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.

Miscellaneous Apollo Memos and Documents

The documents below are courtesy of Eldon C. Hall.

Integrated Circuits in the Apollo Guidance Computer

Eldon Hall

   The decision, in 1962, to design the AGC using integrated circuit logic devices was critical to Apollo Computer’s success and a key moment in the history of computing. Eldon Hall's Journey to the Moon recounts this decision process.
   Following are copies of integrated circuit purchase orders for components required in the evaluation processes and the view graphs used to report the evaluation’s conclusions to the NASA Program Office.

Adequacy of the Guidance and Control System

To: Honorable James Webb, Administrator, National Aeronautics and Space Administration

From: Joseph E. Karth, House of Representatives, Washington, D.C.

Date: February 15, 1965


As you are aware, several months ago, I raised certain questions with NASA regarding the adequacy of the guidance and control system presently scheduled for the APOLLO and Lunar Excursion vehicles.  It was my understanding at the time that the MIT system had insufficient flexibility, capability and reliability to best accomplish this mission.

Considerable time has now elapsed since I initially expressed my concern.  As best I can determine, little progress has been made in solving these problems.  The latest information I have indicates that while some work has been done to improve the system, it is still inadequate and a replacement system, or at least a capable backup system, should be developed.  It is also my understanding that the system is heavy and expensive.

I believe it would be most helpful if you would answer several questions listed below which have been bothering me for some time.


To: Honorable Joseph E. Karth, House of Representatives, Washington, D.C.

From: James E. Webb, Administrator

Date: March 8, 1965

Cover Letter

The attached information is submitted in reply to the questions you raised on the Apollo and LEM guidance systems in your letter of February 15, 1965.  It has been classified CONFIDENTIAL because of the information contained in the answer to Question No. 5.

I am most appreciative of your continued interest in our Apollo Program, and can assure you that we are doing everything possible within our resources to guarantee the success of this program.

Should you have additional questions, I would be pleased to provide whatever information you desire.

Portafam System Operational Description

Jonathan Leavitt
July 1969

The purpose of this report is to provide an operational description of the Portafam system. Special attention is paid to the master control logic and front panel controls.
Portafam is designed to provide a compact electrically alterable core rope simulator to replace the fixed memory or rope memory in the Apollo guidance computer (AGC). The construction and operation of this system is described on a subsystem level.  The master control logic controls the operation of all subsystems in Portafam and therefore an understanding of this subsystem logic is necessary to operate the complete system. This logic is described in detail including a description of the interfacing pulses to other Portafam subsystems.   Front panel operation procedures are also analyzed in order to further explain the proper operation of the complete system.

Auxilary Memory System Final Report on Phase I

D. J. Bowler
April 1968

Summary of the Study to 8 February 1968
The major effort to date is reported in Appendix A. This section will try to summarize that appendix to tie the AGC and the Auxiliary Memory (AM) into the Block 2-1/2 entity that they are. The Block 2-1/2 extension is appropriate because the AM has access to the bus structure of the AGC and its proposed addressing structure is an augmentation of the bank concept already in use in the AGC.
     This combination can make the AGC into a more advanced and flexible processor. We shall demonstrate this flexibility in six steps. In fact, the major decisions to be made will be to decide how much of this flexibility one does want. At the conclusion of this section is a list of contributions or results of this study.

Table of Contents

1 Introduction
2 Summary of Study 8 February 1968
   2.1 Multi-job, Multi-task Capability
   2.2 Address and Control Flexibility
   2.3 Interface and Multiprocessor Capability
   2.4 Relocation
   2.5 Additional Functions
   2.6 Programming Impact
   2.7 Advantages of the Auxiliary Memory
3 Specific Contributions of this Study
4 Use of the Raytheon Built ACM Prototype
5 Decision Areas
6 Future Auxiliary Memory Work

APPENDIX A APM 1818, Auxiliary Memory Progress Report #1
APPENDIX B D.D. Menlo #381, Reliability Aspects of Auxiliary Memory

Technical Evaluation of the Raytheon Proposal for Mechanical Redesign of the AGC, Contract NAS 9-498

To: Mr. J. Epperly, APCAN

From: David W. Gilbert; Manager, Spacecraft Systems, Guidance and Control

Date: retyped August 23, 1963


The following is submitted in response to your request for a Technical Evaluation of the task related to the subject proposal:

Contract NAS 9-153; LEM Guidance and Control configuration

To: Massachusetts Institute of Technology/Instrumentation Laboratory, MB. Trageser, Director, Apollo G&N Program

From: David W. Gilbert; Manager, Spacecraft Systems, Guidance and Control

Date: November 30, 1964


This transmittal serves to define the integrated guidance and control system desired by the MSC.  The selected configuration is based on the results of the joint effort undertaken by GAEC, MIT, and MSC during the recent program definition phase to optimize the design and operational flexibility of the LEM guidance and control system.

The configuration selected is defined herein in terms of basic features and of changes desired to the pre-definition phase mechanization.  The other details of the system and its peripheral functions and interfaces remain as defined in the minutes of the implementation meetings.

A primary control path and an independent abort contro path, both having manual control capability, are to be provided.

