NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.

Notes for the 2005 MAPLD International Conference Invited Mishap and Lessons Learned Talk: "Computer Overload and The Apollo 11 Lunar Landing," part of Session G: "Digital Engineering and Computer Design: A Retrospective and Lessons Learned for Today's Engineers"   --  Presentation

Jack receiving an award from Chris Kraft.

NASA S-68-32272

AGC Support Console

Jack Garman is the second from the end on the left, front row.  The fellow with the heavy framed glasses, coat and tie, standing behind the center front-row console is Steve Bales.

NASA S-69-34209

Pencil-written checklist that was under the Plexiglas on Garman's console during the Apollo 11 landing.  This was the official result of the quick study of all alarms we did following the aborted simulation just a few weeks prior to the flight.

"No word processors, just typewriters, ergo no easy way to make drawings, tables, etc., except by hand on grid-paper so we did!"

Jack receiving an award from Alan Shepard.  George Low is in the middle of this picture.

NASA S-71-24439

Console Audio of Apollo 11 Landing

Audio File Size (megabytes) Compression
  apollo_11_descent.mp3 60 Low
  apollo_11_dsc.mp3 15 Medium
apollo_11_dsc2.rm apollo_11_dsc2.mp3 3 High

Additional Reading

Tales from the Lunar Module Guidance Computer

Don Eyles
27th Annual AAS Guidance and Control Conference
February 4-8, 2004, Breckenridge, Colorado
Paper # AAS 04-064


The Apollo 11 mission succeeded in landing on the moon despite two computer- related problems that affected the Lunar Module during the powered descent. An uncorrected problem in the rendezvous radar interface stole approximately 13% of the computer's duty cycle, resulting in five program alarms and software restarts. In a less well-known problem, caused by erroneous data, the thrust of the LM's descent engine fluctuated wildly because the throttle control algorithm was only marginally stable. The explanation of these problems provides an opportunity to describe the operating system of the Apollo flight computers and the lunar landing guidance software.

Recovery from Transient Failures of the Apollo Guidance Computer

Edward M. Copps Jr.
August 1968 

     In the Apollo Guidance Computer, nearly one hundred thousand word transfers occur each second. A random error rate of one in 1012 actions would be considered good in today’s technology, but at that rate an error might occur within several hundred hours. Added to the random error rate are externally induced errors, including power and signal interface transients, program overloads, and operator errors. The incidence of induced errors has so far been very much greater than random errors (if indeed there have been any at all) in AGC experience.
     The AGC is a control computer, and has been designed to detect and to recover from random or induced transient failures. The techniques used are the subject of this paper.
     The paper outlines the character of the computer and its system software to a depth sufficient for the discussion. The built-in malfunction alarm logic is discussed, along with the software based computer Self Check. These systems, upon detection of a failure, force an involuntary reordering of the signal interface, and an involuntary transfer to the restart program logic. From this point, the software, without recourse to suspect information in the central processor, reconstructs the output interface conditions, and reconstitutes the control processes in progress at the time of failure. There are a number of ground rules which limit the recovery capability, based on presumptions about the nature of the failure, These are presented.
     The software associated with a restart is described, with a typical program flow derived from an Apollo Mission Program. The amount of memory assigned to restart protection is stated. Several interesting sidelights are briefly discussed such as manual break-in to prevent restart looping, the adoption of the restart technique for scheduling the termination of active programs, and the use of the failure recovery technique to remove a temporary computer overload.

Journey to the Moon: The History of the Apollo Guidance Computer

Eldon C. Hall, Reston VA: American Institute of Aeronautics and Astronautics, 1996, 221 pages, ISBN 1-56347-185-X (softbound).

See pp. 165-166 and 179-180. 

Notes: Details history and design of the computer that enabled Apollo astronauts to land on the moon. Written for experts as well as lay persons. Contents: History, Computer Hardware, Computers, MIT Instrumentation Laboratory, Apollo Hardware, Requirements, In the Beginning-Apollo Computer, Winds of Change were Blowing, Block I Computers, System Integration, Naysayers and Advice from Outside Experts, Next Generation-Block II, Naysayers Revisited, Reliability, Apollo Software, Software Development, Mission Software, and Finale.

Annotations to Eldon Hall's Journey to the Moon.

In February of 1997, Hugh Blair-Smith wrote a series of annotations to Eldon Hall's book about the history of the AGC.

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