Over 400,000 people worked on the Apollo project.
As we near the 50th anniversary of the Apollo 11 mission, I thought
it fitting to post about one of my many favourite pieces of engineering,
the Saturn V ST-124 inertial guidance platform.
The Saturn V Instrumentation Unit or "IU" was a ring that stood atop
the third stage (picture 1 below).
The IU's role was to provide guidance and control to the Saturn V from liftoff
up to the point of Trans-Lunar Injection (TLI) when it and the third stage
were then discarded after the Command Service Module docked with and
extracted the Lunar Module from the Lunar Module Adapter.
From that time onward, guidance and navigation would be performed by
the Apollo Guidance Computers (AGC) on-board both the Command Module
and Lunar Module.
The IU housed a digital computer manufactured by IBM and an inertial
platform designated ST-124, designed at the Marshall Space Flight Center
and manufactured by Bendix.
"ST" stood for "Stable Table" and this remarkable piece of engineering
included three gyros, three accelerometers, two pendulums and two
prisms.
Almost frictionless nitrogen gas bearings were deployed and a spherical
cylinder supplied the nitrogen at the requisite 15psi.
Much of the ST-124 was machined from anodized beryllium which is light
but stiff.
By contrast, the rotors of the gyroscopes were fabricated out of a
copper–tungsten alloy known as Elkonite, which is very dense and strong.
One of the most remarkable attributes of the ST-124 was its ability
to be calibrated seconds prior to launch.
There was a small window in the side of the Saturn V IU.
Whilst on the launchpad, there was a theodolite on the ground some 213
metres away from the base of the vehicle. The theodolite had an
instrument that could send a modulated infrared beam that was
directed through that window to the prisms within the ST-124.
Commands from a ground based system were transmitted to the ST-124
in a way analogous to the infrared remote control of a television set.
Picture if you will the spinning gyroscopes within the ST-124. Their
rotational inertia maintains their fixed orientation to space.
But whilst the countdown to launch is proceeding the Earth is rotating
around its own axis and it is orbiting the Sun.
At the instant prior to launch, the infrared beams of light would
essentially be communicating to the ST-124, "this is where we are now,
this is where we are now ...".
Specifically the theodolite would send commands to the ST-124's
azimuth alignment subsystem.
That way when the Saturn V launched, the ST-124 would "know" the
correct starting orientation taking into account the Earth's rotations.
During the Apollo 11 launch, you hear Jack King say, "T minus 15 seconds,
guidance is internal".
What he is referring to is the instant the ground based theodolite has
transmitted its final commands via beams of light through the little
window in the side of the Saturn V to the ST-124 and that the ST-124
is now fully in-charge for knowing "where it is" from now on.
The third stage and IU's of the first four Apollo lunar missions
(8, 10, 11 and 12) were placed in a heliocentric orbit, while those of
subsequent missions (13 on) were targeted at the moon.
So out there, orbiting for who knows how long, are at least four ST-124's.
"A GENERAL DESCRIPTION OF THE ST124-M INERTIAL PLATFORM SYSTEM"
by Herman E, Thomason, George C. Marshall space Flight Center.
Huntsville, Alabama, NASA document, Sept 1965 (pdf) :-
https://ntrs.nasa.gov/archive/nasa/c...9650024833.pdf
Jack King Apollo 11 countdown, "Guidance is internal" :-
https://www.youtube.com/watch?v=o4OBKOlgmfo
Pictures :-
1) Saturn V Instrumentation Unit (IU) ring
2) Technician with uncovered ST-124
3) An ST-124 with its beryllium casing
4) An uncovered ST-124