posted 07-16-2026 04:19 PM            

I am assuming for both the Apollo lunar module and the command module they were used to indicate the lunar horizon. But I really don't know*. It's my understanding the early astronauts being largely test pilots, lobbied hard for the inclusion of these.

posted 07-16-2026 09:22 PM               

In reality, the instrument can be referenced in any position. As there is no up in space, but there needs to be an up reference, the FDAI can be zeroed so that its idea of up matches the Earth, moon, or anything. Once you have a reference to which way you need to use as up, you can navigate, maneuver, or take measurements.

Imagine a space plane that can fly to space and dock with a space station. The FDAI is set on the ground before flight to indicate zero (straight and level) when the vehicle is positioned level on the ground. Up indication matches up in real life.

You launch the space plane and climb to space, but when you try to dock with the space station, it would be helpful if your idea of up matches the space station's idea of up, so you align the FDAI to the same plane of reference as the space station. Now you can control the space plane using the same orientation as the space station.

During Apollo, there was a requirement for the Command Module and the Lunar Module to dock together. The Command Module pilot used a docking target that was designed to provide alignment information in three axis so that the crew could approach and dock in the correct orientation so that the Command Module matched the Lunar Module. The docking Target was a simplified way of achieving navigation between two bodies without losing the navigation system's "up" reference.

The Space Station's position "moves" East with reference to Earth during orbit. In reality, the Earth moves under the space station as the space station moves above Earth. If the space plane launches as the space station passes overhead, and they both have the same calibrated up, their attitude indicators match. However, if the space station passes to one side or the other of the position the space plane launched, their "up" references do not align. Having an instrument that allows the "up" reference to be changed makes navigating and controlling the space plane much easier.

During Apollo, the Command Module "up" reference was on the back of the FDAI eight ball on the pad, aligned with the Z axis of the rocket. As the rocket climbed to orbit, the "up" reference changed until "up" aligned with what was Command Module Y axis on the ground.

posted 07-17-2026 10:25 PM               

The Command Module Guidance and Navigation Computer was coupled to the DSKY display and to the FDAI. The instrument can usually be slaved to information from the navigation computer or external radio signals.

A gimbal is a ring mounted on a pivot that allows an object to rotate in 3D space. Most mechanical systems use three nested gimbals to track or achieve all possible orientations (like pitch, yaw, and roll), although a more simple system can use two gyros to achieve a similar result.

Gimbal lock is the loss of one degree of freedom in a 3D rotation system. It happens when two rotational axes align parallel to each other, causing the system to treat them as a single axis.

posted 07-18-2026 01:33 PM               

First, the orientation of the Apollo spacecraft was tracked by the IMU (Inertial Measurement Unit, aka the "stable platform"), which is the bit of equipment that housed the three gyroscopes. The FDAIs ("8-balls") were driven electrically by the AGC (Apollo Guidance Computer) which continuously integrated the data from the IMU. So IMU to AGC to FDAI. Maybe not "interesting," but necessary for a slightly deeper understanding of the operation of the FDAIs, specifically the next bit...

So the FDAI is showing the orientation of the spacecraft in relation to fixed space — theoretically useful and satisfying, but not intuitive when orbiting the Earth or Moon. That's where the ORDEAL (Orbital Rate Drive, Earth and Lunar) comes in. The ORDEAL was an add-on box that added an offset to the data from the AGC that controlled orientation of the FDAI. This offset tracked the orbit around the Earth or Moon. In this way, the FDAI could, in fact, act like a "artificial horizon."

posted 07-22-2026 04:48 AM               

quote:

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Originally posted by Robert Pearlman:
This might help...

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Nope.

To understand gimbal lock, an understanding of gyroscopes, gyroscope precession, and 3-axis stabilisation is warranted.

This is probably more that can be covered in an online forum.

At the basics, If you try to tilt a spinning gyroscope, the force reacts at a 90-degree angle to the direction of the spin.

If you align two 3-axis gyroscopes, the two react to the same input from the same plane, so the information from the plane of rotation that the upset gyro was originally aligned is lost.

