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The Apollo Guidance Computer

Updated on October 19, 2013
Astronaut Edwin E. Aldrin during checkout and inspection of the Apollo 11 Lunar Module. The AGC can be seen at lower right.
Astronaut Edwin E. Aldrin during checkout and inspection of the Apollo 11 Lunar Module. The AGC can be seen at lower right. | Source

In 1962, when President John F. Kennedy announced that the United States would put a man on the Moon and return him safely to Earth by the end of the decade, he set in motion one of the greatest feats of human engineering of all time. While the challenges of building rockets and spacecraft and docking ports and life support systems were daunting, these challenges paled in comparison to the technological hurdle presented by the project.

Guiding a spacecraft into lunar orbit and then landing on the Moon required some sophisticated computing power. However, computers of the time were still room-sized affairs, not easily loaded onto a cramped space capsule. This being the height of the Cold War, the threat of Soviet interference ruled out the possibility of ground-based computers performing all the calculations necessary for guidance and navigation and transmitting the data via radio.

This meant that the computing for guidance and navigation would have to be done aboard the Apollo command module and lunar lander. And due to the constraints of space flight, it would need to be done on extremely compact, lightweight, resilient, and reliable computers that used less power than a standard light bulb.

This was the challenge presented to Charles Draper and the Massachusetts Institute of Technology (MIT) Instrumentation Laboratory in 1961 when they began to design the Apollo Guidance Computer.

DSKY Noun-Verb code "cheat sheet" printed on the Command Module control panel
DSKY Noun-Verb code "cheat sheet" printed on the Command Module control panel | Source

The iPhone 5 vs. the Apollo Guidance Computer

iPhone 5
Apollo Guidance Computer
4.9x2.3x0.3 in (12.3x5.8x.7 cm)
24x12.5x6.5 in (61x32x17 cm)
3.9 oz (112 g)
70 lbs (32 kg)
1136x640 pixel retina display
seven-segment numeric indicators
Memory (RAM)
2,048 words
Power Use
0.7 watts
55 watts
$150,000 (estimated)

PCNGS, DSKYs, and LOL Memory

The hardware of the Apollo Guidance Computer (AGC) was state of the art when the design project began, but by the time the missions flew in the late 1960s and early 1970s the private technology industry had long since caught up. The AGC was one of the first to use integrated circuits - circuits printed onto a silicon chip rather than wired together from discrete components. This was a risky gamble at the time as the technology had only been patented in 1959, but necessary to meet the computer's size and weight requirements. Many scholars of computer history credit the Apollo program's pioneering use of integrated circuits in the guidance computer as influencing the adoption of this technology that now lies at the heart of all the electronic devices we use today.

The computers were manufactured by Raytheon Corporation based on MIT's design, and a total of 75 Apollo Guidance Computers were built. One each flew on the Command Module (CM) and Lunar Module (LM) of each mission to the Moon, while others were used on the unmanned test flights and in the training simulators for the Apollo missions. The design of the AGC underwent an upgrade during the course of the Apollo program, from the Block I version that flew on the unmanned Apollo 4 and 6 missions to the more powerful Block II version that flew on the manned flights.

The Apollo Guidance Computer was responsible for monitoring all aspects of the Primary Guidance, Navigation and Control System (PGNCS) for the Command and Lunar modules. This included tasks from calculating the vehicle's speed and orientation to aligning the star tracker navigation system to monitoring radar altitude from the lunar surface and distance between vehicles during docking to firing the thrusters and main engines.

The interface for the Apollo Guidance Computer was the DSKY, an abbreviation of DiSplay KeYboard. The display portion consisted of an array of seven-segment numeric displays (the kind you might see in a digital alarm clock) and a panel of indicator lights. The keyboard itself was calculator-style with some additional function keys such as Enter , Reset , Noun , and Verb , which were used for entering and executing commands.

Astronauts commanded the computer using the Verb or Noun key followed by a two-digit code. Verbs referred to the type of operation to be performed and Nouns were the piece of data to perform the "Verb" operation on. In order to request for the change in velocity needed for an upcoming engine burn, for example, the commander would type Verb 06 to command the computer to display in decimal numbers, then Noun 84 to request the velocity vector. During a typical mission to the Moon, astronauts would use more than 10,000 keystrokes. Though the system seems cumbersome in our current era of point-and-click graphical interfaces, the Apollo astronauts found it very easy to use -- once they'd mastered it over many hours of simulator practice.

Perhaps the most fascinating component of the Apollo Guidance Computer was the core rope read-only memory that served as the AGC's operating system. The programming of the computer's many operations was literally hard-wired into the computer circuits. Thousands of tiny wires were woven by hand to either pass through or bypass a magnetic core to signal a one or a zero in binary code, respectively. Since fixing a bug in the code would involve extensive rewiring, it was critical that the MIT programmers write the software correctly the first time. To manufacture these memory cores, Raytheon hired dozens of highly skilled workers - primarily women with many years of experience as textile workers and watchmakers. The core rope memory units were affectionately dubbed LOL Memory, or "Little Old Lady Memory," in recognition of these dedicated factory workers.

To The Moon And Back

By today's standards, the Apollo Guidance Computer seems cumbersome, confusing, and ridiculously antiquated. This has led many moon-hoaxers to cite it as "evidence" that NASA never sent astronauts to the Moon at all. At the time of its manufacture, however, it was a giant leap forward in computer technology, helping to spawn innovations that led to the devices we take for granted today. This clunky, 70-pound technological marvel was essential to fulfilling President Kennedy's vision -- putting twelve astronauts on the surface of the Moon, and returning all the astronauts of Apollo safely to Earth.


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    • profile image

      Murat 16 months ago

      Is that memory as in hard drive or memory as in RAM in the comparision table. it is a bit confusing

    • somethgblue profile image

      somethgblue 5 years ago from Shelbyville, Tennessee

      And your point was what exactly . . . ?

    • Silwen profile image

      Silwen 5 years ago from Europe

      Well, this is true. The capabilities of computers of those times were really small. But on the other hand, computer were more reliable. The low level programming languages such as assembler used every bit of memory in the way it was needed. Today's computing has one big problem. To much power and to much memory. Operating systems became main usurpers of computer's memory. In many cases we do not need as much resources as are used.

      To confirm that I would suggest you explore some micro controllers. Some of the use quite small amounts of memory but are capable to control various devices such as washing machine, TV and others. Yes, some systems are difficult, but there are more, which are quite simple and need small amount of resources. Everything depends on the way the program is written. The high level programing languages uses translators that translates programming code into machine code. The machine code, generated by the translator can be for e.g. 1 kB long, while the same code written in low level programing language such as assembler may be only 512 B long.

    • MyWebs profile image

      Anthony Goodley 5 years ago from Sheridan, WY

      Thanks for the interesting hub. You did a great job of turning a bit of history into a fascinating read.

      It's amazing the programmers were able to pull off so much with such a limited amount of memory.