Apollo Guidance Computer (#StateOfNASA | #NASASocial

In February, Doug spent a day at NASA’s Armstrong Flight Research Center (AFRC) attending a #NASASocial event dubbed #StateOfNASA.

One of the day’s highlights occurred when Dr. Chrisitian Gelzer (Armstrong’s Historian) brought a tiny, shiny box with black keys on its front face into the room. Doug immediately recognized the device as an object from an earlier age: an Apollo Guidance Computer. Here’s what we learned about this computer in particular and the Apollo Guidance Computer (AGC) generally.

#1. Apollo 15

DOUG-DSKYWhen he first saw the computer, Doug assumed that it was a model, and then Gelzer said, “This is the Apollo 15 Command module guidance computer.” At that point, Doug said, “Can you repeat that?” And Gelzer did. Channeling his inner twelve-year-old, Doug then said, “May I touch it?” And he did. After regaining his senses, Doug realized that what Gelzer had brought out that day was actually the DSKY, the user interface that astronauts used to control the AGC. The actual AGC was a much larger object than the DSKY (roughly three times longer and two times wider and weighing seventy pounds), but it was still quite small for computers of that day-gone-by.

The story that Gelzer told of how the computer that guided astronauts Dave Scott, Al Worden, and James Irwin to the moon and back is charming and serendipitous.

Basically, an AGC was requested by a Dryden (now Armstrong) lead test engineer to use in a new flight-test program. One of the 48 back-up computers for Apollo was sent from Houston, and it was summarily destroyed (Oh, no, blue smoke…) when it was installed into the test plane (see #4 below). A second AGC was requested and received. It was only after it arrived at Armstrong (then Dryden) did anyone there realize that it was the Apollo 15 AGC—the actual, used-in-space Apollo 15 AGC. Of course, NASA HQ wanted it back. Dryden said, No take backs. NASA HQ said, Well, OK. But try not to destroy this one.

#2. Alarm!

Charlie Duke in Houston: not alarmed? (NASA)

Each Apollo mission—save Apollo 8—required two AGCs: one for the Command Module and one for the Lunar Module. One moment of highest tension during the Apollo 11 moon landing occurred when the AGC issued alarms. As the lunar module descended, the AGC issued two types of alarms (1201 and 1202) that resulted from the real-time computer system being overloaded by information it was receiving from the rendezvous radar. After quickly checking the books, Houston decided these alarms were not too alarming and gave Neil Armstrong and Buzz Aldrin the go ahead to land on the Moon.

lunar module landing (NASA)



#3. Rope Core Memory

rope core memory (not from the AGC). (Johann H. Addicks - Eigenes Werk)
rope core memory (not from the AGC). (Johann H. Addicks – Eigenes Werk)

The AGC has several interesting design features, including the first extensive use of Integrated Circuits (ICs) in a general-purpose computer. Another feature was use of rope core technology for the AGC’s Read Only Memory (ROM). ROM is often referred to as nonvolatile memory, meaning that contents of the memory cannot be changed. In rope core memory, this means that the programs were actually woven into the memory.


#4. Fly-by-Wire

F-8 Crusader that flew the Apollo 15 AGC and DSKY (Doug)
F-8 Crusader that flew the Apollo 15 AGC and DSKY (Doug)

The AGC was brought to the California desert (see #1 above) to be a part of a flight test program for digital fly-by-wire aircraft. The program was highly successful and influence most fly-by-wire systems that came after it.



#5. Avalon

In addition, a variant of the AGC was used as the navigation computer on the Navy’s Deep Submergence Rescue Vehicle. That’s a pretty cool example of reuse.


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