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...for small satellites, spacecraft attitude control, and stability
How it works NASA Goddard’s reaction/momentum wheel consists of a motor-driven flywheel placed at one end of a shaft supported by at least two bearings. The motor, flywheel, and shaft all use the same spin axis. The flywheel is cantilevered off the end of the motor shaft, enabling the entire rotating assembly to be balanced to an extremely high degree of accuracy. The wheel can be operated with a current (torque) or speed (momentum) controller. Although mounting provisions for electronics exist inside the housing, the wheel normally uses external drivers. Pressure and temperature can be monitored in the wheel housing. Diester grease eliminates the need for lubrication reservoirs and the shelf-life storage concerns associated with fluid lubricants. When the motor receives input power, the output shaft rotates, which causes the flywheel to rotate. The rotating flywheel stores momentum; when accelerated or decelerated, a resultant torque is created. An equal, opposite reaction torque is created on the satellite, causing it to change attitude. By employing three or more wheels with spin axes in at least three directions and controlling the magnitude of the torque produced by each flywheel, the satellite attitude and instrument aim can be changed and maneuvered. Why it is better NASA Goddard’s reaction momentum wheel creates a very high torque compared to its relatively small size and weight. This enables faster satellite reaction and maneuvering, improving the utility of the satellite itself. The device’s small size is made possible because the motor and bearings are all housed on one side of the flywheel. In other designs, the bearings are located on both sides of the flywheel, necessitating a housing to support them and a cover to encapsulate the system. These requirements increase the bulk of the housing and the weight of the entire device. With a lighter weight, NASA Goddard’s wheel costs less to be put into orbit, enabling lighter satellites to be placed into higher orbits. Once in orbit, a lighter satellite can be maneuvered more quickly than heavier designs. NASA Goddard’s wheel design also helps to minimize dynamic unbalance forces, which can cause noise in the satellite output signal. In traditional reaction/momentum wheel designs, the flywheel is balanced while detached from the rest of the rotating system and mounted onto the shaft during assembly. This means that any imbalances in the bearings, shaft, and motor remain unaccounted for, increasing dynamic unbalance forces. In contrast, NASA Goddard’s design ensures that the entire rotating system is balanced, minimizing dynamic unbalance forces in all rotating parts. This helps reduce noise and, because unbalance forces add to the load carried by the bearings, may help prolong bearing life. Further, NASA Goddard’s technology helps to minimize drag torque, reducing the amount of power needed to achieve a particular rate of speed or acceleration. Using less power is important to minimize the generated heat that can cause failure of bearings, sensors, or the motor. Most traditional wheel designs employ oil-lubricated bearings to reduce drag torque, but oil lubrication usually requires extra components, such as oil reservoirs, channels, and seals, which adds to the size, cost, and complexity of the device. Pressurization is also required to prevent the oil from seeping or evaporating out of the bearings. NASA Goddard’s wheel uses diester grease for lubrication, eliminating the need for reservoirs and other components. Testing and Flight HeritageNASA Goddard’s technology has undergone extensive life testing in the laboratory, where two wheels have been operating continuously in alternating directions and horizontal/vertical orientations since October 1994. In addition, the reaction/momentum wheel has performed successfully aboard several scientific and military spacecraft:
Specifications
NASA Goddard Space Flight Center has patented this technology (U.S. Patent No. 5,723,923). (Link opens new browser window.)
This technology is part of NASA’s Innovative Partnerships Program Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the Apparatus for Providing Torque and Storing Momentum Energy (GSC-13649) for further development and for commercial applications. For information and forms related to the technology licensing and partnering process, please visit the Licensing and Partnering page. (Link opens new browser window) If you are interested in more information or want to pursue transfer of this technology, please contact: Innovative Partnerships Program Office
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NASA Goddard Space Flight Center invites companies to license its unique reaction/momentum wheel design. The design is small and low weight and features extremely low residual imbalance and large, highly controllable torque. The technology helps significantly lower the risks and financial investments normally associated with introducing a new spacecraft component. This innovation has received NASA’s 
Ideal for attitude control and stability of small spacecraft, Goddard’s technology was developed for NASA’s Small Explorer (SMEX) program—an initiative to develop highly focused and relatively inexpensive scientific spacecraft. The SMEX program’s Submillimeter Wave Astronomy Satellite (SWAS) required a reaction/momentum wheel with a highly controllable torque of greater magnitude than any comparably sized, commercially available wheel. In addition, because sensors on scientific satellites require an extremely low pointing jitter to resolve distant images, the reaction/momentum wheel needed an extremely low residual imbalance. Accordingly, Goddard researchers developed this wheel with unique specifications that are ideal for other small spacecraft. 
