161 Space Station Control Moment Gyroscope Lessons Learned 

Charles Gurrisi*, Raymond Seidel*, Scott Dickerson*, Stephen Didziulis∗∗,   Peter Frantz** and Kevin Ferguson∗∗∗ 

Abstract   Four 4760 Nms (3510 ft-lbf-s) Double Gimbal Cont rol 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 c ontrol. These CMGs with a life expectancy of  approximately 10 years contain a fl ywheel 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 y ears. Both units were returned to earth for failure  investigation.   This paper describes the Space Station Double Gim bal 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 less ons learned and design improvements  are not only applicable to CMGs but can be applied to spacecraft mechanisms in general. 

Introduction  2   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 und ertaken. 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 cond ucted 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 comple tely non-propulsive control, the CMGs  provide the ISS with the microgravity environment that is required for science payloads.  The US GN&C system uses a one-, two-, or thr ee-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 f our 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 operati on. 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.                                                        

* L-3 Communications Corporation Spac e & Navigation Division, Budd Lake, NJ  ∗∗ The Aerospace Corporation, El Segundo, CA  ∗∗∗ The Boeing Company, Houston TX 

Proceedings of the 40th Aerospace Mechanisms Symposium, NASA Kennedy Space Center, May 12-14, 2010  

NASA/CP-2010-216272