Test run

Testing the Apollo Guidance Computer 

Part one of this article detailed how a group of four men who restore historically significant, vintage computers — Carl Claunch, Ken Shirriff, Mike Stewart, and Marc Verdiell — connected with Jimmie Loocke, a former technician at the NASA Manned Spacecraft Center (now Johnson Space Center). Loocke had purchased two tons of scrap Apollo hardware at a recycler’s warehouse in 1975. Included in this purchase was a rare Lunar Module Apollo Guidance Computer, or AGC.

The AGC is the computer that navigated Apollo spacecraft to the moon and back, including the landing and ascent of the Lunar Modules to the moon’s surface. Part 1 ended with Loocke and Stewart realizing the AGC was in good enough condition to possibly be restored. That’s when they contacted Claunch, Shirriff, and Verdiell.

The restoration team needed to get their hands on the AGC, but Loocke did not dare ship his precious computer to California. So the team traveled to Houston and sequestered themselves, their expensive and sensitive electronic diagnostic equipment, and the AGC in a hotel suite for two weeks.

The ACG is divided into two trays, A and B. Tray A contains mostly logic modules, while Tray B is mostly the memory modules and associated support electronics. The AGC had a modular design because NASA didn’t trust the reliability of computers at the beginning of the program.

The team first removed the A modules and tested each one. As you can imagine, this was incredibly detailed, painstakingly slow work. After one week of testing, they were surprised to determine that all A modules appeared to work and were safe to power up.

When powered, the logic modules worked well enough that the computer went through the first instructions of its boot sequence. It is extremely unusual that a computer of this age would try to boot on its first attempt without a month of debug and countless repairs. This was a testament to the quality of its design and workmanship.

For example, the system voltage reference source was specified at 5 volts. When powered, the system came to 4.99 +/- 0.01 volts. Think about that: This is 50-plus-year-old equipment that had been in storage for decades, and it was running within 0.20 percent of the original specification!

Next up were the memory modules, which proved more challenging. The team had already discovered a wire break in the module, but locating and repairing the break was extremely difficult for several reasons.

First, this was one of three potted modules in the AGC, so they could not see or reach inside it for testing. Furthermore, this core memory module contained over 32,000 microscopic ferrite core rings suspended in thousands of fragile 4 mil wires, probably the densest core memory module ever built at the time.

The module was brought to Samtec in New Albany to be inspected with the company's sophisticated X-Ray/CT equipment. Scans revealed there would be no chance to repair the original wire.

Fortunately, for extra reliability, the memory was designed with an extra bit in the form of a parity bit, used solely for error detection. It was decided to reuse the parity bit to replace the faulty bit, and disable parity checking. The next steps had to be performed in a properly equipped lab.

The team began exploring with Loocke the possibility of bringing the entire AGC to Marc’s lab in California to complete its restoration. Special arrangements were made to fly the AGC, in a seat next to Jimmie, from Houston to San Francisco. However, when Jimmie arrived at the Houston airport, the crew would not allow the AGC on the plane, despite Jimmie showing the approval documents and special tickets issued by the airline.

Jimmie reluctantly luggage-checked his rare, irreplaceable piece of history and entrusted it to the baggage handlers of a major airline. Sure enough, when his flight arrived in San Francisco, the AGC case was nowhere to be found. Panic and much arguing with the airline personnel ensued, until it magically reappeared hours later; it had been mistakenly loaded onto another flight.

At Marc’s lab, the team discovered three more faults in another potted memory system module. Eventually, after all the modules were repaired and installed in the AGC, it passed all logic and memory tests.

Finally, they began running the original computer programs, the most interesting being P63, the iconic program that controlled the descent of the Apollo 11 lunar module “Eagle” for the actual moon landing. The P63 computerized flight control was essential to a successful lunar landing.

NASA had deemed it impossible for humans to fly the lunar module efficiently enough, given the barely sufficient amount of fuel allocated. Although Neil Armstrong famously assumed manual control of the ship for the last few minutes of the landing to avoid a rock field, Eagle successfully landed with 24 seconds of fuel left in the tank, mostly thanks to the AGC.

The AGC’s data output and monitors were nothing like the video games of today. The restoration team, just like the original Apollo astronauts, were looking at a small screen with a black background and green block letters showing altitude and speed. As Marc later said, “They also had two stick controls. This and the obtuse (keyboard) makes for an extraordinarily entertaining video game!”

Verdiell stresses that Samtec’s sponsoring of the connectors for the project was essential to the restoration. The team needed replacement connectors throughout the AGC. Samtec’s Custom Connector Group tooled these products based on original 1962 Manned Spacecraft Center prints.

These connectors were in the critical interface between the restored computer and monitor, which was necessary for all system checks, communications, and simulations.

Samtec donated the molds, stamping dies, base metals, plating, assembly equipment, and labor to provide thousands of socket and terminal pin assemblies.

The group took the AGC on tour this week at events commemorating the anniversary of the first lunar landing. First, they attended a private event with Eldon Hall in Tampa, Florida. Hall is the original NASA chief architect of the AGC. They demonstrated the AGC by running moon landing simulations on the computer.

The next stop was the Cradle of Aviation Museum in Garden City, New York, on July 18. They again ran demonstrations and simulations.

The last stop is Saturday at MIT in Cambridge, Massachusetts, for the 50th Apollo Landing Anniversary event sponsored by the MIT Museum. The AGC was developed at MIT.

At the end of our last conversation, I asked Marc why he did all of this. I assumed his answer would be something like he did it because the lunar landing is arguably one of the greatest human achievements of all mankind, or because it is living history.

But his first response was “because it was fun!” He later acknowledged the historical significance, but then he added, “You can’t top this one. This is where computers went from the size of an entire room to their current size. It’s the first computer used to fly a machine, let alone to fly one into space. It’s obvious it was made by an A+ team. When you restore it, you feel like you’re there in the 1960s again. You get to relive Apollo; that’s priceless.”

— For more information about the Apollo Guidance Computer, the team, and the connector system, go to www.blog.samtec.com/AGC

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