High resolution millimeter wave imaging of space shuttle external fuel tank spray-on foam insulation

The Space Shuttle Columbia's catastrophic structural failure was caused by a dislodged piece of spray-on foam insulation (SOFI) from the external fuel tank. Separation of SOFI pieces may be caused by the presence within the foam of anomalies such as voids, delaminations ami disbonds. These unwanted discontinuities may occur during the staying of the foam onto the external tank or may be due to post-spray damage. High spatial resolution millimeter wave nondestructive testing and imaging techniques have shown great potential and overall system utility for evaluating internal SOFI structures. This paper presents the basic design of a 150 GHz imaging reflectometer system and discusses the results of a critical investigation of the capabilities of this system that examined a SOFI panel with subtle embedded delaminations and voids without sharp boundaries, and another special panel possessing unknown natural discontinuities (the result of intentional manufacturing processes). Finally, the information obtained from the millimeter wave images was correlated with information obtained from post-imaging dissection of the panel. It was shown that 91% of the discontinuities (such as extended and localized wids and delaminations) were successfully identified using this millimeter wave imaging system.