General information

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.

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.

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.