The No-Gimbal-Lock incident - Apollo 10

The mission objective of Apollo 10, may 1969, was to perform a stage rehearsal for Apollo 11, the first mission to land a man on the Moon. Apollo 10 would pave the way for Apollo 11, completing the full mission-plan, only excluding the actual landing-phase.

Apollo 10 was a complete success, but like the other Apollo-missions, it had its share of iffy incidents, where the mission success and indeed the lives of the astronauts were balanced on a knife-edge.

A well-known incident (see fx. the memoirs of Cernan[3], Stafford[4]) happened at 102 hours, 44 minutes and 49 seconds into the mission. For some 4.5 hours, Tom Stafford and Gene Cernan, had been flying the Lunar Module separated from the Command Module piloted by John Young.



Gene Cernan(L) and Tom Stafford(R) during a training session

By an erroneous flip of a switch (AGS to AUTO [4]), the astronauts lost control of their spacecraft. You don't wanna do that. The Lunar Module went into a very unexpected, wild and unplanned gyration. Now, ordinary people (like yours truly, I'm afraid!) would have panicked completely, facing this sudden, violent and obviously life-threatening circumstance. But commander Tom Stafford was made of another fabric and managed, through a fast and brilliant series of decisions, to regain control and stabilize the vessel back onto a controlled course, 36 seconds after the tumbling started. (Stafford did an emergency descend stage separation, with the idea to loose weight, thus gaining traction for the LM RCS, and then using the LM RCS to stabilize the spacecrafts motion).

The AGS switch that was accidentally flipped to AUTO, causing the incident.

This was an incredible feat, and once again proved that these “men of the right stuff” possessed the knowledge and cold-bloodedness to handle almost any situation. Gene Cernan concludes his account of the event with an understated but unambiguous expression of deep felt admiration for his former commander: “Old Mumbles do know how to fly.” [3].  Stafford’s nickname among his colleagues was “Old Mumbles”, a nickname their witty colleague Michael Collins (Apollo 11) had come up with while Stafford was an instructor in flight test training. At the time Stafford’s fellow astronauts, then his trainees, had trouble understanding his soft Oklahoma drawl.

Immediately after the emergency situation it was discovered, to great surprise, that the gyroscopes which constitute the navigation system’s mechanical sensors had NOT locked in during the wild maneuvers. A roll to either side of more than 85 degrees would have caused a so-called "Gimbal Lock" of the gyroscopes, leaving the navigation system useless.

Normally it was a constant balancing act to keep the gyroscopes away from the positions in which they would lock. Mission Control and the astronauts always kept a watchful eye on these highly sensitive mechanical marvels, that were so essential to navigation. If the gyroscopes had locked, the two astronauts would have been in imminent danger, alone in deep space with no known location, course or direction to the Command Module, their lift home.

What is not mentioned in the many various accounts of this incident, is the impression it made that they were so darn lucky. As the two astronauts passed the far side of the Moon outside radio contact with Earth they couldn’t contain their amazement at the event, and they agreed that it was God’s providence that had protected them.

26th of May 1969. The far side of the Moon. Out of radio contact with Earth: 

• 103:37:38 Cernan : “Am I glad we didn't torque up the platform.” 
• 103:37:41 Stafford: “How in the h*ll did we miss that? Still don't know.” 
• 103:37:49 Cernan: “We had to have someone on our side that time.”  
• 103:37:51 Stafford: “Yep.” 

Just how lucky they were is very difficult to actually estimate analytically: What is the probability that a pseudo-random tumbling for 36 seconds will not cause a rotation around the Z-Axis (roll) of more than 85 degrees to either side?

We do know that the roll reached somewhere between 75 and 85 degrees, because the gimbal lock warning light came on (which it did at 75 degrees), so we know it got very close indeed.

But obviously, the astronauts themselves, who knew the probabilities of the case by guts and experience, better than anyone, believed, that it was but by the grace of God, that they avoided the gimbal lock, which could well have cost them their lives. 

The two level-headed engineers and test-pilot astronauts believed they had experienced a miracle.

See also this 41-second YouTube-movie, where you can hear the actual audio recordings from the tiny, fragile spacecraft flying at the bleak, desolate back-side of the Moon:

Sources
[1]: The Apollo 10 Mission log and audio recordings, NASA archives, Apollo Flight Journals
[2]: Richard W. Orloff and David M. Harland, Apollo. The Definitive Sourcebook, ISBN: 9780387300436
[3]: Eugene Cernan and Don Davis, The Last Man on the Moon, ISBN:9780312263515
[4]: Tom Stafford, We Have Capture, ISBN: 1588341011