Inside the Apollo "8-Ball" FDAI (Flight Director / Attitude Indicator)
26 comments
·June 14, 2025kens
_dwt
Great article. I'd never thought about a spacecraft ADI having a third axis. Sadly, a nitpick - Bill Lear's F-5 autopilot was not, as far as I can tell, in any way connected to the Northrop F-5 fighter jet.
kens
Thanks. You are correct about the F-5 autopilot, so I fixed that. It turns out that it was used in planes such as the C-47, C-60, C-45, and B-26, but is unrelated to the F-5.
garaetjjte
>The Command Module for Apollo used a completely different FDAI (flight director-attitude indicator) that was built by Honeywell.
That's surprising. Was there any requirement that necessitated them to be different parts, or it's just because different suppliers were chosen by Grumman/North American?
kens
It's probably a combination of different suppliers being chosen, and everyone wanted a piece of the pie. But it's annoying when I figure out how something works in the Lunar Module and then discover that the Command Module is completely different. Not to mention that the Saturn V is a whole different world.
rbanffy
I remember a similar thing from the, IIRC, F-104.
johng
I mainly remember this because he refers to it as the 'frappin 8 ball' in the Apollo 13 movie, if my memory serves.
kens
Yes, in the movie, Lovell says "What's the frappin' attitude?" as the 8-ball rolls out of control. The actual Apollo 13 transcript has nothing like that, interestingly enough.
Links: https://archive.org/details/apollo1319959231994/page/n92/mod... https://www.nasa.gov/wp-content/uploads/static/history/alsj/...
mcpeepants
same here, he sure does
jschveibinz
Back in the day, this would be have been a good homework assignment for an EE analog controls class.
wafflemaker
That's a 'kunst' of UI (a gem?). One look and you instantly know the orientation of your craft.
As an amateur astro-pilot (1000h in KSP and 200+ in Flight of Nova, both flight simulators with realistic orbital mechanics) I'd like to say that in modern cockpit of the fusion propelled ships in FoA, the one thing I'm missing from Apollo-style flight instruments of KSP is the Nav-Ball.
The jet-fighter-like "ladder" style attitude meter can't be read with just one look. You need to focus to see the numbers next to the ladder steps. And then another look at the compass for a full reading. 3s of focus (away from controlling the ship) vs. 0.5 (that your subconscious has most likely already interialized).
To put that 3s into perspective, according to ship readings, Apollo 11 had <20s fuel left when it touched down on the moon.
null
WillAdams
This was actually mentioned in a recent talk by Freya Holmér --- I believe this one:
johnsutor
Brings me back to playing Kerbal Space Program
chiph
kens - Are the collectors of the output transistors on the amplifier boards connected to the metal can? I can see from the photo that the heatsinks don't touch (there's a gap between them for the capacitors). Did they use nylon screws to prevent an electrical path through the frame?
kens
Unfortunately, I don't have the FDAI handy to check this.
CamperBob2
For TO-5 bipolars, it was common for the collector to be connected to the case. I wouldn't say that's universally true but I don't recall any exceptions off the top of my head.
jart
Ken once again proves he's one of the greatest publishers on Hacker News.
timewizard
I wonder if that simulator was OV-095 at SAIL.
https://spaceflightblunders.wordpress.com/2017/03/31/ov-095-...
EDIT: Ah. It almost certainly was:
https://www.superstock.com/asset/oct-astronauts-frederick-ri...
kens
There are many different Shuttle simulators. The simulator photo in my post is one of the Shuttle Mission Simulators (SMS), now at Stafford Museum in Oklahoma. The Shuttle Avionics Integration Laboratory (SAIL) is a different simulator for avionics testing (rather than astronaut training) and is currently in Houston.
dmd
The strong impression I always get from the entire Apollo program is "they didn't know it couldn't be done at the level of technology available, so they did it anyway".
null
jsrcout
> 3. The FDAI's signals are more complicated than I described above. Among
> other things, the IMU's gimbal angles use a different coordinate system from
> the FDAI, so an electromechanical unit called GASTA (Gimbal Angle Sequence
> Transformation Assembly) used resolvers and motors to convert the
> coordinates.
null
userbinator
1960s technology, designed and made in the USA. It seems that people back then were far more clever at making do with what they had.
null
Author here for your Apollo questions :-)