ELi5: What is gimbal lock and why is it bad? (Been watching Apollo 13)

Question

ELi5: What is gimbal lock and why is it bad? (Been watching Apollo 13)

In: 1

Answer 1

The mission logs actually have a pretty good description of what it is: https://history.nasa.gov/afj/ap13fj/12day4-approach-moon.html

A gimbal allows two boxes (one inside and one outside the gimbal) to rotate separately in three dimensions using three axis of rotation.

I’m the case of Apollo, it was used, IIRC, as part of the inertial measurement system (basically something that measure acceleration).

If two axis of rotation align, then the gimbal has one axis it cannot rotate around anymore (the wikipedia article has a pretty good illustration: https://en.wikipedia.org/wiki/Gimbal_lock along that axis anymore. Which means, there is a direction in which we can’t measure acceleration anymore. So we don’t know where we’re going.