Floating in the shuttle Discovery's airlock, astronauts Stephen Robinson and Soichi Noguchi switched their spacesuits to internal battery power at 4:42 a.m. to officially begin a high-priority spacewalk to replace one of the international space station's gyroscopes.

"All right! Let's get started," said Robinson as the two headed out into the shuttle's cargo bay.

This is the 60th spacewalk devoted to space station assembly and maintenance by 40 U.S. astronauts, 10 Russian cosmonauts, one Frenchman, one Canadian and one Japanese (Noguchi). Going into today's excursion, station spacewalk time totaled 355 hours and five minutes.

Here is an approximate timeline of today's activities, zeroed to the official start time of the spacewalk:

• 04:42 a.m...Egress and setup; control moment gyroscope No. 1 (CMG-1) thermal shroud removal; robot arm foot restraint setup
  
  
• 05:52 a.m...Remove CMG-1: Demate connectors; loosen bolts; unbolt CMG-1
  
  
• 06:32 a.m...Remove new gyro: Noguchi, on the station's robot arm, is moved back down to the shuttle's cargo bay to temp stow CMG-1; the new gyro is unbolted and temp stowed to make room for CMG-1, which will be returned to Earth
  
  
• 08:02 a.m...CMG-1 is mounted in Discovery's cargo carrier; Noguchi, on the station arm, carries the new CMG up to the Z1 truss where it will be installed; Robinson cleans up cargo bay work site and joins Noguchi at the Z1 truss
  
  
• 09:02 a.m...Install new CMG: remate electrical connectors; re-attach thermal shroud
  
  
• 10:12 a.m...Cleanup and airlock ingress

Along with saving fuel, the 800-pound gyros, spinning at 6,600 rpm, allow station crews and flight controllers to reorient the outpost and keep it stable without using rocket firings that would jar sensitive microgravity experiments.

But on June 8, 2002, CMG-1 suffered a malfunction and shut down. Station astronaut Carl Walz reported hearing an unusual noise inside the Unity module. He said the noise appeared to be coming from the module's zenith area. Mission control then told Walz engineers were working an issue with a spin bearing in CMG No. 1. Walz said the noise was quite noticeable inside the module.

"We're hearing a pretty loud, audible noise, kind of a growling noise, from inside the node," Walz reported.

"It looks like we have a mechanical failure of the spin bearings on CMG-1," an astronaut in mission control replied. "It's currently spinning down right now. The growling noise is undoubtedly due to vibration."

The station's orientation, or attitude, can be controlled by just two CMGs in a worst-case scenario. And indeed, a second gyro, CMG-2, was knocked off line last year because of trouble with a circuit breaker. The circuit breaker was replaced during a station-based spacewalk, but the new unit malfunctioned in March, taking CMG-2 off line once again. During their first spacewalk Saturday, Robinson and Noguchi wired around the faulty breaker to restore CMG-2 to service.

The replacement CMG was carried into orbit on a support structure at the back of Discovery's cargo bay. Robinson and Noguchi, on the end of the station's robot arm, first will float up to the Z1 truss, unfasten thermal blankets, disconnect electrical cables and remove CMG-1 by loosening six bolts. They will carry it back down into the cargo bay and mount it on a temporary support fitting called a ball stack. The new CMG then will be unbolted and mounted on a different ball stack while CMG-1 is locked into the payload bay support carrier for return to Earth.

At that point, Wendy Lawrence and James Kelly, operating the space station's robot arm, will move Noguchi and the new CMG up to the Z1 truss for installation. After Robinson and Noguchi complete electrical connections and re-fasten the thermal blankets, engineers in mission control will begin preparations for spinning up the new gyro.

The gyroscopes are critical to station operation. Here's a description from a NASA press kit:

The motion control subsystem (MCS) hardware launched as part of the Z1 element includes the CMGs and the CMG assemblies.

The CMG assembly consists of four CMGs and a micrometeorite/orbital debris shield. The four CMGs, which will control the attitude of the ISS, have a spherical momentum storage capability of 14,000 ft-lb/sec, the scalar sum of the individual CMG wheel moments. The momentum stored in the CMG system at any given time equals the vector sum of the individual CMG momentum vectors.

To maintain the ISS in the desired attitude, the CMG system must cancel, or absorb, the momentum generated by the disturbance torques acting on the station. If the average disturbance torque is nonzero, the resulting CMG output torque is also nonzero, and momentum builds up in the CMG system. When the CMG system saturates, it is unable to generate the torque required to cancel the disturbance torque, which results in the loss of attitude control.

The CMG system saturates when momentum vectors have become parallel and only momentum vectors change. When this happens, control torques perpendicular to this parallel line are possible, and controllability about the parallel line is lost.

Russian segment thrusters are used to desaturate the CMGs.

An ISS CMG consists of a large flat wheel that rotates at a constant speed (6,600 rpm) and develops an angular momentum of 3,500 ft-lb/sec about its spin axis. This rotating wheel is mounted in a two-degree-of-freedom gimbal system that can point the spin axis (momentum vector) of the wheel in any direction.

At least two CMGs are needed to provide attitude control. The CMG generates an output reaction torque that is applied to the ISS by inertially changing the direction of its wheel momentum. The CMG's output torque has two components, one proportional to the rate of change of the CMG gimbals and a second proportional to the inertial body rate of the ISS as sensed at the CMG base. Because the momentum along the direction of the spin axis is fixed, the output torque is constrained to lie in the plane of the wheel. That is why one CMG cannot provide the three-axis torque needed to control the attitude of the ISS.

Each CMG has a thermostatically controlled survival heater to keep it within thermal limits before the CMGs are activated on Mission 5A. The heaters are rated at 120 watts and have an operating temperature range of -42 to -35 F.

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