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

Posted July 01, 2015 12:00 AM
Pathfinder Tags: challenge questions NASA

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.

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#1

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

07/01/2015 12:05 AM

He wanted another dimension to use for guidance.

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#2

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

07/01/2015 7:35 AM

Collins' comment was a pun. The term 'gimbal' is a homonym for the defunct but once famous 'Gimbel' department store in New York, known for its famous Christmas displays.

'Gimbal lock' occurs when two of the three maneuvering rockets are fired in a way that they are working in parallel. Thus, a 3-axis system functions as only a 2-axis system.

Asking for a 'fourth gimbal' is like a kid getting his magic Christmas wish to own a 'Gimbel' department store - or in this case, to gain an extra dimension for maneuvering the spacecraft.

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#9
In reply to #2

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

07/07/2015 7:46 PM

Or a redundant system, in this case, gimbal, but he should have put an extra (2) gimbals so he would have complete set of (3) on his list to make the redundancy complete.

But hey, I'm just passing through.

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#3

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

07/01/2015 7:58 AM

He was making a wish list. Gemini had four gimbals. The extra gimbal for reference can eliminate gimbal lock. The design engineers argued over the need for the forth because of time constrants. Collins had been a part of the Gemini 10 crew. Had a bit of experience with the for gimbal system for positioning.

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#4

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

07/01/2015 11:42 AM

Gymbals should be used in pairs....

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#5

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

07/01/2015 1:14 PM

There is a good description of Gimbals on "How stuff works"

It includes a brief description of gimbal lock and how a fourth gimbal can be used to avoid lock.

Here's a full record of the comms.

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#6

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

07/02/2015 6:23 AM

Here is a good description of gimble lock. It basically occurs when rotation about 1 axis of rotation happens to make the other two axes of rotation coincide, effective leaving only one axis of rotation about which the object can then rotate.

https://www.youtube.com/watch?v=zc8b2Jo7mno

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#7
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Re: Apollo 11 Communications Log- Gimbal Lock: Newsletter Challenge (July 2015)

07/02/2015 6:56 AM

Great explanation, thanks!

Really well and very simply put!

Even I understood it far better than when I did my Gyro course in the '60s....but we did not have such great videos then.....

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#8

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

07/07/2015 11:09 AM

It was a joke.

An extra gimbal would create a "4th axis" to rotate on. Even if 2 axes were locked he would still have 2 working independently and could rotate out of the locked planes.

Would make a nice Christmas gift.

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#10

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

07/11/2015 4:06 PM

one for the man in the moon?

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#11

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

07/21/2015 9:06 PM

Here's an astronaut explaining it

https://www.youtube.com/watch?v=OmCzZ-D8Wdk

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