Redundancy in control

The designers of the Apollo spacecraft were always careful to build redundancy into their systems to ensure that a single point failure could not put the crew in jeopardy a philosophy that extended to the guidance and navigation system. The designers were very aware that any of its exotic components could fail at any point in the mission. To this end. the command module had a second control system which, although it shared many components with the G&N system, could operate entirely independently. This stabilisation and control system (SCS) could maintain attitude and allow’ the crewr to make accurate manoeuvres and, if necessary, even manually control the SPS engine. Like the G&N system, it used gyroscopes, but these w:ere arranged in a different way to the gyroscopieally stabilised IMIJ.

The gyroscopes for the SCS were not attached to a stabilised platform. Instead, they were fixed to the spacecraft structure and therefore had to move with it. Being mounted in this way prompted the name body-mounted attitude gyros (BMAGs). Like all gyros, they had a tendency to want to remain in one attitude, and when the spacecraft rotated, they exerted a force on their mountings. As this force was a measure of the rate of rotation, the BMAGs were very suitable for measuring how fast the spacecraft was rotating rather than yielding absolute attitude. However, by processing the rate information within electronic boxes it was possible to derive the absolute attitude. The resultant values for attitude were highly prone to drift, much more so than those from the IMIJ, so it was important to regularly realign them to match. Prior to intensive use of the SCS. the crew would press a button to update the BMAGs’ electronics with the spacecraft’s attitude from the IMlJ’s platform. If the IMU ever became unusable, the crew had an emergency procedure whereby the attitude information from the BMAGs could be aligned by sighting on the stars.