Non-Destructive Testing of Rocket Fuel Tank Thermal Insulation by Holographic Radar

The Space Shuttle Columbia disaster occurred on February 1, 2003, killing all seven crew members. This and other incidents (e.g. with the Russian return vehicle which fortunately did not lead to catastrophic consequences) have aroused interest in the development of new methods for non-destructive testing of insulation and thermal protection coatings of spacecrafts and fuel tanks. In the opinion of NASA investigators, one cause of the Columbia disaster was defects in the thermal protection coating of the shuttle’s external fuel tank containing cryogenic components. To reduce fuel vaporization and prevent icing of the tank surface which could damage the shuttle, the tank is covered with insulating polyurethane foam. Methods of ultrasonic diagnostics, which are widely applied for non-destructive testing of different constructions, are ineffective for foam insulation due to polyurethane’s high porosity, which leads to high levels of acoustic attenuation. Similar considerations apply to the silicate fiber tiles that shield the outer surfaces of the USA Space Shuttle and Russian Buran. High porosity of the fiber tiles also produces incoherent scattering and attenuation. Microwave diagnostics using holographic subsurface radars could be a good alternative to ultrasonic testing. The basic advantage of microwave in comparison with ultrasonic methods is the fundamental difference in physical properties effecting the propagation of electromagnetic versus acoustic waves in heterogeneous media. Electromagnetic waves reflect from heterogeneities only when their dielectric contrast is sufficient. Thus, electromagnetic waves propagate practically without loss in foam insulation in which the matrix polyurethane has approximately the same permittivity as the air in the pores. A new method for using the holographic subsurface radar RASCAN 5/15000 to reveal internal defects of foam materials was proposed, and experiments on models of thermal insulation coatings were performed. The experimental results were displayed in the form of radar images on which defects in the heat insulation were shown to provide a good contrast and effective detection.