Entry Guidance Authority over Thermal Protection System Bondline Temperature

Thermal protection systems are sized against the temperature of the structure beneath them, yet entry guidance enforces heating rate, heat load and load factor and leaves structural temperature to offline margins. This paper asks how much guidance can do about the bondline temperature of a reusable winged vehicle that must reach a fixed landing site. A drag-energy analysis shows that, at fixed range, heat load is a concave functional of the inverse drag profile: the smooth profiles of predictor-corrector guidance sit near its broad maximum, and reductions require flight on the corridor boundaries. With verified Shuttle-class models and a transient conduction model of the windward tile stack, a thermal-relief segment that rides the load-factor boundary is embedded in a bondline-constrained predictor-corrector with a conformally calibrated margin. The segment lowers the peak bondline temperature by about four percent of its rise, 5.8 K at 1.9 g, whereas uncertainty in tile conductivity, heating or initial temperature moves it by 7 to 11 K (one standard deviation). In 1000 dispersed flights the law reduces the probability of exceeding the 449.8 K limit from 5.8 to 3.3 percent, worth about one millimeter of tile; a two-coefficient heat-load proxy performs identically.