Active Acoustic Launch Load Alleviation
The MIT Space Systems Lab (SSL) is teamed with Air Force Phillips Lab and McDonnell Douglas Aerospace on the Active Acoustic Launch Load Alleviation (AALLA) project. The goal of the project is to reduce the acoustic loads on payloads during launch by controlling the transmission and reflection of sound through the payload fairing.
An impedance matching control method is being developed for this project. This method is unique in that it does not require detailed knowledge of the acoustic behavior inside the fairing, but only requires knowledge of the fairing structure and local acoustic coupling. In addition, sensors are only required on the surface of the fairing, not on the payload where they may interfere with deployment or performance. Currently, research at MIT is focused on proving the impedance matching concept through computational simulation and experiments in an acoustic test chamber.
The computational portion of the project invloves simulating the controlled response of the acoustic chamber using a finite element model (FEM) and control algorithms in Matlab. There has been significant progress to date using 1D, 2D and simple 3D structural acoustic finite element models. Current work is focused on expanding the 3D FEM capabilities using the ANSYS finite element program. A short term goal is to be able to create and solve the finite element portion of the simulation in ANSYS; then import the results into Matlab for implementation with the controller. Future goals of the FEM portion of AALLA include exploiting the symmetry of the test chamber to greatly reduce the degrees of freedom required for an accurate model.
Experimental work thus far has focused on the implementation of an impedence matching controller on a 1D acoustic wavguide. Significant reduction in the acoustic energy has been achieved for the region near the first and second acoustic modes. The unmodeled higher order dynamics of the actuator prevent a broadband reduction in the acoustic energy. Therefore, experimental work is now concentrated on the modeling of these higher order actuator modes to servo the actuator, allowing a low order impedence matching compensator to be implemented for broadband attenuation of the acoustic energy.