High Resolution Millimeter Wave Inspecting of the Orbiter Acreage Heat Tiles of the Space Shuttle

The physical cause of Space Shuttle Columbia's catastrophic failure was a breach in its thermal protection system, caused by a piece of external tank insulating foam (SOFI) separating from the external tank and striking the leading edge of the left wing of the orbiter. Consequently, there is an urgent need for a rapid, robust and life-circle oriented nondestructive testing (NDT) technique capable of inspecting the SOFI as well as the orbiter's protective heat tiles and its fuselage prior and subsequent to a launch. Microwave and millimeter wave NDT methods have shown great potential to achieve these goals using real focused reflectometer techniques and synthetic aperture focused techniques. This paper presents recent results of an investigation for the purpose of detecting anomalies such as debonds and corrosion in the structurally complex multi-sectioned protective heat tiles using a real focused 150 GHz (D-band) reflectometer and wide-band millimeter wave holography at 18-26.5 GHz (K-band) and 33-50 GHz (Q-band). The results of these investigations clearly show the utility of millimeter wave NDT methods for detecting such anomalies. Both methods provide a significant amount of information about the nature of an anomaly including size and location.