A deep dive into the Apollo Guidance Computer, and the hack that saved Apollo 14

verb-noun throwdown–.

How on Earth do you patch the software on a computer system orbiting the Moon? Extremely carefully.

Frank O’Brien


Frank O’Brien


In the eight months considering that the painful flight of Apollo 13, engineers made a number of changes to the spacecraft to minimize the chance of another surge happening.
The unintended pause also permitted time for some software application updates to be contributed to
the lunar module computer; an especially welcome addition was the capability of the computer to acknowledge changes in

the height of the surface area during the technique to the landing website.
With this brand-new ability, the computer would not be confused by the undulating surface as the automobile headed towards landing.


Apollo14 main team picture, taken in December1970


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    Mitchell, Shepard, and Roosa get ready for a simulation perform at the Kennedy Area Center on January26,1971 Launch was simply five days away.

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    Pre-launch breakfast on the morning of January31,1971 Clockwise from the left are LMP Ed Mitchell, Chief Astronaut Tom Stafford, CMP Stu Roosa, CDR Al Shepard, Flight Team Operations

    Chief Deke Slayton, backup LMP Joe Engle, and backup CMP Ron Evans.

  • .

    CDR Shepard during suit-up prior to launch.

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    The Apollo14 crew leaving of the hallway towards the transfer van.

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    The team boarding the transfer van, which will take them to their waiting Saturn V launch vehicle.

    The team standing in the White Room simply outside their capsule with Pad Leader Guenter Wendt In the traditional pre-launch present exchange, the crew got Wendt aColonel Klink helmet, and Wendt got Shepard a walking stick– a nod to the reality that if all worked out, 47- year-old Shepard was about to end up being the earliest human to walk on the Moon.

  • What is past is prologue

    In the afternoon of January 31, 1971, the flight roared far from the Kennedy Space Center on its Saturn V launch lorry after just a short 40 minute hold for weather condition. After rebooting the S-IVB 3rd stage for trans-lunar injection( TLI), the command module Kitty Hawk and her crew were on their way to the Moon.

    A very serious issue surfaced practically instantly after TLI, as Kitty Hawk attempted to dock with the mission’s lunar module, Antares Fingernail-sized latches on the docking probe used to link the command module to the lunar module failed to capture, and the 2 spacecraft were unable to dock. Only after duplicated efforts was Cat Hawk able to record and safely connect Antares Later on, the S-IVB was sent on its way to a lonely but magnificent death and the combined Apollo 14 spacecraft continued the voyage to Fra Mauro.

    The 4 days in transit and the time spent in lunar orbit were uneventful– or at least as uneventful as a flight to the Moon could be.

    However, less than 4 hours prior to the set up landing, controllers noticed that according to the indicators on their consoles in Objective Control, the LM’s Abort pushbutton appeared to have actually been pressed. When asked via radio, Shepard validated that no one on board Antares had pushed the Abort button– which meant there was a short-circuit or other electrical problem someplace inside the LM’s complicated guts.

    This was potentially a mission-ending problem: if the button was pushed and the engine was shooting, the LM would immediately start its abort treatment as soon as the lunar descent started, making a landing difficult.

    • SA-509, the Saturn V that would send Apollo 14 to the Moon, rolls out of the VAB on the Crawler-Transporter, headed to Pad 39 A This photo was handled the early morning of November 9,1970


    • SA-509 at Pad 39 A during the Countdown Presentation Test on January 19,1971


    • SA-509 claws her method slowly skyward on January 31,1971


    • Picture of the Apollo 14 launch taken from a camera high up on the red Release Umbilical Tower Command module Kitty Hawk is clearly noticeable at leading, shrouded in her white Increase Protective Cover.

    • The 3rd phase of SA-509 vents propellent simply prior to LM extraction. Lunar module Antares shows up still nestled into the Spacecraft Lunar Module Adapter area at the front of the phase.

    • Wide-angle of Mission Operations Control Room # 2 at the Manned Spacecraft Center in Houston throughout LM extraction troubleshooting. Keep in mind the right-most screen at front of room, showing Antares still awaiting extraction from the S-IVB.

    Under difficult time pressure, the ground had to rapidly determine what was incorrect and devise a workaround. What they came up with was the most dazzling computer hack of the entire Apollo program, and potentially in the entire history of electronic computing.

    To describe exactly what the hack was, how it functioned, and the issues dealing with the developers throughout its development, we need to dig deep into how the Apollo Guidance Computer worked. Keep your hats, Ars readers– we’re going in.

    The Apollo Guidance Computer system laid bare

    It’s common to discover that the AGC is typically explained as a simple calculator, or compared to a controller chip appropriate for a watch or microwave.

    In describing a “computer system,” one anticipates that the system would consist of the abilities we attribute to modern computer systems– the ability to run a number of programs at once, for example, or to present an easy yet user-friendly user interface, to control a wide variety of gadgets, and to with dignity recover from application mistakes.

    The concept of such capabilities being readily available almost 60 years ago stretches credulity, but the Apollo Guidance Computer system had these functions and more. Yes, the AGC is slower and has far less memory, but that is only due to its regrettable timing at birth, being at the incorrect end of the Moore’s Law curve.

    Although the processor at roughly 80,00 0 instructions per second was not especially fast, it is difficult to overemphasize the effect that its scarce memory had on AGC software developers. Think about the limitations the programmers were under: all the software application for the flight to the Moon and back had to suit 36 K words (15 bits long, plus 1 bit for parity) of read-only core rope memory As “bytes” were not a concept in the AGC, all 15 littles a word were accessed at the same time with no simple way to break the word into smaller divisions.

    A number of IBM 2314 disk drives (white) and an IBM 254src Card Reader / Punch, photographed in 1968.

    Enlarge/ A number of IBM 2314 hard disk (white) and an IBM 2540 Card Reader/ Punch, photographed in 1968.

    Secondary storage was not an option: disk systems, then the size of washing makers, could not even suit the spacecraft. Tape storage, while a trusted and feasible choice, was thought about far too late in the development cycle to be included in any styles. The AGC’s software application was completely consisted of within core-rope modules housed inside the AGC itself, a 70 lb (about 32 kg) box measuring 61 cm long, 32 cm large, and 17 cm high.

    In addition to the 36 k words of read-only memory for the core programs, the AGC had an insignificant 2k words of RAM– required for the operating system, process management, healing, and worldwide variables for all objective stages. Inserted amongst this meager amount of RAM were dedicated memory areas utilized by application programs: the software that performed the guidance and navigation jobs, landing on the Moon, or rendezvous.

    With these restraints in mind, it’s easy to be cynical when facing the job of installing the current multi-gigabyte application on our laptops.

    Listing image by Frank O’Brien/ Aurich Lawson

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