Among the most critical spares being transferred from the shuttle to the International Space Station on STS-129 are two 600 lb. control moment gyros built by Boeing and L3 Communications.

Spare Boeing/L3 Control Moment Gyro for ISS is prepared at Kennedy Space Center prior to installation on Express carrier. Credit: NASA

One gyro was transferred on Nov. 18 on the first of two ExPRESS Logistics Carriers (ELC1) lifted from Atlantis by the shuttle arm then handed off to the station arm for positioning on the station's truss. The second gyro was transferred on ELC2 on Saturday, the sixth day of the mission.

The gyros are needed to back up the four operating gyros already in place to ensure station attitude control throughout next 11 years of envisioned station life. The gyros are used to hold station attitude and to torque change station attitude when that method is more efficient than using Russian attitude control thrusters.

The two spare gyros have been upgraded with more advanced lubrication, metals, and internal force settings as a result of the investigation into why they earlier failed.

Each of the previous two failures occurred years earlier than their 8 year contracted specifications.

This raised concern among station managers that additional failures might occur after phase out of the shuttle program in 2011 when there will be no way to get additional spares aloft.

Some studies have indicated that a single large gyro could be carried in a Russian Progress resupply ship, but Greg Vajdos, manager of guidance, navigation and control operations for Boeing said it is better to launch all the rest of the gyros to the station at one time, which has now been done by Atlantis. Each gyro measures 45 x 48 x 54 in.

The first failure occurred in early June 2002 after only 1.6 years of operation. The second was shutdown in April 2004 to prevent what telemetry indicated was a fast approaching catastrophic failure.

The two failures resulted in some station assembly being conducted with just two gyros operational, an undesirable situation because the ISS was often in an asymmetrical aerodynamic configuration with an off nominal center of gravity.

The two gyros that were operating combined with the Russian thrusters enabled the assembly to proceed, however, although there were no shuttle visits to the ISS between 2003-2005 because of the need for the shuttle program to recover following the Columbia accident.

The gyros that failed initially were brought back to Earth and traded out with refurbished or new units. The units launched on Atlantis have the maximum upgrades of any of the systems, says Charles Gurrisi, principal mechanical engineer for the ISS control moment gyro program at the Space and Navigation Division of L-3 Communications at Budd Lake, N.J., where the gyros were built.

Gurrisi, who also worked on development of the Skylab space station gyros, says the two systems have many commonalities, although the avionics control capability is much greater today. The biggest upgrade in avionics and computer control came when NASA in 2000 transitioned the shuttle to standard 1553 processors, enabling the gyros for ISS to use the same software configuration.

One of two STS-129 spare ISS gyros is packaged in protective insulation as it is hoisted onto Express spares pallet. Gyro could spend years on pallet before needed, if ever, to replace one of four operating now. Credit: NASA

Each CMG consists of a single 25 in. dia. 220 lb, stainless steel flywheel that rotates at a constant speed of 6,600 rpm developing 3,600 ft.-lb.-sec. of force about its spin axis.

The rotating wheel is mounted in a two-degree of freedom gimbal system that can position the momentum vector in any direction.

At 6,600 rpm the flywheel is normally spinning so fast it takes hours to slow and stop when shut down. When the first gyro failed, it shook the entire station that weighed 300,000 lb. at that time. The CMG container located in the Z-1 truss above the U.S. lab also emitted a long audible groan as the parts destroyed themselves. The flywheel that normally took about 12 hr. to decelerate this time took less than an hour as it ground up the parts around it.

As a result of the earlier failures, the two CMGs launched to the ISS this week have the following upgrades:

• Better use of real time data: Gyro telemetry settings have been adjusted so more information is received on gyro parameters. The system is more vigilant for vibration and electrical transients that can occur when station attitude control is shifted from the Russian thrusters to the gyros. The initial failure occurred during such a transition and such electrical and vibration transients have been seen at other times, including when the second gyro began to fail.
• Bearing load reduction: The flywheel's center shaft, an axle-type structure, contacts a spin bearing on each end. "We found that the spin bearing likes to be preloaded to maintain contact with the bearing," Gurrisi says. "And we found there were deficiencies in how we applied and maintained that preload," he said. The initial gyro failure caused substantial internal damage to the bearings and surrounding area, which "looked like it had been in the great train wreck," Gurrisi said.
• Lubrication: The primary lubrication was changed from an oil to a grease that more readily sticks to the parts in contact.
• Maneuver rates: Attitude control maneuvers using the gyros were normally conducted at 3 deg. per sec. Now sometimes those maneuvers are conducted at about 2 deg. per sec. to reduce wear and tear on the CMG.

