Apollo 11 Communications Log- Gimbal Lock: Newsletter Challenge (July 2015)

This month's Challenge Question: Specs & Techs from IHS Engineering360:

CapCom - "Columbia, Houston. We noticed you are maneuvering very close to gimbal lock. We suggest you move back away. Over." Michael Collins (CMP) - "Yes. I am going around it...How about sending me a fourth gimbal for Christmas?"

Why did Michael Collins ask for a fourth gimbal for Christmas?

And the answer is:

The full communication log can be found here: http://apollo11.spacelog.org/page/04:08:40:46/
In the discussion above, mission control (CapCom) is communicating to Michael Collins who is piloting the Apollo Command/Service Module, that he is nearing gimbal lock. This is in reference to the Inertial Measurement Unit (IMU) which has Outer, Middle, and Inner gimbals (see diagram below). The outer gimbal is mounted on the navigation base which in turn is rigidly mounted to the spacecraft.

Michael Collins had flown in Gemini 10, which had a fourth or "redundant" gimbal. This redundant gimbal was mounted outside the normal outer gimbal. Normal operation would then be to use the inner three gimbals to drive the stabilizing gyro error signals to zero while the fourth gimbal could be used to keep the middle gimbal near zero and away from the gimbal lock orientation.

The general lesson is, rotations in 3D in space are tricky. That is why 3D simulators are usually based on quaternions rather than Euler Angles. Quaternions have a built-in extra degree of freedom that avoids gimbal lock. Essentially adding a fourth (redundant) gimbal does the same thing.