The Apollo Guidance Computer

Guiding A Spacecraft To The Moon

The ability to exit the Earth's atmosphere in a spacecraft alone was not enough to get a man to the moon. The ship had to be guided there using computers that had dramatically less processing power than a modern smartphone.

Museum Exhibit: DSKY (Pronounced “Dis-Key”)

The Apollo Guidance Computer was compact for its time. The version that flew on the Block II Apollo spacecraft was 24 by 12.5 by 6 inches, weighed 70.1 pounds, and required 70 watts at 28 volts DC to work. And to interact with the computer, the astronauts used display and keyboard units, abbreviated DSKY (pronounced “dis-key”). These units had no functionality on their own, but they were a vital part of an astronaut’s communication with his spacecraft. This is one of those DSKYs, an unflown AGC interface identical to the ones installed in both the CSM and the LM.


Using the DSKY

Measuring 8 by 8 by 7 inches and weighing 17.5 pounds, the DSKY was the astronauts’ interface with their onboard computers, directly hooked to the inertial measurement unit and, in the CM, to the optical units. Astronauts could call up a specific piece of information by entering commands called verbs and nouns; verbs were used to request a piece of information and nouns signified what that information related to. Each verb and noun was assigned a number, so calling up information was as simple as entering two numbers. For example, to see the mission’s ground elapsed time, an astronaut would call up verb 16 and noun 36, then hit enter. The GET would appear on the DSKY. The shorthand in checklists was simplified: V16N36E.

DSKY

Packed and Stored Data

The Apollo Guidance Computer (AGS), for all its sophistication, was fairly simplistic. MIT chose a system that used 16-bit word size at a time when many scientific computers were using 24-bit or longer word lengths. Generally speaking, longer is better: longer words increase the precision of calculations. But shorter words come with the benefit of faster processing power, and two or three consecutive words in a program can offer increased reliability. This was the system MIT chose. Physically, the AGS was packaged rather simply. The computer itself consisted of circuits divided in two trays consisting of 24 modules. Each module had two groups of 60 flat packs, each of which had 72-pin connectors and held two logic gates. Tray A held the logic circuits, interfaces, and the power supply while tray B had the memory, memory electronics, analog alarm devices, and the clock. Memory in the AGS was about 64Kbyte, roughly a hundred thousand times less than found in a basic desktop computer of today. The AGS operated at 0.043MHz, pokey by modern standards, and it consisted of a segment of erasable core and six modules of core rope fixed memory. To keep everything humming every unit was hermetically sealed.

Packed and Stored Data

Fun Fact 2: The Second DSKY

There were two DSKYs in the Command Module, one on the main control panel and one near the optical instruments at the navigator's station. The latter had a “mark” button on the display that would lock onto a star for navigation measurements. There was a single DSKY display in the Lunar Module.

The Second DSKY

The Apollo Guidance Computer paved the way for what’s known as fly-by-wire flight control systems in, among other places, commercial aviation. While traditionally pilots controlled their aircraft by physically pulling on control surface, FBW passes pilot input into a computer and the computer moves the flight control surfaces. While the long term gains to worldwide aviation were far off in 1969, the Apollo astronauts benefitted from the precision navigation. Only the AGC could ensure the spacecraft would reenter the Earth’s atmosphere at exactly the right point at the right time.