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Part 8: Fixing the space station's gyro system
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 11, 2005

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
   
   07/18/05
   Mon  04:51 AM...04...13...00...STS crew wakeup
   Mon  05:21 AM...04...13...30...ISS crew wakeup
   Mon  05:46 AM...04...13...55...Kelly exercises
   Mon  06:16 AM...04...14...25...Robinson exercises
   Mon  06:46 AM...04...14...55...Noguchi exercises
   Mon  07:01 AM...04...15...10...ISS daily planning conference
   Mon  07:21 AM...04...15...30...Transfer review
   Mon  07:51 AM...04...16...00...EVA tool cleanup and stow
   Mon  07:51 AM...04...16...00...Transfer operations
   Mon  07:51 AM...04...16...00...Lithium hydroxide exchange
   Mon  07:51 AM...04...16...00...Camarda exercises
   Mon  08:51 AM...04...17...00...Middeck preps
   Mon  09:06 AM...04...17...15...Lawrence exercises
   Mon  09:21 AM...04...17...30...EVA tools config
   Mon  10:21 AM...04...18...30...Collins exercises
   Mon  10:51 AM...04...19...00...EVA pistol grip tool setup
   Mon  10:51 AM...04...19...00...ISS: Krikalev exercises
   Mon  10:51 AM...04...19...00...ISS: Phillips exercises
   Mon  12:01 PM...04...20...10...PAO event (CDR, PLT, MS5)
   Mon  12:21 PM...04...20...30...Joint ISS/STS  meal
   Mon  01:21 PM...04...21...30...Transfer operations
   Mon  01:56 PM...04...22...05...PAO event (CDR, MS1, MS2, ISS: Krikalev)
   Mon  02:36 PM...04...22...45...ISS: Krikalev exercises
   Mon  03:21 PM...04...23...30...Crew choice television
   Mon  03:51 PM...05...00...00...EVA-2: Procedures review
   Mon  04:46 PM...05...00...55...Transfer review
   Mon  04:46 PM...05...00...55...ISS: Phillips exercises
   Mon  05:21 PM...05...01...30...ISS egress
   Mon  05:46 PM...05...01...55...10.2 psi cabin depressurization
   Mon  05:51 PM...05...02...00...Thomas exercises
   Mon  06:06 PM...05...02...15...ISS daily planning conference
   Mon  08:21 PM...05...04...30...STS/ISS crew sleep begins
The space station uses four massive control moment gyroscopes to maintain the lab's orientation in space without having to tap into limited supplies of on-board rocket fuel. They are housed in the Z1 truss, which was attached to the Unity module's upward-facing, or zenith hatch - hence the name - during shuttle mission STS-92 in October 2000.

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 the first spacewalk, Robinson and Noguchi plan to wire around the faulty breaker to restore CMG-2 to service.

While the overall system remains fully operational, 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
   
   07/19/05
   Tue  04:21 AM...05...12...30...STS crew wakeup
   Tue  04:51 AM...05...13...00...ISS crew wakeup
   Tue  06:21 AM...05...14...30...EVA-2: Preps begin
   Tue  06:51 AM...05...15...00...ISS daily planning conference
   Tue  07:21 AM...05...15...30...Collins exercises
   Tue  07:26 AM...05...15...35...ISS: Phillips exercises
   Tue  07:51 AM...05...16...00...Transfer review
   Tue  07:51 AM...05...16...00...Kelly exercises
   Tue  07:51 AM...05...16...00...EVA-2: EMU pre-breathe
   Tue  08:21 AM...05...16...30...Camarda exercises
   Tue  08:21 AM...05...16...30...ISS: Krikalev exercises
   Tue  08:26 AM...05...16...35...ISS: Phillips exercises
   Tue  08:51 AM...05...17...00...Lawrence exercises
   Tue  09:16 AM...05...17...25...EVA-2: Airlock depress
   Tue  09:26 AM...05...17...35...EVA-2: Airlock egress and setup
   Tue  09:26 AM...05...17...35...14.7 psi cabin repressurization
   Tue  09:51 AM...05...18...00...ISS ingress
   Tue  09:51 AM...05...18...00...Transfer operations
   Tue  10:36 AM...05...18...45...EVA-2: CMG removal and replacement
   Tue  11:31 AM...05...19...40...Crew meals begin
   Tue  12:36 PM...05...20...45...Transfer operations resume
   Tue  02:56 PM...05...23...05...EVA-2: Cleanup and ingress
   Tue  03:06 PM...05...23...15...STS crew leaves ISS
   Tue  03:56 PM...06...00...05...EVA-2: Airlock repress
   Tue  04:21 PM...06...00...30...ISS: Krikalev exercises
   Tue  06:06 PM...06...02...15...ISS daily planning conference
   Tue  06:36 PM...06...02...45...Transfer tagup
   Tue  08:21 PM...06...04...30...STS/ISS crew sleep begins
The replacement CMG will be mounted on a carrier truss at the back of Discovery's cargo bay. Robinson and Noguchi first will float up to the Z1 truss, unfasten thermal blankets, disconnect electrical cables and remove CMG-1. They will maneuver it to a temporary stowage location and lock it in place. Then, using the SSRMS, the replacement CMG will be removed from the cargo bay truss and moved 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 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.

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