Gimbal lock

Factual Questions
1

Can someone explain what this means, exactly?

I’m watching Apollo 13 right now, and the discussion of gimbal lock comes up right after the stirring of the oxygen tanks caused the explosion that altered the mission from a moon landing to a survival mission.

I’ve read about it on such things as wikipedia, among other googled links, and I still don’t have a feel for what it is.

I know it’s bad. But “locked” indicates (to me at least) that it is irreversible. Is this necessarily so? And if so, why is this?

I hope someone can explain this. I’ve wondered about this for a while… so I’d love to understand what it is.

I did a search, and I didn’t see any thread asking about gimbal lock.

thanks

2

Think of a gimbal as a universal joint, if you’ve ever played with one of those. (For example, with a socket wrench.) Within a certain range, you can move one of the shafts up, down, left, right, and in a smooth circle, ellipse, or wherever. But if you bend the U-joint too much, then its structure will hit itself as you move it. Bend it too far, and you only have one hinge available and can only move that hinge until you reduce the angle on the other one. It’s not permanently ‘locked’, but it has reached a limit whereby full freedom of movement can no longer be achieved.

In a gimbaled rocket motor, the bell can be moved in a circle – unless you turn it too far. Then you get gimbal lock and lose freedom of movement. AIUI (and I should know this for sure, since I grew up with Apollo – but memory fades) the spacecraft was trying to compensate for its rotation but the gimbal didn’t have enough range of movement.

Anyway, that’s my quick-and-dirty-from-memory explanation. Stranger on a Train is the expert, and I hope he will drop by the thread.
.

3

The gimbals of concern are not on the motor, but on the Inertial Measurement Platform, which measures where the ship is pointing.

If you got to the point that two of the gimbals locked, you would lose the ability to measure accurately where you are pointing.

Here’s a diagram
and here’s a NASA document

4

Thanks for that.

5

Normally, a gimbal lock wouldn’t have been that big a deal on an Apollo mission. More of a nuisance. They would have had to re-align the system by taking star sightings.

Trouble was, on Apollo 13 they couldn’t do that because of all the debris floating around after the explosion. So it became important that they not allow it to happen.