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 3.6-4.0 GHz. One promising application of the device is nondestructive 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 continuouswave 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 nondestructive 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 materials 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
Experiments have been carried out to evaluate holographic subsurface radar (RASCAN) for non-destructive evaluation (NDE) of subnominal bond conditions between the Space Shuttle Thermal Protection System tiles and the aluminum substrate. Initial results have shown detection of small voids and spots of moisture between Space Shuttle thermal protection tiles and underlying aluminum substrate. The characteristic feature of this device is the ability to obtain one-sided radar soundings/images with high sensitivity (detecting of wire of 20 micron and less in diameter), and high resolution (2 cm lateral resolution) in the frequency band of 3.6-4.0 GHz. JPL's advanced high-speed image processing and pattern recognition algorithms can be used to process the data generated by the holographic radar and automatically detect and measure the defects. Combining JPL's technologies with the briefcase size, portable RASCAN system will produce a simple and fully automated scanner capable of inspecting dielectric heat shielding materials or other spacecraft structures for cracks, voids, inclusions, delamination, debonding, etc.. We believe this technology holds promise to significantly enhance the safety of the Space Shuttle and the future CEV and other space exploration missions.
The Space Shuttle Orbiter Thermal Protection System (TPS) and Leading Edge Structural Subsystem (LESS) materials, design approaches associated with each material, and the lessons learned and operational performance experienced during 135 flights are described. The overall Space Shuttle Program proved that the thermal and structural design requirements were overall successful for the thermal protection system. Lessons learned from 135 flights are documented so that they can be applied to future human spaceflight systems.
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