A report presents a method of optimizing the orientations of three reaction wheels used to regulate the angular momentum of a spacecraft. The method yields an orientation matrix that minimizes mass, torque, and power demand of the reaction wheels while maximizing the allowable duration between successive angular-momentum dumps. Each reaction wheel is parameterized with its own unit vector, and a quadratic cost function is defined based on requirements for torque, storage of angular momentum, and power demand. Because management of angular momentum is a major issue in designing and operating an orbiting spacecraft, an angular-momentum-management strategy is parameterized and included as part of the overall optimization process. The report describes several case studies, including one of a spacecraft proposed to be placed in orbit around Europa (the fourth largest moon of Jupiter).
This work was done by David S. Bayard of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "An Optimization Approach to Orienting Three Spacecraft Reaction Wheel Actuators with Application to the Europa Orbiter," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Machinery/Automation category.
NPO-30526
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Optimization of Orientations of Spacecraft Reaction Wheels
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Overview
The document presents an optimization-based approach to orienting three reaction wheels on an orbiting spacecraft, specifically in the context of the Europa Orbiter mission. Authored by David S. Bayard from the Jet Propulsion Laboratory (JPL), the report outlines a method aimed at minimizing the mass and power requirements of reaction wheels while ensuring a specified maximum time between momentum dumps.
The core of the optimization problem is nonlinear, involving both cost and constraints. The approach utilizes a QR factorization of the wheel-to-body transformation, allowing for separate optimization of the rotation matrix (Q) and the skewness matrix (R) of the reaction wheel frame. Additionally, the initial momentum bias (b) is optimized to enhance momentum management. The optimization of the Q matrix is performed analytically, while the R and b parameters are optimized using Sequential Quadratic Programming (SQP).
The report includes several case studies that demonstrate the convergence and performance of the optimization method, highlighting its application to the Europa Orbiter mission. The results indicate that the per-wheel requirements are systematically reduced with continued iterations of the algorithm, leading to optimized orientations that are intuitively reasonable and favor the body axis with the most stringent requirements.
While the study focuses on three reaction wheels, it acknowledges that the Europa Orbiter is designed to include a fourth wheel, which serves as a backup in case of a single wheel failure. This introduces a more complex orientation problem that will be addressed in future studies, as it requires consideration of the fourth wheel's orientation to meet operational requirements in various failure scenarios.
Overall, the document emphasizes the significance of the new optimization method, which not only improves the performance of the reaction wheels but also has broader implications for a wide range of orbiting spacecraft. The innovative approach is expected to contribute to advancements in spacecraft attitude control, enhancing mission success and operational efficiency. The work was conducted under NASA's contract with JPL, underscoring its relevance to ongoing space exploration efforts.
