The Saturn Instrument Unit

The ring shaped Instrument Unit is about to be placed on top of the third stage of the Saturn V stack. This particular IU (IU-502) was used for the unmanned Apollo 6 mission.

credit to NASA
Scanning credit to Kipp Teague

General information

This ring shaped, 2 metric Tons heavy, Instrument Unit was manufactured by IBM and was the electronic heart of the Saturn V stack. The instrument ring was a load bearing structure, it had to support the Apollo spacecraft (including the Lunar Module) on top with its weight of about 45 Tons. It contained 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 supply of power.

The Instrument Unit (IU) main tasks during flight were:

  1. Navigation
    Determination of attitude, position, velocity and acceleration of the launch vehicle;
  2. Guidance
    Computation of manoeuvres 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 were:
  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-IC 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 large F-1 thrust engines, 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-II 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. But occasionally they were also mounted in de the S-IVB aft skirt and the S-IC forward skirt. The necessity for having additional rate gyros, was depending on the Saturn V configurations which were related to the various payload demands. Different configurations could result in different bending mode characteristics.

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
Occasionally control accelerometers were added into the loop of control. Like the EDS rate gyros, they were body mounted, but never in the IU but somewhere lower inside the launch vehicle (the S-IVB aft skirt and the S-IC forward skirt) in order to sense the bending motions of the Saturn V adequately. According to Ref. 19, accelerometers were employed when additional attitude control signals were considered neccesary to reduce wind loads during atmospheric flight.

The hardware configuration of the instrument ring didn't vary much among the different Saturn V 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.

Diagram based on figure 8 in Ref. 3
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.

Vibration tests on the Saturn V have revealed that 4 bending modes and 4 torsional modes of the Saturn V were relevant.

In this picture are only depicted the first two bending modes to illustrate the strategy behind the choices for the locations of the control rate gyros and the control accelerometers inside the launch vehicle.

For some more information about the location of the rate gyros in the instrument unit.

S-IU-503 ascent
This picture shows the flight path of the SA-503 (Apollo 8) during ascent. Major events which were initiated and controlled by the Instrument Unit are shown in the plot. Until 206 sec. after lift-off the Saturn V followed a programmed flight path with a predetermined pitch rate. After those 206 sec., some 40 sec. after iginition of the S-II stage, the launch vehicles was actively guided. Flight path deviations in the early boost phase, which were stored in the onboard computer, were retrieved and taken into account.

During orbital coast flight, the navigational program preserved the desired orbital conditions. Navigation and guidance information could be updated in the onboard computer by digital data transmission from ground stations. This two way communication link between the launch vehicle and the mission control center, is called the CCS (Command and Communication System).

Diagram based on pictures IBM-DR-7 and IBM-DR-8 in "Saturn V News Reference"
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 manoeuvres 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 provided 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 in order to sense the bending motions of the Saturn V adequately. According to Ref. 19, accelerometers and additional rate gyros 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-501

thumbnail picture S-IU-501

Instrument Unit for SA-502

Instrument Unit for SA-503 and 504

Instrument Unit for SA-505, 506 and 507

Instrument Unit for SA-508, 509, 510, 511 and 512

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

  2. Saturn V News Reference
    compiled by NASA, The Boeing Company, Douglas Aircraft Company, IBM, Rocketdyne

  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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