Comparison between Impulse and Holographic Subsurface Radar for NDT of Space Vehicle Structural Materials

Abstract — A subsurface holographic radar using a multi-frequency signal has been developed for inspecting dielectric construction materials. The characteristic feature of this device is the ability to obtain one-sided radar soundings/images with a high sensitivity and high resolution (2 cm) in the frequency band of 3.6–4.0GHz. One promising application of the device is non-destructive evaluation of the heat protection system and other materials on the U.S. Space Shuttle, and proposed crewed exploration vehicle (CEV). The advantages of this continuous-wave holographic radar over traditional impulse subsurface radars are discussed and illustrated by experimental results. The disastrous loss of the space shuttle Columbia, as well as even more recent dangerous incidents that were thankfully resolved, have aroused interest in possible new methods and devices for non-destructive testing and evaluation of the Space Shuttle Thermal Protection System, the external fuel tank insulating foam, and other materials and structures on the shuttle (see Figure 1), proposed CEV, and other space vehicles. Voids in or under the external tank insulating foam are considered potential sites for “cryopumping ” where water seeps in and then evaporates explosively at altitude, pulling the foam from the tank (Figure 2). One of the possible means for non-destructive testing (NDT) and evaluation of structural ma-terials is subsurface radar. This method is based on the propensity of electromagnetic waves to be reflected at permittivity contrasts. Up to now, the use of radar for NDT has been hindered by Figure 1: Suspect flaws on the external hy-drogen tank coating [1]. Figure 2: Close-up of voids in the external tank insulating foam [2]