Last week was the forty-sixth anniversary of the Apollo 11 moon landing—the first of the six crewed landings on our nearest celestial neighbor. In the years between 1969 and 1972, 12 human beings walked on the surface of the moon: Neil Armstrong, Buzz Aldrin, Pete Conrad, Al Bean, Alan Shepard, Ed Mitchell, Dave Scott, Jim Irwin, John Young, Charlie Duke, Jack Schmitt, and Gene Cernan. Each Apollo landing by necessity leapfrogged the previous by some notable amount, because even as Apollo 11 was preparing to lift off it was obvious that the money wasn’t coming and Project Apollo might be the only chance to visit the moon—perhaps for a long, long time.
Even though Apollo 10’s “dress rehearsal” had taken NASA through all but the final phase of the lunar landing two months before, there were still a large number of unknowns in play when Neil Armstrong and Buzz Aldrin separated Eagle from Columbia, leaving Michael Collins to watch his crewmates descend to the lunar surface—perhaps to stay there forever.
And as it turned out, the first landing on the moon almost did encounter disaster. Shortly after Eagle entered one of the most complicated stages of the descent, the guidance computer began throwing off alarms—very serious alarms, of a type no one in mission control or on the spacecraft was immediately familiar with. Back at MOCR2 in Houston, the burden to determine whether or not the alarms were benign—and therefore the decision to determine whether to abort the landing, blow the Eagle in half, and make an emergency burn to try to make it back up to Columbia—fell on the shoulders of two people: guidance controller Steve Bales and backroom guidance specialist Jack Garman.
No common pocket calculator
It’s an accepted axiom that the Apollo missions flew to the moon on a computer variously described as “less powerful than a pocket calculator” or “less powerful than a digital watch” or other similarly deprecatory statements, but that’s a half-truth. While shockingly primitive at first glance to modern eyes, the Apollo Guidance Computer was a capstone of engineering achievement in the context of the 1960s; further, the software that ran on it was almost miraculously sophisticated by the standards of the day. The executive system—written in part by software geniuses like Hal Laning at MIT—pioneered many of the ideas behind real-time computing, and many of the principles first put into effect in the various revisions of the Apollo Guidance Computer’s software are still used in real-time systems today.
The Apollo Guidance Computer really isn’t a general purpose computer at all. It had no need to address a complicated set of peripherals through some kind of hardware abstraction layer; it had no need to parse English-style commands; it had no high-level programming language to interpret. As explained by Woods in How Apollo Flew to the Moon, the Apollo Guidance Computer can be best understood as a sophisticated embedded controller, built and wedded to the hardware of its host vehicle. The AGC in the Command Module was built to control the Apollo spacecraft as it flew toward the moon, keeping track of where it was and where it was going (in the form of its state vector relative to one of several different points of reference, which changed as the mission progressed).
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One of the non-flown Apollo Guidance Computers, which provided computation and electronic interfaces for guidance, navigation, and control. The actual computer is to the left, and the “DSKY” (display-and-keyboard, pronounced “dis’-key”) is to the right.
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A close-up on the Apollo Guidance Computer display plus keyboard assembly (DSKY). The DSKY contained a five-line electroluminescent display, a number of feedback lights, and a small keyboard. Ground controllers had a lot more insight into the status of the spacecraft than the astronauts did, and this was by design.
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Close-up of one of the core rope modules that contained the AGC’s hardcoded programming. Though hopelessly outdated by today’s standards, core rope storage like this was extremely resilient against many of the hazards of space flight.
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The AGC DSKY in the Lunar Module. Each spacecraft had its own AGC running different software.
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Another view of the LM’s DSKY, with a fold-out panel showing common instructions. Astronauts used the AGC by inputting a system of “nouns” and “verbs,” where verbs (usually) triggered actions and nouns (usually) referred to objects that were acted upon or displayed.
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The Command Module had two DSKYs—one in the main instrument panel and one down in the lower equipment bay. This image, from the command list near the lower equipment bay DSKY, shows the AGC commands most commonly used when aligning the CM’s astro-navigation equipment and sighting in on stars.
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In the event that the Lunar Module’s main guidance computer failed, the crew in the LM would switch to using the Abort Guidance System (AGS) to get them back up to the Command Module. The AGS was a completely separate system and used a much simpler interface, called the DEDA (Data Entry and Display Assembly). Lacking even the DSKY’s system of nouns and verbs, the primitive single-line DEDA operated by letting the Lunar Module Pilot either read from or write to memory addresses directly.
The AGC in the Lunar Module, on the other hand, had an entirely different set of tasks focused on getting the lander out of orbit and onto the lunar surface and then back up again. Though the two computers were similar from a hardware perspective—each using blocks of integrated circuits and rope core memory to store and operate on 16-bit words—each ran very different sets of hardcoded software.
However, one thing in common between the two computers’ software was their basic bifurcated structure. Split into two “halves,” each guidance computer ran a main program called the “Executive,” which O’Brien calls “a priority-scheduled multiprogramming operating system,” and another called the “Interpreter,” which created sort of a “virtual computer” that allowed complex program actions (including work with complex data types, vector math, transcendental functions, and many other things) to be accomplished using the limited hardware instructions available.
It’s the Executive that we’re going to look at very briefly—because the design of the Executive almost certainly saved Apollo 11.
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SA-506, the designation of Apollo 11’s massive Saturn V rocket, at the moment of main engine ignition on July 16, 1969.
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SA-506 lifts Apollo 11 off of the pad and yaws away from the red LUT (Launch Umbilical Tower).
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SA-506 claws its way skyward, powered by five enormous Rocketdyne F-1 engines that gulped one ton of fuel and two tons of oxidizer per second—each. As the Saturn V climbed higher, atmospheric pressure dropped and the exhaust plume expanded enormously, even climbing forward up the back of the first stage.
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