Initial Structural-Acoustic Modeling and Control Results for a Full-Scale Composite Payload Fairing for Acoustic Launch Load Alleviation

1 Launch loads, both mechanical and acoustic, are the prime driver of spacecraft structural design. Passive approaches for acoustic attenuation are limited in their low frequency effectiveness by constraints on total fairing mass and payload volume constraints. Active control offers an attractive approach for low frequency acoustic noise attenuation inside the payload fairing. Smart materials such as piezoceramics can be exploited as actuators for structural-acoustic control. In one active approach, structural actuators are attached to the walls of the fairing and measurements from structural sensors and/or acoustic sensors are fed back to the actuators to reduce the transmission of acoustic energy into the inside of the payload fairing. In this paper, structural-acoustic modeling and test results for a full scale composite launch vehicle payload fairing are presented. These analytical and experimental results fall into three categories: structural modal analysis, acoustic modal analysis, and coupled structural-acoustic transmission analysis. The purpose of these analysis and experimental efforts is to provide data and validated models that will be used for active acoustic control of the payload fairing. In the second part of the paper, this closed-loop acoustic transmission reduction is implemented and measured on a full-scale composite payload fairing. INTRODUCTION The structural design of expendable launch vehicle (ELV) payloads is driven by the severity of the launch environment. The loads transmitted to the payload from the ELV in the first few minutes of flight are far more severe than any load that a payload experiences on orbit. Therefore, payloads are qualified by subjecting them to loads whose magnitude and frequency content are representative of the launch environment. A more severe environment increases the cost of placing the payload into orbit because it must be designed to withstand higher launch loads. Reduction of these loads offers several benefits to the spacecraft designer and manufacturer, such as smaller lifetime cost due to the ability to use more off-the-shelf components and extend mission lifetime by carrying fuel in place of structural mass; and enhanced spacecraft survivability through the reduction of flight qualification loads. Passive vibration isolation has made tremendous progress during the last several years in reducing the levels of transmitted mechanical loads from the launch vehicle through the payload adapter to the payload, with the transition to flight demonstration currently taking place. As vibration isolation gains widespread use, the emphasis will shift to attenuation of acoustic loads. Acoustic loads are a major component of the launch environment for ELVs. Exterior sound pressure levels on an ELV can reach 150 dB depending on the vehicle and the launch configuration. The magnitude of the acoustic loads transmitted to the payload is a function of the external acoustic environment as well as the design of the payload fairing and its sound absorbing treatments. Several Roger M. Glaese, Project Engineer, CSA Engineering Inc., 2565 Leghorn St., Mountain View, CA 94043. Eric H. Anderson, Associate Principal Engineer, CSA Engineering Inc. Report Documentation Page Form Approved