Glen Stephan
Sep 2, 2018 · 5 min read

The REAL Apollo Missions at 50 - Part 1: Navigation computing

The facts debunking the myth that in the 1960s we just didn’t have computers good enough to run a spacecraft to the moon.

With less than a year until the 50th anniversary of the Apollo 11 moon landing, it’s time to learn the true facts that debunk and destroy every argument the various flavors of deniers have bullet pointed as "proof" that the Apollo program was faked, a scam pulled on the American public and the world.

The Myth
Today we’ll start off with the denier claim that in the mid to late 1960s the computer technology just did not exist to stick a mobile computer that was both small enough and smart enough to control and navigate something as sophisticated as a trip to the moon on board a cramped spacecraft that could barely hold three astronauts.

Computers back then were the size of entire rooms, ran on tubes, and could barely do what a calculator did ten years later. And yet we were allegedly able to use an on-board computer to navigate a spacecraft to the moon? Hogwash! Fake news!

The Short Answer
In fact, the Apollo program had no shortage of computing power for their moon missions. Each spacecraft actually had multiple state of the art on-board computer systems comparable in power to the first personal computers commercially available to the public twenty years later. Moreover, these systems were backed up by an entire floor of the Mission Control building in Houston dedicated to a computer room that ran several of the fastest IBM mainframe computers available at the time. These mainframes did most of the navagational computing heavy lifting for the spacecraft.

The Apollo Guidance Computer
The on-board navigation computers on Apollo (there were two of them, one for the command module and one for the lunar module) were custom designed and built using the latest (at the time) integrated circuit and core memory technologies, and were plenty smart enough to calculate position and navigation using the fairly basic math of Newtonian mechanics.

The Apollo Guidance Computer, or AGC, and its DSKY (display and keyboard) user interface that were installed onboard the Apollo spacecraft were custom developed specifically for Apollo by the MIT Instrumentation Laboratory and built by Raytheon Corporation in the early '60s. They were, as is typical of what comes out of MIT’s labs, custom state-of-the-art technology for the time. The AGC was one of the first integrated circuit-based computers, a true 16bit computer built of RTL (resistor-transistor logic) integrated circuit technology, with 32K of software in a read-only core rope memory and an additional 2K of read-write core memory, all clocking in a clock speed of one megahertz.

Core memory - which is a basically a physical matrix of wires strung through magnets - is rather bulky and heavy, so while the AGC, which was about the size of a small, carry-on suitcase, had specs very roughly comparable to the first generation of personal computers such as the Altair 8800 or Imsai 8080 some ten years later, it weighed in at a hefty 70 lbs, much heavier and less portable than it’s commercial, fully integrated circuit memoried descendants.

Every Apollo moon mission except Apollo 8 was equipped with two AGCs, one in the Command Module and one in the Lunar Module (Apollo 8 had no Lunar Module). Each of these computers was programmed with the Apollo PGNCS (Primary Guidance Navigation Control System), referred to as "Pings".

"Pings" was a self-contained inertial guidance system that allowed Apollo spacecraft to navigate on it's own whenever communication with Earth was lost, as when going around the back side of the moon, or when, as when actually landing on the moon, too much needed to be monitored too fast in real time to depend upon Earth.

But for all that, Pings and the AGC were actually not the main navigation computers guiding an Apollo mission. Primary navigation for Apollo spacecraft was actually done here on Earth!

Apollo spacecraft position and velocity were tracked by using data from NASA/JPL’s Deep Space Network (the ground dishes used to communicate with spacecraft) and analyzing that data with the computers back at Mission Control. The resulting position calculations were actually more accurate than those produced onboard Apollo with Pings and it’s inertial guidance measurements. These measurements from the ground were periodically uploaded to the spacecraft and inputed into the onboard AGCs as refined position and velocity vector data.

Those computers at Mission Control in Houston were located in the Real Time Computer Complex (RTCC). Initially there were five IBM Model 7094 mainframe systems there, but shortly into the Apollo program they were upgraded to five IBM System/360 Model J75 mainframes, all running the latest real-time versions of the IBM OS/360 operating system, with applications programming written in Fortran. Those computers had a total storage capacity of about 1 megabyte each and could execute a million instructions per second. These computers interfaced to the controllers in Mission Control (the part you always see on TV and in the movies) via several Univac 490 and (later) 494 mainframe systems.

During each flight, one computer was used as support (Dynamic Standby), i.e. that it performed the same operations as the other flight control computer, but was used as emergency system. Another computer was used for commands simulation.

So…?

Most of this detailed information on Apollo’s computing capabilities will be lost to the hard core deniers as just more "blah blah blah", but for those on the fence or honestly unsure folks who might be swayed by the denier’s argument, there’s sure to be enough in here to sway them back to reality.

All images are public domain or CC licensed and courtesy of NASA, Jet Propulsion Laboratories (JPL) or Wikipedia.

Glen Stephan
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