page_title: Saturn V instrument unit
section_title: Guidance and control
hierarchical_section_title: Saturn V instrument unit / Subsystems / Guidance and control
caption_attribution_description: English: The ST-124-M3 has three gimbals, of which the outermost is attached to the Instrument Unit and the innermost is fixed in inertial space. As the vehicle rolls, pitches and yaws during flight and orbit the inner gimbal remains oriented toward the same point in space at which it was pointed at the moment of launch. The inner gimbal carries accelerometers which measure vehicle accelerations in this fixed, inertial coordinate system. Three gyros also attached to the inner gimbal sense any rotations of this gimbal and send signals to feedback circuits which drive servotorque motors that counter the rotations by nulling the gyro outputs. The resolvers measure the angles of the three gimbals, providing the attitude data of the vehicle to the navigation system. The prisms are part of the system for aligning the inner gimbal just before launch.
context_page_description: The Saturn V instrument unit is a ring-shaped structure fitted to the top of the Saturn V rocket's third stage and the Saturn IB's second stage. It was immediately below the SLA panels that contained the Lunar Module. The instrument unit contains the guidance system for the Saturn V rocket. Some of the electronics contained within the instrument unit are a digital computer, analog flight control computer, emergency detection system, inertial guidance platform, control accelerometers, and control rate gyros. The instrument unit for Saturn V was designed by NASA at Marshall Space Flight Center and was developed from the Saturn I IU. NASA's contractor to manufacture the Saturn V Instrument Unit was International Business Machines.
  One of the unused instrument units is currently on display at the Steven F. Udvar-Hazy Center in Chantilly, Virginia. The plaque for the unit has the following inscription:
  The Saturn V rocket, which sent astronauts to the Moon, used inertial guidance, a self-contained system that guided the rocket's trajectory. The rocket booster had a guidance system separate from those on the command and lunar modules.
context_section_description: The Saturn V launch vehicle was guided by navigation, guidance, and control equipment located in the IU. A space stabilized platform (the ST-124-M3 inertial platform at location 21) measured acceleration and attitude. A launch vehicle digital computer (LVDC at location 19) solved guidance equations, and an analog flight control computer (location 16) issued commands to steer the vehicle.
  The attitude of the vehicle was defined in terms of three axes:
  The roll axis (X) runs from tail to nose and was vertical at time of launch.
  The pitch axis (Y) is at right angles to the roll axis, and is marked on the exterior of the IU by +Y above the viewport, outside location 21.
  The yaw axis (Z) is at right angles to both the pitch and roll axis, and is marked by +Z outside location 3.
  The ST-124-M3 inertial platform contains three gimbals: the outer gimbal (which can rotate 360° about the roll or X axis of the vehicle), the middle gimbal (which can rotate ±45° about the yaw or Z axis of the vehicle), and the inner or inertial gimbal (which can rotate 360° about the pitch or Y axis of the vehicle). The inner gimbal is a platform to which are fixed several components:
  Two vertical alignment pendulums sent signals before launch to ground support equipment, which generated signals to the platform gyro torque generators to level the inner gimbal. The vertical alignment system levelled the platform to an accuracy of ±2.5 arc seconds.
  Two prisms, one fixed and one servo-driven, were used with an external theodolite which sighted through the viewport outside location 21 to set the azimuth of the inner gimbal before launch. The azimuth could be set to an accuracy of ±5 arc seconds.
  Three single-degree-of-freedom gyroscopes have their input axes aligned along an orthogonal inertial coordinate system. Three signal generators, fixed to the output axis of each gyro, generated electrical signals proportional to the torque disturbances. The signals were transmitted through the servo electronics which terminated in the gimbal pivot servotorque motors. The servoloops maintained the inner gimbal rotationally fixed in inertial space. That is, while the vehicle rolled, pitched, and yawed, the inner gimbal remained in the same attitude to which it was set just before launch. Though it was being translated during the launch and orbit process, it was rotationally fixed.
  Three integrating accelerometers measured the three components of velocity resulting from vehicle propulsion. The accelerometer measurements were sent through the launch vehicle data adapter (LDVA at location 19) to the LVDC. In the LVDC the accelerometer measurements were combined with the computed gravitational acceleration to obtain velocity and position of the vehicle.
  The angular positions of gimbals on their axes were measured by resolvers, which sent their signals to the LVDA. The LVDA was the input/output device for the LVDC. It performed the necessary processing of signals to make these signals acceptable to the LVDC.
  The instantaneous attitude of the vehicle was compared with the desired vehicle attitude in the LVDC. Attitude correction signals from the LVDC were converted into control commands by the flight control computer. The required thrust direction was obtained by gimbaling the engines in the propelling stage to change the thrust direction of the vehicle. Gimbaling of these engines was accomplished through hydraulic actuators. In the first and second stages (S-IC and S-II), the four outboard engines were gimbaled to control roll, pitch, and yaw. Since the third (S-IVB) stage has only one engine, an auxiliary propulsion system was used for roll control during powered flight. The auxiliary propulsion system provides complete attitude control during coast flight of the S-IVB/IU stage.
caption: Drawing on the ST-124-M3 inertial platform
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