Space Station Control Moment Gyroscope Lessons Learned

Four 4760 Nms (3510 ft-lbf-s) Double Gimbal Control Moment Gyroscopes (DGCMG) with unlimited gimbal freedom about each axis were adopted by the International Space Station (ISS) Program as the non-propulsive solution for continuous attitude control. These CMGs with a life expectancy of approximately 10 years contain a flywheel spinning at 691 rad/s (6600 rpm) and can produce an output torque of 258 Nm (190 ft-lbf) 1 . One CMG unexpectedly failed after approximately 1.3 years and one developed anomalous behavior after approximately six years. Both units were returned to earth for failure investigation. This paper describes the Space Station Double Gimbal Control Moment Gyroscope design, on-orbit telemetry signatures and a summary of the results of both failure investigations. The lessons learned from these combined sources have lead to improvements in the design that will provide CMGs with greater reliability to assure the success of the Space Station. These lessons learned and design improvements are not only applicable to CMGs but can be applied to spacecraft mechanisms in general. The International Space Station (ISS) is currently the largest man-made object to ever orbit the Earth and represents one of the greatest engineering and integration efforts the National Aeronautics and Space Administration (NASA) has ever undertaken. The Guidance, Navigation, and Control (GN&C) system is composed of both a US non-propulsive attitude control system and a Russian thruster attitude control system. Nominal operations are conducted under US control using its four Control Moment Gyroscopes (CMGs), shown in Figure 1, with the Russian system providing momentum desaturation through thruster assists. When configured for completely non-propulsive control, the CMGs provide the ISS with the micro- gravity environment that is required for science payloads. The US GN&C system uses a one-, two-, or three-axis Torque Equilibrium Attitude (TEA) seeking controller to minimize the amount of momentum required to maintain attitude control. Due to thermal issues with external components, several of the assembly stages require a biased attitude to minimize sun exposure to specific surface regions. Analysis of the momentum necessary to maintain the required attitude envelope has shown that the momentum of four CMGs is required for much of the assembly phase. This paper describes the configuration of the ISS CMGs, their operations, a description of CMG1's "hard" bearing failure after approximately 1.3 years of operation. Included also are the findings for CMG3's "soft failure" that led to its removal from service and the corresponding most probable causes and lessons learned.