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This paper discusses the adaptation and implementation of a modified two-level corrections process as the onboard targeting algorithm for the Trans-Earth Injection phase of the Crew Exploration Vehicle. Unlike earlier Apollo missions to the Moon, Project Orion intends to land near the polar regions, a task that can lead to a substantial plane change maneuver prior to Earth return. To reduce the fuel expenditure associated with this plane change, a three-maneuver sequence is employed during the return phase. First, the initial maneuver raises apolune at departure. The subsequent maneuver executes a plane change. The third and final maneuver places the spacecraft on its final approach to Earth while targeting the entry state for precision landing. An autonomous onboard targeting algorithm is sought in the event of loss of communication with the ground. This last scenario presents a very unique challenge, one never required of any Apollo vehicle. The Apollo missions also benefited from flexible entry requirements in contrast to Orion. Precision targeting in multi-body regimes has only been previously demonstrated in unmanned sample return missions, like Genesis. The two-level corrector formulation presented here ensures the entry constraints are met without violating the available fuel budget.

I. AbstractIn earlier investigations, the adaptation and implementation of a modified two-level corrections process as the onboard targeting algorithm for the Trans-Earth Injection phase of Orion is presented. The objective of that targeting algorithm is to generate the times of ignition and magnitudes of the required maneuvers such that the desired state at entry interface is achieved. In an actual onboard flight software implementation, these times of ignition and maneuvers are relayed onto Flight Control for command and execution. Although this process works well when the burn durations or burn arcs are small, this might not be the case during a contingency situation when lower thrust engines are employed to perform the maneuvers. Therefore, a new version of the modified two-level corrections process is formulated to handle the case of finite burn arcs. This paper presents the development and formulation of that finite burn modified two-level corrections process which can again be used as an onboard targeting algorithm for the Trans-Earth Injection phase of Orion. Additionally, performance results and a comparison between the two methods are presented. The finite burn two-level corrector formulation presented here ensures the entry constraints at entry interface are still met without violating the available fuel budget, while still accounting for much longer burn times in its design.

The present investigation focuses on one aspect of the autonomous targeting process used onboard during the Orion trans-Earth injection phase, specifically, a fast and robust algorithm that identifies a feasible return trajectory, one that meets the entry constraints without exceeding the fuel available. Unlike earlier Apollo missions, Orion seeks to land near the polar regions of the moon. Thus, a substantial plane change maneuver is required before returning to Earth. To reduce the fuel expenditure associated with this plane change, a three-maneuver sequence is employed during the return phase. An autonomous onboard targeting process for precision entry (one that incorporates multiple coordinated trans-Earth maneuvers) is sought in the event of loss of communication with the ground. The latter scenario presents a very unique challenge: one never before required of any Apollo vehicle. The Apollo missions also benefited from flexible entry requirements in contrast to Orion. Precision targeting in multibody regimes has only been previously demonstrated in unmanned sample return missions such as Genesis. The formulation presented here ensures that the entry constraints are met without violating the available fuel budget.

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