The Instrument Unit for the Saturn IB

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 information

This 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:

  1. Navigation
    Determination of attitude, position, velocity and acceleration of the launch vehicle;
  2. Guidance
    Computation of maneuvres necessary to achieve the desired flight path;
  3. Control
    Execution of the guidance maneuvres by controlling the actuators of the thrust engines;
  4. Launch Vehicle Management
    Controlling flight events and monitoring the condition of the entire launch vehicle.
The IU systems taking care of Navigation, Guidance and Control are:
  1. the inertial platform;
  2. the Launch Vehicle Data Adapter (LVDA);
  3. the Launch Vehicle Digital Computer (LVDC);
  4. the analog Flight Control Computer (FCC) and
  5. the rate gyros to measure the change rates of yaw, pitch and roll angles.
The three-axis stabilized inertial platform was equiped with gyros and accelerometers. It acted as a fixed spatial onboard reference. The inertial platform gave information on attitude and acceleration. In conjunction with a computer, the platform provided information on the launch vehicles' attitude, location, velocity and acceleration. The LVDA is the input-output device of the LVDC, it is able to convert analog signals into digital information and vice versa. Data was processed and calculated by the LVDC. The FCC converted attitude correction data into command signals for the actuators of the thrust engines to control the thrust vector.

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
The rate gyros provided information on the internal motions of the launch vehicle. The Saturn launch vehicles could not be considered as rigid bodies because of their sheer size. Internal motions like torsional and bending motions had to be reckoned with. The information from the rate gyros provided the FCC the ability to stabilize the attitude of the launch vehicle and dampen out the internal motions to preserve the mechanical integrity of the launch vehicle. The rate gyros were body mounted inside the IU.

Control-EDS Rate Gyros for emergency detection
The rate gyros were also part of the EDS (Emergency Detection System). The EDS system was meant to detect whether the launch vehicle was leaving the range of control. It involved parameters like thrust engines performance, pressure values of propellant tanks, vibration levels and attitude. With the rate gyros, the EDS was able to detect whether angular rate boundary values were exceeded. If that was the case, there was a real danger that the launch vehicle would break up, an abort procedure was therefore initiated. This abort procedure was meant to separate the crew cabin from the launch vehicle first, in order to bring the crew into safety, and next to disperse the propellants of the launch vehicle by ripping open the propellant tanks with explosive charges.

Control accelerometers for attitude stablilization
Also Control accelerometers were used for attitude stabilization, but only during the burn of the first stage (S-IB stage). Like the EDS rate gyros, they were body mounted inside the IU.

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.

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.

For some more information about the locations of the rate gyros and the control accelrometers in the instrument unit.

S-IU-200 navigation, guidance&conrtol

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
Guidance was performed by the LVDC by comparing the actual flight path derived from the navigation data with the desired flight path according to the guidance program, which was stored in the LVDC. Based on these differences, the necessary maneuvres were computed by the LVDC to meet the required flight trajectory conditions.
Control was executed by the FCC based on the guidance data from the LVDC and the signals from the angular rate gyros (which provides information on the instantaneous flight behaviour of the launch vehicle). The information from the angular rate gyros made that command signals from the FCC to the actuators of the thrust engines resulted in smooth flight path corrections.

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. 3 accelerometers were employed when additional attitude control signals were considered neccesary to reduce wind loads during atmospheric flight.

Instrument Unit for SA-201

Instrument Unit for SA-202


Instrument Unit for SA-203

Instrument Unit for SA-206

Instrument Unit for SA-207

Instrument Unit for SA-208


  1. Skylab Saturn IB Flight Manual
    MSFC-MAN-206, September 1972

  2. Saturn V Flight Manual SA-503
    George C. Marshall Space Flight Center

  3. Description and performance of the Saturn launch vehicle's Navigation, Guidance and Control System
    NASA TN D-5869, July 1970
    by Walter Haeussermann

  4. Saturn V/IB Instrument Unit Description and Component Data
    MFSC No. III-5-509-4, IBM No. 66-966-0006, June 1966, January 1970
    by IBM, Federal Systems Division

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