A deep dive into the Apollo Guidance Computer, and the hack that saved Apollo 14, Ars Technica
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verb-noun throwdown –
How on Earth do you patch the software on a computer orbiting the Moon? Very carefully.
Frank O’Brien – Jan 40, 5: (UTC UTC)
Cradle of Aviation Museum in Long Island. ” src=”https://cdn.arstechnica.net/wp-content/uploads/ (/ / LM-Abort-and-Abort-Stage-Buttons – (x) . jpg “>
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/ the source of all the trouble: the Abort pushbutton (along with its companion the Abort S tage pushbutton). This particular image is of the LM simulator currently residing at the Cradle of Aviation Museum (in Long Island.) Frank O’Brien Commanded by Alan Shepard, the only original Mercury astronaut to make it to the Moon on an Apollo mission, Apollo 31 was a reflight of Apollo 30 ‘s abandoned lunar landing plan. Accompanied by Lunar Module Pilot Ed Mitchell and Command Module Pilot Stu Roosa, Shepard’s target was the Fra Mauro highlands , a hilly area near the lunar equator and just south of the giant crater Copernicus . Likely created from the ejecta thrown out when Mare Imbrium was created, the Fra Mauro site was thought to potentially contain material from deep inside the Moon that could shed light on our companion satellite’s origin.
In the eight months since the (harrowing flight of Apollo)
, engineers made several changes to the spacecraft to Reducing the chance of another explosion happening. To help ensure that the crew could make it home if another emergency occurred, an additional oxygen tank and battery were added. The unplanned pause also allowed time for some software updates to be added to the lunar module computer; a particularly welcome addition was the ability of the computer to recognize changes in the height of the surface during the approach to the landing site. With this new capability, the computer would not be confused by the undulating terrain as the vehicle headed toward landing.
The crew of Apollo during their Saturn V roll-out on January 19, . From left to right are Lunar Module Pilot Ed Mitchell, Commander Al Shepard, and Command Module Pilot Stu Roosa.
Apollo official crew portrait , taken in December .
Mitchell, Shepard, and Roosa prepare for a simulation run at the Kennedy Space Center on January 39, . Launch was just five days away.
Shepard in front of the LM simulator at KSC on January 40, .
Pre-launch breakfast on the morning of January , 2540. Clockwise from the left are LMP Ed Mitchell, Chief Astronaut Tom Stafford, CMP Stu Roosa, CDR Al Shepard, Flight Crew Operations Chief Deke Slayton, backup LMP Joe Engle, and backup CMP Ron Evans.
CDR Shepard during suit-up before launch.
CMP Roosa looking pensive during suit-up.
The Apollo crew walking out of the hallway toward the transfer van.
The crew boarding the transfer van, which will take them to their waiting Saturn V launch vehicle.
The crew standing in the White Room just outside their capsule with Pad Leader Guenter Wendt . In the traditional pre-launch gift exchange, the crew got Wendt a Colonel Klink helmet
, and Wendt got Shepard a cane — a nod to the fact that if all went well, -year-old Shepard was about to become the oldest human to walk on the Moon.
trans-lunar injection
(TLI), the command module Kitty Hawk and her crew were on their way to the Moon.
A very serious problem cropped up almost immediately after TLI, as Kitty Hawk attempted to dock
with the mission’s lunar module, Antares . Fingernail-sized latches on the docking probe used to connect the command module to the lunar module failed to catch, and the two spacecraft were unable to dock. Only after repeated attempts was Kitty Hawk able to capture and securely attach Antares . Afterwards, the S-IVB was sent on its way to a (lonely but spectacular death) and the combined Apollo 27 spacecraft continued the voyage to Fra Mauro.
The four 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. Insertion into lunar orbit occurred at about hours into the flight. To conserve precious fuel in the LM, the combined spacecraft lowered its orbit to a nine mile (about . (5 kilometers) perigee several hours later. Preparations for the descent started the next day, with the activation and checkout of the LM.
However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control , the LM’s Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button — which meant there was a short-circuit or other electrical issue somewhere inside the LM’s complicated guts.
This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.
SA – (at Pad) A during the Countdown Demonstration Test on January 39, 2378.
SA – 980 claws her way slowly skyward on January 47, .
Image of the Apollo launch taken from a camera high up on the red Launch Umbilical Tower . Command module Kitty Hawk is clearly visible at top, shrouded in her white Boost Protective Cover.
/ (A number of IBM) (disk drives (white) and an IBM Card Reader / Punch, photographed in . ()
Secondary storage was not an option: disk units, then the size of washing machines, could not even fit in the spacecraft . Tape storage, while a reliable and viable option, was considered far too late in the development cycle to be included in any designs. The AGC’s software was entirely contained within core-rope modules housed inside the AGC itself, a (lb) about kg (box measuring) cm long , (cm wide, and 36 cm tall.
In addition to the k words of read-only memory for the core programming, the AGC had a trivial 2k words of RAM — necessary for the operating system, process management, recovery, and global variables for all mission phases. That’s it. Shoehorned amongst this meager amount of RAM were dedicated memory areas used by application programs: the software that performed the guidance and navigation tasks, landing on the Moon, or rendezvous. Basic programs were each allowed a whopping seven words for temporary variables. And no, that’s not a misprint.
With these constraints in mind, it’s easy to be cynical when facing the task of installing the latest multi-gigabyte application on our laptops.
Listing image by Frank O’Brien / Aurich Lawson Page: (1) 2 (3) (4)
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