ELi5: What is gimbal lock and why is it bad? (Been watching Apollo 13)
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Mechanical gyroscopes (such as the one found in the Inertial Measurement Unit of the Apollo guidance computer) consist of three gimbals (x, y and z).
Gimbal lock occurs when two of these end up in the same plane (ie parallel to one another). When this occurs, you can only now measure in two axes, rather than three. Not great when you’re trying to navigate round the 3-D void of space with few landmarks to give you an accurate position fix.
It used to be an issue in fighter aircraft too, before the introduction of laser ring gyros, as extreme nose up or nose down attitudes could cause it.
When you’re walking around or driving around it’s easy to know where you are because you have eyes and can see things like street signs and landmarks. When you’re in something like a submarine or a spaceship, it’s much harder. So how do you ever know where you are if you can’t look out a window?
You use a series of rings called gimbals which spin around and they can basically tell you which direct you’re pointing in. If you combine that knowledge with something like how hard your engines are firing, or elapsed time, you can do math to figure out where you must be from a starting point. Gimbals like this are also called a gyroscope (each gimbal is one ring, 3 rings make a gyroscope).
In the case of the Apollo missions they ran into issues with the gimbals they sent up on the space ships where they “locked”, for whatever reason they stopped accurately reporting what direct the ship was facing. This is obviously suuuuper bad because the tiniest miscalculation meant the spaceship would likely just fly off into space forever or crash into the Earth or Moon. So they were forced to use a sextant and plot their position via the stars just like old timey sailors.
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.
On the spacecraft, they were using a gyroscope/gyrocompass to help navigate. Gyrocompasses are good for navigation and other things because they remain [rigid in space when spinning.](https://www.youtube.com/watch?v=xQb-N486mA4&ab_channel=EuropeanSpaceAgency%2CESA). So, if you get a gyroscope spinning, you use to keep track of your positioning since it stays in place. You tell what is relatively up and down if you align your gyro in that way. This is important as astronaut may need this info to properly line themselves up a course between points in space.
Gimbal lock is when you apply a force to a gyro that causes the rotating axies of the gyro (also known as the gimbals) to line up and “lock up” with eachother. When this happens the gyro no longer maintains its rigidity and becomes useless. [In this video, you can see that the gyro does like a full flip when the rings ling up.](https://youtu.be/oj7v3MXJL3M?t=106). Imagine trying to navigate, and your instrument literarily flips upside down.
In Apollo 13, the spacecraft was shimmying violently in all direction after the explosion. This movement applied force on the gyro. If enough force was applied, the rings of the gyro would align, they would lock up, and gimbal lock occurs. This ruin the gyro’s ability to give the astronauts directional data. With gimbal lock, they wouldn’t be able to determine what is up, down, sideways, etc.
[This video explains it a bit more](https://www.youtube.com/watch?v=OmCzZ-D8Wdk&ab_channel=TheVintageSpace)
A gimbals that whirly metal round thing that you often see in labs on space related movies.
I don’t know anything regarding spacecrafts use of them, but they are used in lots of everyday equipment too, like stage lights that rotate in multiple different ways, which I know a lot more about.
In that context, ‘gimbal lock’ is when the light refuses to move out of a certain position, and therefore the light becomes next to useless unless the gimbal can be replaced.
I imagine on a spacecraft, gimbal lock could be a lot more serious than a lightshow fucking up.
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