Abstract <p>The external structure of launch vehicles is made up of metals or composites, which are insulated with multilayer materials to create a thermal protection system. In some instances, cork phenolic of the required thickness is bonded to the external structures to serve as a thermal protection system. Single-layer thermal insulation paint is applied over the cork phenolic sheet to further enhance thermal insulation. In some instances, multiple thermal insulation paints function as a thermal protection system and are coated by spraying or brushing over external structures. Conventional non-destructive testing methods, such as the eddy current technique, ultrasonic testing, and magnetic gauges, cannot be used for thickness measurement of single-layer and multilayer coatings over carbon fiber-reinforced plastic substrates. Terahertz-time domain spectroscopy in reflection mode is employed in this study for the thickness measurement of cork phenolic/single and multilayer thermal insulation systems over carbon fiber reinforced plastic substrates. A sparse deconvolution method resolves the reflected echo from each interface. The calculated thickness measurements of thermal protection systems closely match the optical microscopy reading.</p>