Instrument Unit description

Atop the S-IVB stage, a ring 1 metre high and the same diameter as the stage, called the IU (Instrument Unit), carries the guidance and control electronics for the Saturn V. Particularly important among these are the LVDC (Launch Vehicle Digital Computer) and the inertial platform, the ST-124, which has to be aligned to the intended trajectory before launch. The angle between north and the groundtrack of the vehicle's flight path is called the flight azimuth. For this launch, it is to be 80.088° or slightly north of east. The gyroscopically stabilised platform at the heart of the ST-124 is aligned with the intended azimuth just prior to launch by rotating the X stable member with reference to a theodolite mounted some distance away from the launch pad. There is a small window in the skin of the Instrument Unit for this purpose. (ap15fj)