any given position on the vehicle for each plume slice and for each 1/3-octave band, inclusive of the effects of directivity.
  11. Calculate the sound-pressure level at any given position on the vehicle for all plume slices by logarithmic summation of contributions from each slice.
  12. Finally, compute the overall sound-pressure level (OASPL) by logarithmic summation for all plume slices and all 1/3 octave bands.

The program includes the input parameters and the computed outputs. Among the computed outputs are the 1/3-octave band number, the center frequency of each band, the width of the frequency band, and the sound-pressure level in that band. A plot of sound-pressure level for each 1/3-octave band number is also generated for use in developing specifications for qualification tests.

This work was done Raoul E. Caimi of Kennedy Space Center and Ravi N. Margasahayam of Dynacs, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Mechanics category.


NASA Tech Briefs Magazine

This article first appeared in the February, 2001 issue of NASA Tech Briefs Magazine.

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