Microwave and Millimeter Wave Nondestructive Testing of the Space Shuttle External Tank Insulating Foam


The space shuttle Columbia's catastrophic failure has been attributed to a piece of external fuel tank insulating foam (spray on foam insulation) striking the leading edge of the left wing of the orbiter causing significant damage to some of the protecting heat tiles. The accident emphasizes the growing need to develop effective, robust and life cycle oriented methods of nondestructive testing (NDT) of complex conductor backed insulating foam and protective acreage heat tiles used in the space shuttle fleet and in future multilaunch space vehicles. The insulating spray on foam is constructed from closed cell foam. In the microwave regime, this foam is in the family of low permittivity and low loss dielectric materials. Near field microwave and millimeter wave NDT techniques were one of the techniques chosen for testing this material. (Microwaves are electromagnetic waves with frequencies between 0.3 and 300 GHz. Those in the frequency range above about 30 GHz are generally referred to as millimeter waves because their wavelengths in free space are conveniently measured in millimeters.) To this end, several flat and thick spray on foam insulation panels, two structurally complex panels similar to the external fuel tank and a blind panel were used in this investigation. Several discontinuities such as voids and disbonds were embedded in these panels at various locations. The location and properties of the embedded discontinuities in the blind panel were not disclosed to the investigating team prior to the investigation. Three frequency bands were used in this investigation, covering a frequency range of 8 to 75 GHz. Moreover, the influence of signal polarization was also investigated. Overall, the results of this investigation were very promising for detecting the presence of discontinuities in different panels covered with relatively thick insulating spray on foam. Different types of discontinuities were detected in foam up to 229 mm (9 in.) thick. Many of the discontinuities in the more complex panels were also detected. When investigating the blind panel, no false positives were detected. Discontinuities in between and underneath bolt heads were not easily detected. This paper presents the results of this investigation along with a discussion of the capabilities of the technique used.


Electrical and Computer Engineering

Keywords and Phrases

Disbond; Insulating Foam; Millimeter Waves; Composite Testing And Evaluation; Microwave And Millimeter Wave Imaging; Disband; Microwaves; Near Field; Void; Near-fields -- Measurement; Nondestructive testing

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Article - Journal

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© 2005 American Society for Nondestructive Testing, Inc., All rights reserved.

Publication Date

01 Mar 2005

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