We used to cage gyroscopes before doing aerobatic maneuvers to prevent upsetting or tumbling the gyroscopes, before strap gyros, accelerometers, and laser gyros were a thing.

posted 07-22-2026 07:13 PM               

The AJB-3 bombing/navigation system, which included the ID-811/AJB-3 Attitude Director Indicator ("8-ball"), was introduced on the production F4H-1/F-4B, entering service around 1961–1962, not on the original prototypes.

posted 07-22-2026 08:45 PM               

quote:

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Originally posted by MartinAir:
The first three axis "8-ball" attitude indicator was used on the X-15, right?

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Lawrence Sperry tested gyroscopic instruments during World War I. In 1929, U.S. Army Lieutenant James Doolittle achieved the first-ever "blind" flight, taking off, navigating, and landing using solely Sperry’s artificial horizon and directional gyro.

Direction indicators and artificial horizon instruments became mainstay instruments for aviation. During WW2, most Allied aircraft had gyroscopically stabilised instruments, often “powered” by vacuum air, sourced from external venturis mounted on the outside of the fuselage, which meant that the gyroscopes could not be spun up until the aircraft was airborne. The venturis were susceptible to icing.

Later instruments were powered electrically, mostly by 3 phase 115 volts AC that was derived from a 24 vold DC system via inverters. Due to the risks associated with losing an inverter during instrument flight, aircraft generally had more than one inverter that sourced power from more than one buss bar.

Over time, additional information was added to the instrument display, including VOR, navigation, TACAN, and glideslope information, which was useful for instrument navigation and instrument approach and landing.

Manufacturers added more information to predict flight paths, which became known as flight directors.

These systems were around before the X-15 and F4 Phantom. The instruments traditionally had a blue half representing the sky and a black half representing the ground; however, it was found that having a white/black background worked better when red cockpit lighting was used. The military versions also had the ability to withstand inverted and aerobatic attitudes, which was rarely needed in the civilian world unless things had gone horribly wrong.

Bill Lear solved an indicator problem for the U.S. Air Force: the supersonic F-102 Delta Dagger interceptor (1953) could climb at steep angles, but existing attitude indicators could not handle nearly vertical flight. Lear developed a remote two-gyro platform that drove the cockpit indicator while avoiding "gimbal lock" during vertical flight. For the experimental X-15 rocket-powered aircraft (1959), Lear improved this indicator to handle three axes: roll, pitch, and yaw.

These systems were replaced by EFIS systems that used electronic screens. This allowed information such as speed, altitude, and vertical speed to be incorporated, which was where heads-up displays came from.

Today's instruments have moving maps, synthetic radar, traffic alert, terrain alert, navigation routes and boundaries, radio information, weather radar, and a myriad of other data overlaid on the one instrument. The military overlays some of this and other information in the pilot helmet visor, and “gyroscopes” no longer have moving parts.

posted 07-28-2026 08:10 PM               

quote:

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Originally posted by Ted Peterson:
I will never grok this.

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Even better, somewhere in the collection of oral histories, some engineer who worked on such systems has possibly spoken about the challenges and their often unique solutions to specific problems. They provide a better context and reasoning to the problem at hand than you get from just reading what piece of equipment or instrument the spacecraft had.

posted 07-29-2026 01:03 AM               

The result is the platform will be "dumped" (i.e. physically moved) meaning the platform which is "truth" in inertial navigation is no longer holding the reference position it had. Once the platform is dumped you no longer have a stable reference to measure angles and accelerations for navigation.

Relative to "powering thru it," that is not possible because the angle measuring resolvers mounted at the gimbal pivot points to measure the rotational motion of the platform gimbals are no longer measuring the angular movement of the lost axis because of the gimbal lock. When the spacecraft moved and dumped the platform, there is no measurement of how much the spacecraft moved. So the stable inertial reference is lost, unless some external measurement can be made such as a star sighting. One way to be immune to gimbal lock is to add a fourth gimbal. But extra mass requires more fuel, etc. It all adds up.

If you really want to get a good understanding, I recommend building a simple three-ring gimbal model and playing with it.