Investigation of external acoustic loadings on a launch vehicle fairing during lift-off.

Abstract

During the lift-off of a launch vehicle, the acoustic pressure fluctuations caused by the engine exhaust gases produce high noise levels inside the cavity of the fairing and can damage the payload inside the fairing. Hence reducing the noise transmitted into the payload bay is an important area of research. Work presented in this thesis investigates the external acoustic pressure excitations on the fairing of a launch vehicle during the lift-off acoustic environment. In particular, it investigates the external sound pressure levels in the low frequency range from 50Hz to 400Hz, on the fairing during the lift-off of a launch vehicle. This study establishes theoretical and numerical models for the prediction of external sound pressure loading on composite structures representing launch vehicles, such as a large composite cylinder referred to as a Boeing cylinder and a Representative Small Launch Vehicle Fairing (RSLVF). To predict the external sound pressure loading, various incident wave conditions were investigated, including incident plane waves, oblique plane waves and oblique plane waves with random phases that strike the circumference of the composite structures. For the theoretical model, both the incident and scattered sound pressure fields due to incident plane waves; perpendicular to an idealised long cylinder were investigated. The results show that the scattered sound pressure field plays a major role in determining the total circumferential sound pressure field at the surface of the cylinder and cannot be ignored for the launch case. The theoretical model was developed further for a point source, line source and oblique incident waves, and modified to determine the incident, scattered and total sound pressure fields away from the cylinder. The approach developed overcomes some limitations of previous analytical derivations. An experiment was undertaken to determine the sound pressure patterns at the surface of a cylinder at various frequencies due to a point source positioned at a finite distance from the cylinder surface. The experimental work confirmed the accuracy of the theoretical model for a point source at a finite distance from the cylinder. The Boundary Element Method (BEM), approach was used for the numerical investigation of the acoustic loadings. The numerical analysis was developed for various acoustic loading conditions and verified with the theoretical results, which showed that the numerical and theoretical models agree well. Both models were extended to a Boeing composite cylinder and an RSLVF for various acoustic loading conditions. The complex acoustic environment generated during the lift-off of a launch vehicle was investigated and used as a basis for the acoustic loading on an RSLVF. To predict the acoustic excitations on an RSLVF, two different source allocation techniques were investigated, which considered acoustic sources along the rocket engine exhaust flow. The investigations were conducted both numerically and analytically. Both results agree well and show that it is possible to predict the acoustic loads on the fairing numerically and analytically.