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Old 15-06-2019, 02:20 PM
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"Guidance is internal". The remarkable Saturn V ST-124 inertial platform

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

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.

by Herman E, Thomason, George C. Marshall space Flight Center.
Huntsville, Alabama, NASA document, Sept 1965 (pdf) :-

Jack King Apollo 11 countdown, "Guidance is internal" :-

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
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Click for full-size image (ST-124_1.jpg)
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Click for full-size image (ST-124_3.jpg)
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Click for full-size image (ST-124_uncovered_.jpg)
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Old 15-06-2019, 03:08 PM
Startrek (Martin)
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Brilliantly written and documented Gary
I visited the KSC last year for 3 days and saw a cut a way section of an IU and a mock up of the Apollo inertial guidance system to scale
I have an electrical engineering background and I was astounded by the simplicity of its design and basic electronics ( mostly electromechanical components) This little “box and ball” precisely guided that enormous Saturn 5 rocket with 7.5 million pounds of thrust from the launch pad into low earth orbit and again towards the moon after TLI
You can’t beat old technology , big , ugly , robust , slow , but it damn worked and it put man on the moon !!
Great post, well done !!
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Old 15-06-2019, 03:44 PM
Startrek (Martin)
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Internal guidance was the brainchild of Charles ( Doc) Draper who came up with idea of using gyros and accelerometers to determine the exact location and position relative to the ground of a moving object in flight in the late 40’s and early 1950’s
He designed and installed the first usable inertial guidance system in a old B52 aircraft which flew from Massachusetts to Los Angeles in 1953 without a pilot in control to navigate. The aircraft navigated itself via the IGS perfectly across the US which paved the way for further development in the newly formed Draper Lab and ultimately with Apollo to the Moon

A bit of history about the inertial guidance system
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Old 15-06-2019, 07:40 PM
Dazzled by the Cosmos.

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I just love reading about this stuff Gary.


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Old 15-06-2019, 07:55 PM
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RobF (Rob)
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Yep, thanks for sharing....
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Old 16-06-2019, 08:41 AM
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multiweb (Marc)
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Great post Gary.
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Old 16-06-2019, 06:29 PM
Multiverse (Grant)
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Great post - thanks Gary.

After the SaturnV IU had completed its job, the Apollo Guidance Computer (AGC) that you mention took over the inertial guidance duties, it also has a fascinating development story. It interfaced with a gyroscopic Inertial Measurement Unit, the AGC included hardware utilising a series of newly invented I.C's (with an unbelievable six transistors on each IC!), non-erasable 'software' (which was actually hardware) installed from 'woven' çore-rope memory & a cumbersome but robust looking new development - a display/keyboard unit. All these components brought together from Doc Draper. Fascinating reading in the link below:

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