The Discovery astronauts will spend the day after the first spacewalk transferring more supplies and equipment to the station from the logistics module. Robinson and Noguchi will service their spacesuits and prepare the tools that will be needed for the second spacewalk to install the replacement control moment gyroscope. Two televised crew interviews are planned as the astronauts gear up for the critical station repair work.

Flight Day 6 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/19/05
   Thu  04:11 AM...04...12...00...STS crew wakeup
   Thu  04:41 AM...04...12...30...ISS crew wakeup
   Thu  07:11 AM...04...15...00...Spacesuit servicing
   Thu  07:11 AM...04...15...00...Transfer operations resume
   Thu  07:41 AM...04...15...30...TPS/EVA tool transfer
   Thu  10:41 AM...04...18...30...EVA tools configured for use
   Thu  10:51 AM...04...18...40...Public Affairs event with crew
   Thu  12:11 PM...04...20...00...Joint ISS/STS  meal
   Thu  03:36 PM...04...23...25...Public Affairs event with crew
   Thu  03:56 PM...04...23...45...EVA-2: Procedures review
   Thu  08:11 PM...05...04...00...Crew sleep begins

This illustration shows the spacewalkers installing the new CMG. Credit: NASA TV

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 was knocked off line last year because of trouble with a circuit breaker. But that problem was fixed during a station-based spacewalk and the gyro was returned to service.

While the overall system remains fully operational with three working gyros, NASA wants to replace CMG-1 as soon as possible to provide additional redundancy in case of subsequent failures down the road.

Flight Day 7 highlights:

   DAY..EDT........DD...HH...MM...EVENT
   
   05/20/05
   Fri  04:11 AM...05...12...00...STS crew wakeup
   Fri  04:41 AM...05...12...30...ISS crew wakeup
   Fri  05:26 AM...05...13...15...EVA-2: Preparations begin
   Fri  07:11 AM...05...15...00...Transfer operations resume
   Fri  09:31 AM...05...17...20...EVA-2: Airlock depressurization
   Fri  10:16 AM...05...18...05...EVA-2: Airlock egress
   Fri  11:26 AM...05...19...15...EVA-2: CMG removal and replacement (4:15)
   Fri  03:41 PM...05...23...30...EVA-2: Cleanup and ingress
   Fri  04:36 PM...06...00...25...EVA-2: Airlock repress
   Fri  08:11 PM...06...04...00...Crew sleep begins

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 spacewalkers, meanwhile, will move the old gyro back to the cargo bay truss and lock it down for return to Earth. If all goes well, the new unit will be spun up while they are still in the cargo bay.

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 deg F.

PREVIEW REPORT PART 9 --->