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People first visited the moon in 1969. The last time we went was 1972, more than 50 years ago. Back then, astronauts in the Apollo program made their journeys in spacecraft that relied on remarkably basic electronics that were completely unsophisticated compared to what you’d find in an expensive blender or refrigerator today. At its heart was the Apollo Guidance Computer, which was charged with keeping the craft on course as it traveled to its destination and back.
Marc Verdiell, also known as CuriousMarc, is an expert at restoring old vintage electronics. So when it came time to restore one of these rare and legendary guidance computers, he was ready and willing to take on the task. Even better: he came to the 2023 Hackaday Supercon to tell us how it all went!
Restoration
You may have heard whispers about this effort before, or seen some of it our previous coverage. The effort to resurrect this Apollo Guidance Computer (AGC) began in earnest in late 2018. This particular example of the AGC was found in an electronics recycler in 1976, and the plan was to restore it before the 50th anniversary of the first moon landing in 1969. Marc worked with Carl Claunch, Ken Shirriff and Mike Stewart on the project , and documented much of it on his YouTube channel under the name (CuriousMarc).
As Marc explains, Apollo actually had four main computers. There was the Launch Vehicle Digital Computer (LVDC) that was charged with controlling the Saturn V from launch, along with the Flight Control Computer, which was basically an analog machine. There were then two identical Apollo Guidance Computers (AGC) – one was in the Apollo Command Module (Apollo CM) and one was in the Lunar Excursion Module (LEM). There was also the Abort Guidance System for dealing with situations where things got out of hand. Using old NASA diagrams, Marc shows us where the AGC lived in the Command Module and the LEM.
The AGC shown next to the DSKY.
The Apollo program was the first time humans entrusted the control of a spacecraft or aircraft to a computer system. The AGC was designed by MIT, built by Raytheon, weighed 70 pounds and drew 50 watts. Although it looks bulky and heavy by today’s standards, it was extremely compact and efficient by 1960s standards. The interface for the AGC was the DSKY, short for ‘Display and Keyboard’. It is the way the astronauts controlled and interacted with the AGC during the mission. The AGC was absolutely mission critical. In the words of NASA engineer Dan Lickly, “The AGC did everything.” It was responsible for orienting the spacecraft, checking rocket burns and placing the craft into orbit, and for handling the landing of the lunar module and re-entry into the Earth’s atmosphere.
The AGC is built with a highly modular design.
Marc tells how a copy of the Apollo Guidance System ended up at an electronics scrapyard in 1975, on behalf of one of NASA’s engineers. It soon fell into the hands of hacker Jimmie Loocke, who obtained no less than two tons of Apollo program hardware. Years later, a guy named Mike Stewart contacted Jimmie, hoping to see a real AGC and take some measurements to start working on a replica. When this happened, Jimmie in turn asked if the device could be turned on, and eventually a team was assembled to try to make that happen. The first attempt took place in a Houston hotel room in October 2018, and the project took off from there.
The lecture includes numerous glorious internal shots of the AGC, serial number RAY 14, which was built for a technical test series. The AGC was composed of stacks of neatly arranged modules full of integrated circuits, all connected via an elegant wire-wrapped backplane. In fact, this was one of the first computers to rely on integrated circuits. These were all dual 3-input NOR gates or analog amplifier ICs. Not being flight hardware, most of the modules in this AGC were not pocketed, which had the advantage of being easier to machine. However, two of the core memory modules had potting, which would make them more difficult to work on.
The wire-wrap construction was considered very reliable: crucial for manned space missions.
The repair and restoration of the AGC was supported by official schedules, so the team knew what they were working with. Modules were tested and the core memory (essentially the RAM) was found to be poor. There was also no core cable memory – essentially the ROM for the AGC that stored the program. For this technical test article, a Raytheon rope memory emulator was used instead. This was for ease of development, as it allowed the stored program to be changed more easily than rewiring a core cable memory device. However, the Raytheon emulator device was not documented, which took some reverse engineering to figure out. Still, booting up the device without RAM or ROM was a positive experience: the AGC still tried to boot and there were some small signs of life.
The rest of the lecture outlines how the AGC was fully brought back to life. The team improvised an FPGA memory emulator, x-rayed faulty components, and built replicas of things they couldn’t replace, all in service of the ultimate goal. Sneaky hacks were used to get bits and pieces functional again against the ods. Even connectors had to be made from scratch because the AGC used long-forgotten standards that are no longer in use. The fact that Marc works for connector supplier Samtec was particularly useful in this case. Prepare to choke on your drink when he explains how much it costs.
The team was able to show their restoration work Eldon Hallthe engineer who led the development of the Apollo Guidance Computer (AGC).
We won’t spoil the entire journey here, because Marc’s talk is worth watching from start to finish. There are twists and turns aplenty, and all the while you’re waiting for that ultimate pleasure: the joy every hacker knows when the beleaguered machine finally comes to life. Never mind the beautiful, heartwarming moments at the end.
It’s simply amazing to see a dedicated team bringing this legendary hardware back to life – and even better to see them flying simulator missions while the real AGC hardware does its thing. It’s a fantastic restoration effort that has been documented in meticulous detail, and it’s a joy that we get to see everything that made it possible.