The Instrument Unit for the Saturn IB |
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This view depicts engineers conducting a system test on the Saturn IB instrument unit (IU-206) at International Business Machines (IBM) in Huntsville, Alabama. IBM was a prime contractor for development and fabrication of the IU. This particular IU was used for the Skylab 2 mission. The white-grey box, in front, mounted against the outer skin of the IU, contained various materials to be exposed to prelaunch, launch and space environments to determine degradation effects (M-415 experiment). credit to NASA Marshall Space Flight Center (NASA-MSFC) |
General informationThis ring shaped Instrument Unit was manufactured by IBM and is the electronic heart of the Saturn IB stack. The instrument ring was a load bearing structure, it had to support the Apollo spacecraft on top with its weight of nearly 30 metric Tons. It contains all the necessary instrumentation for guidance, navigation and control. The electronics was mounted onto 16 liquid cooled panels. The coolant was provided by an onboard cooling unit. Electric power was supplied by ground supply equipment on the launch pad until 50 seconds prior to lift off, at that moment onboard batteries took over the power supply.The Instrument Unit (IU) main tasks during flight are:
During flight the IU controlled all flight events like staging, engine ignition, engine cut-off and steering based on acquired flight data. The required sequence commands were send by the LVDC to the several stages of the launch vehicle. Preserving the mechanical integrity of the launch vehicle was one of its most important tasks. Active guidance was therefore suspended during the boost phase of the S-IB stage. The reason for that was, that during that phase the launch vehicle was travelling thought the dense layers of the atmosphere and was subjected to wind sheer and large aerodynamic forces. Additional lateral forces, which are applied as a result of the swivel motions of the four outer H-1 thrust engines (the four inner H-1 engines were fixed) to make course corrections, might jeopardize the vehicle integrity. Therefore the launch vehicle went through a predetermined smooth flight path, controlled by a fixed program in the onboard computer memory. Deviatons from the desired flight path caused by wind sheer, were however sensed, measured and stored in the onboard computer for later retrieval. After ignition of the S-IVB stage in the thin upper atmosphere, the launch vehicle was actively guided and flight path deviations from the early boost phase could be compensated for.
Control-EDS Rate Gyros for attitude stabilization
Control-EDS Rate Gyros for emergency detection
Control accelerometers for attitude stablilization The hardware configuration of the instrument ring didn't vary much among the different Saturn IB flights. Differences in hardware could involve items like an extra battery pack or an extra measurement unit. But because of the different mission profiles, for each Apollo mission, mission dedicated software had to be written for the LVDC. |
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The instrument unit was sending actuator command signals to the thrust engines to maintain the proper flight trajectory, but also to counteract bending and torsional movements of the launch vehicle. To maintain the proper tractory, the Inertial Guidance Platform provided information on the velocity and the acceleration of the launch vehicle. To dampen out the internal motions, the control rate gyros and the control accelerometrs provided information on the flight behaviour of the launch vehicle. |
![]() Diagram based on figures 6-12 and 6-24 from the Skylab Saturn IB flightmanual. |
LVDC: Launch Vehicle Digital Computer (processing flight data and flight sequence control) LVDA: Launch Vehicle Data Adapter (input-output device of the LVDC) FCC: (Analog) Flight Control Computer (to convert processed flight data and control data into thrust engines command signals)
Navigation was performed by the LVDC based on measurements from the Inertial Platform Occasionally control accelerometers were added into the loop of control. Like the EDS rate gyros, they were body mounted, but usually not in the IU but somewhere lower inside the launch vehicle. The rate gyros were located in the IU only. According to Ref. 19 accelerometers were employed when additional attitude control signals were considered neccesary to reduce wind loads during atmospheric flight. The Apollo spacecraft was also equipped with a Guidance & Control system with a digital computer (AGC)and an inertial platform. In the highly unlikely case that the IU inertial platform and the triple redundant LVDC would fail, the Apollo guidance system was able to take over the LVDC's guidance task only. The system also provides the option for manual intervention from the onboard crew. |
Instrument Unit for SA-201
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Instrument Unit for SA-203
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