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IU-514 at Udvar-Hazy Center |
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IU-514 at Udvar-Hazy Center |
The Saturn V Rocket, which carried astronauts to theMoon, used inertial guidance. The booster had its own inertial system, separatefrom the inertial system for the Command, Service, and Lunar Modules. This wascontained in an "Instrument Unit" (IU): a large, thin ring, 1 meterhigh by 6.7 meters in diameter, located between the S-IVB stage of the Saturnrocket and the Service Module. Around this ring were located the basiccomponents of the guidance system: a stable platform, accelerometers, a digitalcomputer, and control electronics to send signals to the rocket enginesthemselves. The ring also contained radar, telemetry, and other units.
The Instrument Unit was developed at NASA's MarshallSpace Flight Center, formerly the Army's Redstone Arsenal, at Huntsville,Alabama. That Unit's stable platform was based on designs that went back to theGerman V-2 of the Second World War (many of the Marshall engineers had workedon the V-2). The Bendix Corporation produced the stable platform, while theFederal Systems Division of the IBM Corporation developed and built the digitalcomputer and assumed the role of prime contractor for the entire InstrumentUnit.
The Instrument Unit performed multiple duties. Beforelaunch it performed check-out of the vehicle. Beginning a few seconds beforelaunch, it began a pre-programmed course to control the 7.5 million pounds ofthrust that lifted the giant rocket--and its three human passengers--off theground. It then controlled stage separation, ignition, and active guidance ofthe next two stages; then a restart of the third stage that set it on a courseto the Moon; and it held the third stage steady during transposition anddocking of the Command-Service Modules (CSM) with the Lunar Module. Its finalact was to guide the spent third stage away from that of the CSM as it coastedto the Moon.
During the countdown, the platform was optically alignedby a theodolite located some distance away from the launch pad, using a beam oflight directed through a window. Controllers on the ground could interact withthe computer up to a few seconds before launch, but after that it operatedautonomously. Astronauts on board did not interact with the system at any pointin a mission.
Initial work began on Saturn guidance concepts in 1958. Amockup IU was completed in 1961. By 1964 IBM had established a facility inHuntsville and had overseen the development of the advanced system used inSaturn V launches. Manned Saturn V launches began with Apollo 8 in 1968 andcontinued through Apollo 17 in 1972. The Skylab Orbital Workshop was launched,using the first two stages of a Saturn V, in May 1973. The Saturn IB, a smallerrocket that carried astronauts into Earth orbit for Apollo and Skylab Missionsthrough July 1975, also used a similar IU.
The computer used "Unit Logic": a hybridbetween discrete and integrated circuits. It contained a memory of 460,000 bits(16K, 28-bit words), in magnetic core, and had a serial, fixed pointarchitecture. Word length was 26-bits + 2 check bits. It operated atapproximately 10,000 operations/second.
This particular unit is one of two presently in the NASMcollections (See Saturn Instrument Ring (2)).It was transferred to the Smithsonian from NASA. The unit was apparentlyshipped directly from Hutsville, Alabama to a museum of transportation inTokyo, to whom it was loaned with other large space artifacts. It was returnedfrom loan, and found to be in good condition, in 1983. It was stored at thePaul E. Garber Facility until October 2004, when it was prepared for displayand moved to Udvar-Hazy Center. This is Instrument Unit 514, one of the lasttwo units built. It may have been procured for Apollo 19, a mission that wascancelled.
Almost everything about the Saturn V rocket is measuredin superlatives, and it continues to be regarded as the ultimate launch vehiclelong after its last flight. Although inertial guidance is itself not anuncommon technology, this system was unique owing to the nature of the Apollomission. It was a crucial subsystem for the Saturn, and remains a technical andhistoric milestone in real-time computing, guidance, and control.
PEC 11/04