Q&A on Chandra's Safe Mode and Gyroscopes, Part II

(Q&A on Chandra's Safe Mode and Gyroscopes, Part I)

On October 21, Chandra returned to science observations, less than two weeks after a glitch in one of its four gyroscopes caused it to go into Safe Mode. To answer some of the questions surrounding Chandra’s gyroscopes, members of the Chandra X-ray Center and its Flight Operations Team have put together this Q&A.

1. How do Chandra’s gyros measure the speed at which the spacecraft is turning?

The Inertial Reference Units (IRU) used on Chandra are strapdown attitude rate sensing units. An IRU contains two gyros and each gyro provides two channels of rate information. The spinning disks in the gyros are used to sense external torques or rotations about their spin axes which are proportional to the rates at which the spacecraft may be turning about its own axes. For a primer on how gyros measure rate information, please see http://www.sensorwiki.org/doku.php/sensors/gyroscope or other comparable sources.

2. How do they help steady the spacecraft?

Chandra uses its gyros to provide movement information and six reaction wheels to hold the spacecraft steady and to maneuver (rotate to a new target). Each of six wheels is tuned to a set speed to keep Chandra pointing at a target or execute a maneuver. The required reaction wheel speeds are determined by the on-board computer using a feedback loop. The gyros are part of this feedback loop. They sense Chandra’s angular rate, that is how fast Chandra is rotating in each of three directions. That rate data is used to determine if the wheels need to speed up or slow down to produce the torques that keep Chandra steady or cause it to rotate to a new desired target. Any rotation caused by the wheels is sensed by the gyros and sent back to the on-board computer to adjust the wheel speeds again.

3. How fast do their wheels spin? Are they immersed in fluid?

The gyro wheels spin very fast (in excess of 10,000 rpm) to provide the sensitivity needed to measure very small disturbances and motion rates. The gyros are not immersed in fluid.

4. Are they the most fragile piece of equipment on Chandra?

All of the equipment on Chandra was designed to withstand the significant stresses of a shuttle launch and subsequent boost to our highly elliptical orbit, so to describe them as fragile is not accurate. The gyros on Chandra do have moving parts and are highly sensitive instruments, so they are subject to more “wear-and-tear” than most spacecraft equipment.

5. How much longer are they expected to work?

Life test data shows units have demonstrated over 25 years (219,000 hours) of on-orbit operations. Other space missions are using gyros similar to Chandra's, however, the Chandra mission now has the most on-orbit use time at this point for this specific model of IRU. At the time of the recent Safe Mode event, the IRU being used had been operating for more than 15 years.