(marcus hall) writes:

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.

I recall that in Michael Collins book _Carrying the Fire_, he mentions
this as being the case (he flew on Gemini 10 and Apollo 11), and related
that when he was Command Mondule Pilot on Apollo 11 he mentioned over
the comm link that he would like another gyro. He also hypothesized
that North American, having won the contract for Apollo 11 after
McDonnell Douglas had built both Mercury and Gemini, didn't seem to want
to use any "lessons learned" from the earlier programs.