The Apollo Guidance Computer: History's Unsung Hero

The Apollo Guidance Computer: History's Unsung Hero

Margaret Hamilton posing next to stacks of software she and her team at the MIT Instrumentation Laboratory developed for the Apollo computer.

We’ve all heard that the computer on Apollo 11 had less memory than modern calculators. And that’s true, but the disparities are even more astounding, and on the 50th anniversary of the first moon landing, I wanted to take a gander at the piece of hardware that made Apollo 11 possible, the Apollo Guidance Computer.

Designed at the MIT Instrumentation Laboratory under Charles Stark Draper (with hardware by Raytheon), the AGC was a behemoth, weighing a robust 70 pounds. Engineers toiled at the AGC for over a decade – the equivalent of 2,000 years of labor – and the finished device used about 4,000 ICs from Fairchild Semiconductor (which invented the first germanium IC back in 1959). And while the AGC provided guidance, navigation, and control of the spacecraft, thousands of IBM engineers and technicians not only built the guiding instrument unit embedded in the 3,000-ton Saturn rockets, but sat alongside their NASA counterparts in Houston to help navigate the Apollo spacecraft back to Earth.

And most of you probably know this, but Apollo’s flight software was a lot more analog than modern computers. Margaret Hamilton’s team at the MIT Instrumentation Laboratory spent the equivalent of 1,400 man-years developing Apollo’s software, all of it painstakingly transferred to punch cards for testing.

The AGC was a true technical marvel, possibly the greatest engineering feat of the 20th century. But it’s worth emphasizing what these “steely-eyed missile men” accomplished with relatively primitive technology.

The Apollo computer had 32,768 bits of RAM memory and 72KB of Read Only Memory (ROM), or 589,824 bits. By contrast, modern phones have 3-4 GB of RAM, meaning they have about 1 million times more memory than the AGC.

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The Apollo Guidance Computer, on display at the American Computer and Robotics Museum in Montana

But it gets crazier – the TI-82 graphing calculator, released in 1993, had 28,734 bytes of RAM (the equivalent of about 229,872 bits). So the little gizmo that helps with advanced equations had more memory than the computer that took astronauts to the moon.

What about power? The AGC needed 70 watts at 28 volts DC. Early model Playstations and X-Boxes drew 150 watts (in play mode). Your garbage disposal uses about 450 watts, while the average toaster requires at least 800 watts. Even a can opener needs about 150 watts.

As I’ve demonstrated, we’ve since surpassed the AGC, technologically, many times over, but it was state-of-the-art in 1969, and its success not only excited the imagination of 530 million viewers, worldwide (many of, whom, undoubtedly pursued engineering), but it served as a grand coming-out party for the IC.

Apollo 11 – and especially the Apollo Guidance Computer – became the most high-profile engineering achievement in human history.