Prediction of the Acoustic Levels During P80 FW Static Firing Test

Abstract

VEGA, the future European small launch vehicle, is a single-body launcher composed of three solid-propellant stages and a liquid propellant upper module. It is approximately 30 metres high, has a maximum diameter of 3 metres, and weights a total of 137 tons at lift – off. The acoustic loads applied to the launch vehicle upon the lift-off and the transonic flight, are broad band and random loads which may be dangerous for the payload and the equipment. A fast and cheap prediction may be obtained by numerical procedures rather than performing measurements on real prototypes. At the lift-off the noise exciting the structure depends on the jet noise and on the interaction between the jet and the launch pad. During the transonic and the supersonic flight the forces are aeroacoustic excitations caused by the interaction between the structure and the air. In particular, the lift-off external acoustic levels are typical of a diffuse field, while the aeroacoustic transonic noise derives from a turbulent boundary layer. The classical prediction method of the lift-off external acoustic field is that presented in the NASA space vehicle design criteria named “Acoustic Loads Generated by the Propulsion System” (NASA SP-8072). On the basis of this technical specification and of Wilby and Wilby “Prediction of External Acoustic Field of Taurus during Lift-off”, Tom Irvine realized a prediction tool, which is particularly able in the prediction of the motor noise during lift-off or during a static firing test in the immediate surroundings of the nozzle exit section. A fundamental step of the development of VEGA Launcher is the static firing test of the first stage which is named P80 FW. This is a 90 tons of solid propellant rocket motor, having a height of about 10 m and a diameter of about 3 m. The burning time is about 105 s. In particular, the P80 FW static firing test represents a fundamental test bed for the acoustic levels which are actually expected at lift-off of VEGA Launcher. The tuned Tom Irvine prediction tool has been used in order to obtain the acoustic levels on the nozzle exit section during the static firing test. The Statistical Energy Analysis (SEA) is, at present, the most useful method for solving the vibroacoustic problems, by providing information on the stored mechanical energy and on the dissipated mechanical power among modal subsystems. The energy of the subsystems is calculated by solving a set of algebraic energy-balance linear equations: the right-hand side quantities are the powers injected into each subsystem and the coefficients depend on the