A comparison of two orion skip entry guidance algorithms

The Orion capsule is required to have landing site access (either on land or in water) inside the Continental United States (CONUS) for lunar returns anytime during the lunar month. This requirement means the vehicle must be capable of flying ranges of at least 5500 nm. For a blunt body entry vehicle, these ranges are easily achieved through the use of a skip entry trajectory. A skip entry guidance algorithm is required to achieve accurate landings at these downranges. Two skip entry guidance algorithms have been developed: the Numerical Skip Entry Guidance (NSEG), developed at NAS A/JSC and PredGuid, developed at the Charles Stark Draper Laboratory. A comparison of these algorithms is presented in this paper. Each algorithm has been implemented in a six-degree-of-freedom simulation called the Advanced NASA Technology Architecture for Exploration Studies (ANTARES). NASA, Lockheed, and Draper engineers have completed several Monte Carlo analyses in order to compare the performance of each algorithm in various stress states. Each algorithm has been tested for entry-to-target ranges to include direct entries and skip entries of varying length. Dispersions have been included on the initial entry interface state, vehicle mass properties, vehicle aerodynamics, atmosphere, and Reaction Control System (RCS). Performance criteria include miss distance at drogue deploy and landing, RCS fuel usage, maximum g-loads and heat rates for the first and second entry, total heat load, and control system saturation. The comparison resulted in the PredGuid algorithm being selected as the primary skip entry guidance algorithm for Orion Preliminary Design Review analyses.