page_title: Saturn IB
section_title: Instrument unit
hierarchical_section_title: Saturn IB / Instrument unit
caption_attribution_description: English: This photograph was taken during the final assembly operation of the Saturn V launch vehicle for the Apollo 4 (SA 501) mission. The instrument unit (IU) was hoisted to be mated to the S-IC/S-II assembly in the Vehicle Assembly Building high bay at the Kennedy Space Center. Note that the access door has been removed (about 8 o'clock), to allow technicians to enter the IU after it is stacked. The access door is location 8. Location 7 is to the right, with the nitrogen purge ducts (used during refueling to prevent the buildup of dangerous fumes). Location 6 has the silver cylinder that is the water accumulator. (This water sublimated to space during a mission, providing cooling for the elctronics in the IU.) To the right, at locations 5 and 4 are shelves to hold batteries. This differs from the configuration for IU-514, on display at the NASM. IU-514 has the water accumulator at location 3, to the right of the battery shelves. At the right (4 o'clock), at location 22, is the silver sphere holding the nitrogen supply for the gyro gas bearings. IU-514 also differs in having another battery shelf at location 24, two places to the left of the nitrogen sphere. Note the −X marking on the side facing us (12 o'clock), indicating that the old coordinate system was in use for this mission.
context_page_description: The Saturn IB was an American launch vehicle commissioned by the National Aeronautics and Space Administration for the Apollo program. It replaced the S-IV second stage of the Saturn I with the much more powerful S-IVB, able to launch a partially fueled Apollo command and service module or a fully fueled Apollo Lunar Module into low Earth orbit for early flight tests before the larger Saturn V needed for lunar flight was ready.
  By sharing the S-IVB upper stage, the Saturn IB and Saturn V provided a common interface to the Apollo spacecraft. The only major difference was that the S-IVB on the Saturn V burned only part of its propellant to achieve Earth orbit, so it could be restarted for trans-lunar injection. The S-IVB on the Saturn IB needed all of its propellant to achieve Earth orbit.
  The Saturn IB launched two uncrewed CSM suborbital flights to a height of 162km, one uncrewed LM orbital flight, and the first crewed CSM orbital mission. It also launched one orbital mission, AS-203, without a payload so the S-IVB would have residual liquid hydrogen fuel.
context_section_description: IBM built the instrument unit at the Space Systems Center in Huntsville, Alabama. Located at the top of the S-IVB stage, it consisted of a Launch Vehicle Digital Computer (LVDC), an inertial platform, accelerometers, a tracking, telemetry and command system and associated environmental controls. It controlled the entire rocket from just before liftoff until battery depletion. Like other rocket guidance systems, it maintained its state vector (position and velocity estimates) by integrating accelerometer measurements, sent firing and steering commands to the main engines and auxiliary thrusters, and fired the appropriate ordnance and solid rocket motors during staging and payload separation events.
  As with other rockets, a completely independent and redundant range safety system could be invoked by ground radio command to terminate thrust and to destroy the vehicle should it malfunction and threaten people or property on the ground. In the Saturn IB and V, the range safety system was permanently disabled by ground command after safely reaching orbit. This was done to ensure that the S-IVB stage would not inadvertently rupture and create a cloud of debris in orbit that could endanger the crew of the Apollo CSM.
caption: The instrument unit, which controlled the Saturn IB and Saturn V
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