In article ,
wrote:

A long time ago I read that the US. Gemini manned spacecraft used an
inertial guidance system that had 4-axis gimbals. Presumably this was to
help prevent gimbal lock. Is this true?

I have been seeing a lot of historical information (on internet, film,
TV, etc.) that suggest the US manned Apollo program used 3-axis gimbal
system, thus all the references to potential gimbal lock. Perhaps even a
reference to that in the movie Apollo 13.

Is it true that earlier Gemini flights had 4-axis but the Apollo was
built with 3-axis, presumably to reduce weight, size, cost, and
complexity?

I have no references to back it up, but my understanding is that the
Gemini system had 4-axis gimbals as you said to prevent gimbal lock.
Aparently the Apollo system was already designed and committed before
the Gemini system came along, so it stayed with a 3-axis system.

marcus hall