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The brain of the Saturn V and Saturn IB booster rockets used in the Apollo project, providing navigation, control and telemetery. It operated independantly of the Apollo Guidance Computer onboard the command module, which is why the Apollo 12 mission was able to reach orbit successfuly, despite being hit with a bolt of lighting at launch that temporarily knocked out many of the command module's electrical systems.

The IU had to fulfill multiple roles. The most basic of these was structural - the IU was a load bearing component of the Saturn boosters. The Saturn IB and Saturn V IU was a ring 1 meter high and 6.6 meters in diameter. It sat on top of the S-IVB booster stage below the adapter that housed the Lunar Module and supported the Commmand and Service modules. The various IU components were mounted around the inside of the ring in sealed and cooled boxes.

The IU's next role was navigation. It used an inertial guidance system that was a direct descendant of the V-2 missile's gyroscope based guidance system, via the redstone and jupiter rockets developed by Wernher von Braun's team at Huntsville, Alabama. The heart of the inertial guidance system was the ST-124, which used three nested gyroscopes to provide a fixed frame of reference. The ST-124 was a large device, massing 53 kilograms and had a diameter of 53 centimeters.

The IU was also responsible for receiving commands from the ground and sending back telemetery of the boosters' performance in flight. This was a vital role during the many test flights of the Saturn boosters; if good data was not returned, a multi-million dollar flight would be wasted.

The IU also assisted in the checkout process before launch and controlled flight after liftoff. A digital computer oversaw the operations of the IU, but an analogue computer was used to to produce the actual flight control commands.

The IU was initially developed at the Marshall Space Flight Center (MSFC), but IBM took over the role of manafacturer in February, 1964. The IU was assembled and tested in clean room conditions at a purpose built facility at the MSFC. However, on one occasion, with deadlines looming, the IU was completed in a make shift clean room onboard NASA's transport barge to the Kennedy Space Center in Florida.

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