Development of a Novel Algorithm for High Performance Reentry Guidance

Kurzfassung

While Europe has done tremendous steps forward in many aspects of space technology, there is still a gap for what regards the development of the technologies for a safe and reliable atmospheric entry. Several US-based programs such as NASA’s Orion, SpaceX’s Dragon, and Sierra Nevada Corporation’s Dream Chaser show that this capability is essential for a fruitful and safe access to space. ESA’s successful Intermediate eXperimental Vehicle (IXV) flight shows that Europe can now make important steps in this direction. In the European frame, the German Aerospace Center plays a main role with the development of the SHEFEX program. While the successful SHEFEX-1 and SHEFEX-2 flights were milestones for experiments associated with navigation and control subsystems, no guidance algorithms for a completely autonomous entry were tested so far. The continuation of SHEFEX, with SHEFEX-3, and its evolution ReFEX, may answer some fundamental questions. Is it possible for a vehicle such as the SHEFEX-3 spacecraft to have a safe and autonomous entry in the presence of a strongly constrained scenario, despite several uncertainties and disturbances acting on the vehicle? Are the standard methods based on drag-tracking (as in the case of the Space Shuttle, or of the aforementioned IXV) still the state of the art, or it is possible to implement new architectures based on the improved capabilities of the modern CPUs? And finally, how can the entry guidance deal with a strong asymmetric scenario? The present work answers these questions by illustrating the strategies conceived for SHEFEX-3, but which can in principle be adopted for future missions, to reduce the mission costs, and at the same time, to increase the reliability and the performance of the atmospheric entry. In this thesis a dual strategy is proposed. In what we call main guidance, novel methods based on optimal control, adaptive trajectory generation, and sliding-mode control, are developed and tested. In what we call backup guidance, a traditional drag-tracking method is applied to the SHEFEX-3 scenario. Some of the hypotheses usually valid for other scenarios, are here rejected. For this reason, the method developed for SHEFEX-3 can be seen as a generalization of the traditional methods used for entry guidance of vehicles like the Space Shuttle or the IXV. The two guidance methods are completely independent, and can run in parallel. Significant uncertainties are considered for testing them, and extensive Monte-Carlo campaigns have been run for the verification of the algorithms. Finally, a proper comparison of the methods is done at the end of the work, together with the conclusions and the lesson learned for the future. Results show that even if both the proposed methods provide good results despite several uncertainties and constraints acting on the vehicle, the main guidance performs better than the backup guidance in terms of final dispersion and range-to-go. For instance, when the perturbed US76 atmospheric model is used, the main guidance allows to have more than 60% of the cases falling within a radius of 50 km w.r.t. the target nominal point, while this percentage decreases to about 55% in case the backup guidance is used. Moreover, when more complex models for the atmosphere are used (e.g., the NRLMSISE-00), 1 . 6% of the cases associated with the backup guidance exceeds the maximum dynamic pressure, while the main guidance always allows to satisfy the constraints, and therefore, does not threaten the success of the mission. The main guidance proposed here can be therefore considered a valid choice in the frame of the future roadmap for the development of key GNC technologies for atmospheric entry, in addition to the existing methods.