Nuts & Volts - September 2005

Micro Memories

by Edward Driscoll, Jr.

Micro Memories

The Computer That Took Man to the Moon

Even though the last Saturn V flew some 30 years ago, the Apollo moon landings remain one of mankind’s greatest engineering efforts. What makes them even more impressive was the era in which they were designed.

During the 1960s, computers occupied whole rooms, but had less power than the PC now sitting in the den of your home. There weren’t even pocket calculators back then. And yet, on September 12, 1962, in front of a packed crowd in Rice University’s football stadium, President Kennedy was willing to propose the building of “a giant rocket more than 300 feet tall, the length of this football field, made of new metal alloys, some of which have not yet been invented, capable of standing heat and stresses several times more than have ever been experienced, fitted together with a precision better than the finest watch.”

At the top of the Saturn was the Apollo spacecraft. And one of the

most important components inside of the Saturn was its guidance computer. As Charles Murray and Catherine Bly Cox noted in their seminal classic Apollo: The Race to the Moon (Simon & Schuster, 1989), “The computer capacity of the mainframes in the Control Center [of NASA’s Manned Spacecraft Center in Houston] was smaller than that of the desktop systems of the 1980s, and onboard computers in the command and lunar modules had less capacity than some pocket calculators.”

Overflowing the Boxes

The "Block 2" version of the Apollo Guidance Computer, designed in 1966, increased erasable memory from 1K to 2K words. Fixed memory was expanded from 24K to 36K.

“Overflow the boxes” was NASA-speak for putting in more requirements than computer memory and computing capacity could handle. That wasn’t very difficult with the Apollo Guidance Computer (AGC), which in retrospect looks pitiful in comparison to today’s Windows XP-equipped PCs with their GHz of memory, 3 GHz processors, and 300 GHz hard drives.

In contrast, the Apollo Guidance Computer, which weighed 70 pounds, had only 36K of RAM and

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2K of ROM. But it was able to guide 27 men to the moon’s orbit and bring them back safely. It consisted of two parts: a small keyboard for the astronauts inside the command and lunar modules, and a separate, slightly larger logic unit.

The AGC was built by Raytheon and used approximately 4,000 discrete integrated circuits from Fairchild Semiconductor. (There were also two other onboard computers on a moon mission — a flight computer onboard the Saturn V booster rocket, and a separate backup computer on the lunar module that could be used in an emergency for a liftoff from the moon should the AGC fail.)

The limitations of the Apollo Guidance Computer resulted in some classic push-pull arguments from Apollo’s flight controllers and engineers trying to fit routines too complex into the limited capacity of these computers. Murray and Cox quote Cliff Charlesworth, one of Apollo’s flight directors, as saying, “Every time we’d get a new capability in the computer systems, the flight controllers would start laying their requirements on it. In short order, we’d overflow the boxes. And [Chris]

Kraft [NASA’s legendary first flight director] would get mad. He’d say, ‘Goddammit, get it back to where it’ll fit, we can’t get any more computers!’”