Newsgroups: sci.space.shuttle
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: What is "gimble lock" in Apollo 13
Date: Sat, 30 Dec 1995 18:09:53 GMT

(Incidentally, posting to "sci.space,sci.space.shuttle" is kind of silly --
sci.space no longer exists.  I've taken it out of the Newsgroups line.)

In article <30E472AC.6D67@ratenet.com> bberry@ratenet.com writes:
>It's obviously a bad thing.  Can someone give me an explanation?

It's hard to do in a few words...

Old-style inertial-guidance systems used a "stable platform", held in a
fixed orientation in space by gyros.  To do this, the connection between
platform and spacecraft needs to be able to rotate in several directions
at once.  This is done by putting several gimbals in between platform and
spacecraft, each one a ring which rotates on pivots supported by the
next-biggest ring and in turn supports the pivots for the next-smallest
ring.

Three gimbals is the minimum for free movement in any direction, since
there are three axes on which rotation can occur.  However, if the gimbals
are moved suitably, you can line up the innermost pivots with the
outermost pivots... in which case those two can only handle one axis, and
so there is an axis which neither those two nor the third can handle.  If
there is then a rotation around that axis, the result is loss of the
platform's stable orientation.  That's gimbal lock. 

Adding a fourth gimbal gives enough extra freedom of motion that the
problem goes away.  However, each gimbal also adds complexity, bulk, and
weight.  For Apollo, the decision was to use only three gimbals and rely
on the astronauts to avoid sequences of rotations that would produce
gimbal lock.  This was normally not a problem, but became a troublesome
complication when Apollo 13 had attitude-control difficulties after the
tank rupture.
-- 
Look, look, see Windows 95.  Buy, lemmings, buy!   |       Henry Spencer
Pay no attention to that cliff ahead...            |   henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: Apollo 13 question
Date: Tue, 23 Jul 1996 22:44:53 GMT

In article <4t01uk$coa@clarknet.clark.net> prb@clark.net (Pat) writes:
>>>...something about the Shuttle's gyros having an extra degree of freedom
>>>to get around this problem.
>>So did Gemini's, in fact.  The designers of the Apollo navigation system
>>took a calculated risk, using one less gimbal to produce a system that was
>>lighter and smaller but vulnerable to gimbal lock if the crew wasn't
>>careful...
>
>I'd have thought the Gemini Flight (Armstrong?/Scott) wheere a
>stuck thruster spun the vehicle out would have shown the problem
>to be real.

Gemini 8 (Armstrong&Scott) was in 1966, at which time the Apollo system's
basic design was long since frozen.  Draper and his crew at MIT, the
designers of the Apollo system, were not stupid -- they knew there was
some risk, but saw it as a necessary compromise.
-- 
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |   henry@zoo.toronto.edu