Development of an Adaptive Passive Vibro-Acoustic Device for Payload Fairings

Abstract

Launch acoustic loads have the potential to damage sensitive payloads within a payload fairing. A typical payload fairing will exhibit cavity resonance modes related to its dimensions. These modes are excited both by the vibrations transmitted through the structure during launch, and by acoustic ground reflections coupled into the system during the initial launch phase. At low frequencies, cavity resonance modes are often responsible for peaks, or local maximums, in acoustic transmission. Conventional acoustic blankets mitigate the acoustic environment within a payload fairing, but are generally only effective above 250 Hz. In order to address this deficiency, the Air Force Research Laboratory Space Vehicles Directorate developed the Passive Vibro-Acoustic Device (PVAD), which employs a passive attenuator and a tuned-mass damper to mitigate a low-frequency acoustic cavity mode and an acoustically-coupled structural mode simultaneously. Each device must be tuned to a specific mode, however, and the cavity mode frequencies are sensitive to winter/summer temperature variations and varying payload volumes. Hence the performance of a purely passive device may vary significantly from launch to launch. In this work we present an Adaptive VibroAcoustic Device (AVAD), which overcomes sensitivity to temperature and other factors by adjusting the passive attenuator to a mode of the current acoustic environment during an autonomous training period shortly before launch. The device also has the added bonus of being able to add damping to several modes. 44th AIAA/ASME/ASCE/AHS Structures, Structural Dynamics, and Materials Confere 7-10 April 2003, Norfolk, Virginia AIAA 2003-1814