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Directed Energy Wind Tunnel Test Methodology


OBJECTIVE: Develop a methodology for directed energy (DE) system testing in transonic wind tunnels that decouples mechanical vibration effects (jitter) from aero-optic aberrations due to density gradients in the shear layer above the DE turret. DESCRIPTION: The development a test and evaluation methodology is needed that maximizes the information return from ground testing before committing to airborne directed energy (DE) systems flight tests. To date, the results of wind tunnel tests at matched flight conditions are of questionable value because of the difficulty in separating the wind tunnel dynamics from the aero-optic measurements of interest. Future ground tests of DE systems will be conducted only if it is possible to characterize the relative displacement and vibration of the laser source, the target, and wavefront sensors in the wind tunnel. During the planning phase for wind tunnel tests, the finite number of options for locating the source, target, and sensors are evaluated. If the dynamic environment of each location is known, a motion damping system could be used to mitigate the jitter present in the wind tunnel. The predicted residual jitter could then be used to estimate the feasibility of each laser shot within the limitations of the optical hardware. Therefore, an effective DE test methodology should include a wind tunnel dynamic environment measurement system capable of measuring the baseline wind tunnel motion over the range of flight conditions and the test peculiar environment of each DE installation. The output of the dynamic measurement system will be the input to existing optical system design software for determining the wind tunnel impact on the waveform. This would allow the actual waveform that has been diffracted and attenuated by the aero-optic features of the turret to be corrected for the wind tunnel contribution. The data protocol for existing DE optical design software and wind tunnel configurations will be made available. The Phase I should produce a measurement system architecture for vibration measurements and process definition for implementing the measurements to an DE optical design software program. Basic system vibrational measurements should be demonstrated in a laboratory environment. The ultimate goal is development of a system prototype that includes the dynamic environment measurement system with output vibration data compatible with existing optical design software. The final system prototype should be demonstrated in a simulated wind tunnel environment. PHASE I: Develop a system design concept with detailed hardware and measurement requirements for quantifying wind tunnel structure vibrations. PHASE II: Develop and demonstrate a prototype system in a relevant environment incorporating the measurements for removal of facility vibration effects from DE aero-optic test data. PHASE III: The military and commercial wind tunnels have needs for highly accurate optical measurement systems with the ability to account for and understand the impacts of tunnel vibrations. REFERENCES: 1. Duffner, Robert W., Airborne Laser: Bullets of Light, Publication New York: Plenum Trade, August 21, 1997. 2. Trolinger, J. D. and W. C. Rose,"Technique for Simulating and Evaluating Aero-Optical Effects in Optical Systems,"36th AIAA Plasmadynamics and Lasers Conference, 69 June 2005, Toronto, Ontario Canada, AIAA 2005-4654. 3. Crosswy F. L., and A. G. Havener,"Aero-Optics Measurement System for AEDC Aero-Optics Test Facility, Technical Report AEDC-TR-90-20, 1991. 4. Hugo, R. J., E. J. Jumper, G. Havener, and C. Stepanek,"Time-Resolved Wave Front Measurements through a Compressible Free Shear Layer,"AIAA Journal Vol. 35, No. 4, pp. 671-677, April 1997.
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