Adaptive Heat-Flux Tracking for Re-entry Guidance

To limit the mass of the vehicle’s thermal protection system, an optimal trajectory that minimizes the total integrated heat load should be flown. In essence this boils down to tracking the maximum heat-flux constraint for as long as possible. A straight-forward and simple implementation for this tracking system could be a (linear) output-feedback controller that has a fast response, although its robustness could be doubtful due to insufficient damping. A possible good alternative is a guidance-tracking system based on so-called simple adaptive control. Such a system can have excellent performance under the influence of rather large uncertainties, although its transient response can be sluggish at times. In this paper the performance of both tracking systems has been compared, as well as an integrated implementation to see whether the individual strong points of the tracking systems can be combined. The system under consideration is a hypersonic test vehicle that has to track a stagnation heat-flux constraint of 1,700 kW/m. The results show that for a nominal mission the performance of the two individual systems is equal. A Monte-Carlo analysis indicates that the tracking error is smaller for the output-feedback controller, but due to its longer tracking time the total heat load is smaller for the adaptive system. Integrating the two systems yields a significant reduction of the tracking error, albeit at the expense of a larger guidance effort.