"We also changed the materials in the preload mechanism to enhance its life and to prevent the preload from exerting force where it is should not have been," said Gurrisi.

Additional coverage for subscribers:
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VIDEO: CBS, NASHVILLE TV AND FOX RADIO INTERVIEWS PLAY

VIDEO: THE FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: THURSDAY'S MISSION STATUS BRIEFING PLAY
VIDEO: SPAEWALKERS HAMMER STUCK BRACE ON TRUSS PLAY
VIDEO: MAINTENANCE WORK ON JAPANESE ROBOT ARM PLAY
VIDEO: ENJOYING THE VIEW OF SHUTTLE AND STATION PLAY
VIDEO: REFURBISHED S-BAND ANTENNA INSTALLED PLAY
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VIDEO: SPACEWALK NO. 1 GETS UNDERWAY PLAY
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VIDEO: THURSDAY MORNING'S FLIGHT DIRECTOR INTERVIEW PLAY

VIDEO: AERIAL LAUNCH AND SRB SPLASHDOWN FOOTAGE PLAY

VIDEO: THE FLIGHT DAY 3 HIGHLIGHTS MOVIE PLAY
VIDEO: WEDNESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY
VIDEO: SPARE PARTS CARRIER INSTALLED ONTO STATION PLAY
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VIDEO: SHUTTLE CREW FLOATS INTO STATION PLAY
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VIDEO: SPACE SHUTTLE PERFORMS 360-DEGREE BACKFLIP PLAY
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VIDEO: SUMMARY OF THE SPARE PARTS PALLETS PLAY
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VIDEO: NARRATED PREVIEW OF RENDEZVOUS AND DOCKING PLAY

VIDEO: THE FLIGHT DAY 2 HIGHLIGHTS MOVIE PLAY
VIDEO: TUESDAY AFTERNOON'S MANAGEMENT TEAM UPDATE PLAY
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VIDEO: LAUNCH REPLAYS: EXTERNAL TANK CAMERA PLAY
VIDEO: POST-LAUNCH PRESS CONFERENCE PLAY

VIDEO: CREW FINISHES GETTING SUITED UP PLAY | HI-DEF
VIDEO: ASTRONAUTS LEAVE CREW QUARTERS PLAY | HI-DEF
VIDEO: LIFTOFF OF SPACE SHUTTLE ATLANTIS! PLAY | HI-DEF

VIDEO: LAUNCH REPLAY: PRESS SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD PERIMETER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: BEACH TRACKER PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: PAD CAEMRA 070 PLAY | HI-DEF
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VIDEO: LAUNCH REPLAY: WEST TOWER SITE PLAY | HI-DEF
VIDEO: LAUNCH REPLAY: UCS-23 TRACKER PLAY | HI-DEF
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VIDEO: LAUNCH REPLAY: PAD FRONT CAMERA PLAY | HI-DEF
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VIDEO: NARRATED REVIEW OF SHUTTLE'S PREPARATIONS HI-DEF
VIDEO: NARRATED REVIEW OF PAYLOADS' PREPARATIONS HI-DEF

VIDEO: STUNNING SUNSET ROLLBACK OF PAD GANTRY PLAY | HI-DEF
VIDEO: A LOOK AT SPACE STATION SCIENCE RESEARCH PLAY
VIDEO: THE STS-129 PRE-LAUNCH PRESS CONFERENCE PLAY
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VIDEO: PAYLOADS DELIVERED TO PAD 39A PLAY | HI-DEF
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VIDEO: FLIGHT READINESS REVIEW SETS LAUNCH DATE PLAY

VIDEO: CREW BRIEFED ON EMERGENCY PROCEDURES PLAY | HI-DEF
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VIDEO: SHUTTLE AND STATION PROGRAM UPDATE PLAY
VIDEO: THE STS-129 MISSION OVERVIEW BRIEFING PLAY
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VIDEO: THE ASTRONAUTS' PRE-FLIGHT NEWS BRIEFING PLAY

VIDEO: SPACE SHUTTLE ROLLOUT IN FAST-FORWARD PLAY | HI-DEF
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VIDEO: CLOSING ATLANTIS' PAYLOAD BAY DOORS PLAY | HI-DEF
VIDEO: FUEL TANK ATTACHED TO SOLID ROCKETS PLAY | HI-DEF
VIDEO: ASTRONAUTS MAKE A VISIT TO THE CAPE PLAY | HI-DEF
VIDEO: ORBITER DOCKING SYSTEM CHECKED OUT PLAY | HI-DEF
VIDEO: ATLANTIS' FUEL TANK UNLOADED FROM BARGE PLAY | HI-DEF
VIDEO: EXTERNAL TANK ARRIVES AT THE LAUNCH SITE PLAY | HI-DEF
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