Managing energy and mode transitions in skip entry guidance for lunar return trajectories

Abstract

Skip entry trajectories provide a technique for returning an astronaut crew from the Moon to a continental United States landing site at any time during the lunar month. This approach to atmospheric entry requires that the guidance system be capable of precisely targeting a vast array of downrange distances. To meet this objective, Draper Laboratory has developed a baseline skip entry guidance algorithm which is a blend of the original Apollo guidance logic and a numeric targeting algorithm, PredGuid, that is used for the skip portion of the entry. The addition of PredGuid greatly improved the algorithm's performance for skip trajectories, but numerous simplifications and Apollo-based empirical relationships still limit the capability of the algorithm and may be unsuitable for other vehicle configurations. This thesis presents enhancements to the Draper baseline which redesign the energy management system and phase transition logic. A model-based predictor has been developed to determine the type of trajectory that is necessary to cover the target range and the appropriate time to transition to the next flight mode, based on the trajectory type. The direct entry capability has been improved and expanded by incorporating a variable constant drag policy to manage energy, and by designing specific direct entry reference trajectories which are used by the path-following controller in the final descent phase. In addition, an intermediate loft regime is introduced to bridge the range capability between direct and skip entries. These upgrades to the baseline algorithm greatly improve its robustness to uncertainties encountered through the entry, as demonstrated via Monte Carlo simulations, using the Crew Exploration Vehicle capsule concept. This algorithm design also offers the ability to optimize a skip trajectory to meet a particular set of objectives by modifying the reference skip bank angle.