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An Apollo Command Module Flight Direction Attitude Indicator (FDAI) manufactured by Honeywell Corporation under subcontract to North American Aviation (Primary Contractor for design/build of the Apollo Command Service Module). Two FDAI (for redundancy) were integrated as part of the CSM Stabilization and Control System (SCS) to support the crew with display of angular velocity (rate), attitude error, and total attitude.
The FDAI or "8-ball" was one of the most important instruments in the Apollo Command Module (and Lunar Module). Designers had originally intended to give the crew three separate displays to show their attitude; one each for roll, pitch and yaw. Being pilots, the crews quickly threw out the three displays for a development of the artificial horizon familiar from aircraft instrument panels.
In an aircraft, the Earth's horizon provides an obvious reference against which the artificial horizon displays attitude. In a spacecraft, such an obvious reference may not exist so the inertial platform at the center of the IMU (Inertial Measurement Unit) provides one - a reference that is constant relative to the stars, known as an 'inertial' reference. The basic function of the FDAI was to display the spacecraft's attitude with respect to the orientation of this platform. Pitch and yaw can be read off the ball directly; roll is shown by a pointer around the edge of the 8-ball. Three meters around the display (black needles) show the rate of rotation around the three axes. Three additional yellow needles in front of the ball graphically display the difference between the current and desired attitude of the spacecraft.
The body rate (roll, yaw, or pitch) displayed on either or both FDAI's was derived from the Command Module Inertial Measurement Unit (IMU) Body Mounted Attitude Gyros (BMAGs). Positive angular rates were indicated by a downward displacement of the pitch rate needle and by leftward displacement of the yaw and roll rate needles. The angular rate displacements were "fly-to" indications as related to rotation control direction of motion requirement to reduce the indicated rates to zero.
Servometric meter movements are used for the three rate indicator needles. Specific functions performed by the Apollo Command Module FDAI:
(1) Provided an inside-out display of the Command Module attitude with respect to a select inertial frame of reference by means of graduated three-axis ball and appropriate reference indexes.
(2) Provided a "fly-to" display of the spacecraft angular position relative to an inertial reference in all three axis by means of attitude-error needles.
(3) Provided a fly-to display of the Command Module angular rate about each of the three mutually perpendicular spacecraft body axis before the .05g level during entry (After 0.05g was reached, the displays provided the angular rate about the spacecraft pitch axis and about the roll and yaw axes.)
(4) Provided a centralized display of attitudes, vehicular rates, and attitude errors.
(5) Provided coarse attitude orientation (obtainable with the ball) and fine orientation using attitude-error needles.
(6) Indicated the IMU maneuver limits and the approach of these limits during maneuvers.
(7) Provided a monitor and a cross-check of reference-equipment conditions by comparing body-axis rates adjacent to body-axis errors.
(8) Provided a monitor of the SCS execution of reference-system commands in three axes by three command needles.
(9) Provided variable scale factors in rate and error indications for varying precision of the maneuvers required during the different mission phases.