PROJECT APOLLO GUIDANCE AND NAVIGATION: A Proposal for a Research, Development, and Space Flight Program

Enclosure "A"
August 4, 1961

   This program will provide support for project Apollo in the general area of guidance and navigation.  In particular, this program will be concentrated on the development and employment of the self-contained guidance and navigation system which the Apollo crew will use aboard the spacecraft.
   Participating in all stages of Project Apollo from the early earth orbital flights through the lunar landing missions will be included in the proposed program.  Participation in the earliest space flights will be centered around two objectives: the development and perfection of the on-board guidance and navigation techniques; and the provision of a source of guidance and navigation information to be either primary or secondary depending on the use of the Mercury ground system for these flights.
   The scope of this program ranges from the development of the basic understanding of the phenomena used in on-board guidance methods to providing guidance and navigation equipment for space flight use.  Between these limits are many areas such as guidance concept development, error analyses, technique development, component design, testing, etc.
The program will accept the responsibility of providing the appropriate guidance and navigation subsystems for the various stages of Project Apollo.  These systems will be provided completely check-out and flight ready in sufficnet number to fulfill the guidance and navigation requirements.  Assistance will be given in the proper installation of these subsystems on the spacecraft and in the conducting of the space flights with the on-board guidance and navigation systems.
   Carrying out the above responsibilities will involve close cooperation with the various NASA centers and with the other Project Apollo contractor(s) and subcontractor(s).  In addition it will be advantageous to capitalize on the capability of industry to produce numbers of guidance components, etc., through the subcontract or through technical direction of prime contracts.


Jayne Partridge, L. David Hanley, and Eldon C. Hall
November 1964

   An approach to reliable systems design through standardization is given.  The approach which allows systems to use predominately the same simple integrated circuit also allows the development and maintainability of the high reliability of integrated circuits.  The approach will be justified by the presentation of integrated circuit failure modes detected to date.  The integrated circuit failure modes will indicate present reliability problems, the need for detection, monitoring, and elimination of the problems, and the necessity of reliability evaluation among suppliers.  the success of the approach will be demonstrated by extended use failure rate data made available through the Apollo program.

Electronic navigator charts man's path to the moon

Albert L. Hopkins
Electronics, January 9, 1967
pp. 109-118


Apollo, our boldest step into space, will be guided by a computer that is both elementary and advanced.  Its memory holds a fixed program but its applications are diverse and flexible.


R-500, Volume 2 of 2
June 1965

PREFACE (Excerpt)
   The material in this book was assembled to support a series of lectures to be given by the authors in Europe in June 1965, under the sponsorship of the Advisory Group for Aerospace Research and Development, an agency of NATO.
   The general subject of Space Vehicle Control Systems is the subject of discussion with particular application to the present Manned Lunar Landing Program.  The man-machine interaction along with requirements of the mission are first described.  These mission requirements in terms of specific hardware along with the performance requirements and underlying reasons for choice are next explained.  Lastly, the theoretical bakground, the system analysis and the derivation of the control functions to integrate the hardware into a precision guidance, navigation and control system are discussed.  The book is organized into seven sections following the pattern of the lectures.
  1. Historical background, fundamental problems of guidance and navigation.
  2. More specific to Apollo.
  3. Analytic foundation for on-board navigation and guidance calculations
  4. Mechanization of the inertial sensor equipment for Apollo
  5. Same visibility into the optical navigation sensors
  6. Background and specific techniques, mechanization of on-board digital computers.
  7. Attitude control under rocket power and free-fall condistions for CSM, LEM, and earth entry return configurations.


Eldon C. Hall
January 1972

   The APOLLO Guidance Computer was designed to provide the computation necessary for guidance, navigation and control of the Command Module and the Lunar Landing Module of the APOLLO spacecraft. The computer was designed using the technology of the early 1960's and the production was completed by 1969. During the development, production, and operational phase of the programs the computer has accumulated a very interesting history which is valuable for evaluating the technology, production methods, system integration, and the reliability of the hardware. The operational experience* in the APOLLO guidance systems includes 17 computers which flew missions and another 26 flight type computers which are still in various phases of prelaunch activity including storage, system checkout, prelaunch spacecraft checkout, etc.
   These computers were manufactured and maintained under very strict quality control procedures with requirements for reporting and analyzing all indications of failure. Probably no other computer or electronic equipment with equivalent complexity has been as well documented and monitored. Since it has demonstrated a unique reliability history, it is important to evaluate the techniques and methods which have contributed to the high reliability of this computer.

*The operational experience includes missions through Apollo 15 which flew in August 1971. The compilation of all other data from this report ended 31 December 1970.

Digitally Controlled Pulse Torquing of Preceision Inertial Instruments

Frank E. Gauntt
September 1957


A test was performed to determine the feasibility of pulse torquing.  A small special purpose digital computer was coupled to the torque motor of a precise inertial gyro by a power switch in place of digital-to-analog conversion equipment.  The idea was to control the average current to the torque motor by holding its magnitude constant and reversing the direction of current flow in some prescribed cycling manner.  A brief sketch comparing a pulse torque system to an analog torque system is given in Appendix A.

Apollo Lunar-Descent Guidance

Allan R. Klumpp
June 1971
(from NTRS)

Abstract (excerpt)

This report records the technology associated with Apollo lunar-descent guidance.  It contains an introduction plus five major sections:

  1. Braking-phase and approach-phase guidance
  2. Terminal-descent-phase guidance
  3. Powered-flight Attitude-maneuver Routine
  4. Throttle Routine
  5. Braking-phase and Approach-phase Targeting Program

Apollo 11 Mission  Report

Mission Evaluation Team
NASA Manned Spacecraft Center


On May 25, 1961, this nation made a commitment: to land men on the Moon before the end of the decade.  On July 20, 1969, the commitment was met.  American astronauts left the following message on the lunar surface: "Here men from the planet Earth first set foot upon the Moon, July 1969 A.D.  We cane in peace for all mankind."

This achievement belongs to all mankind.  But those that made it possible deserve our special thanks.  First, there are three especially brave men -- Neil Armstrong, Mike Collins, and Buzz  Aldrin.  They were backed up by thousands of men and women in NASA, in other government agencies, in industry and in universities, and in the Congress.  All of them were dedicated to the cause of Apollo, and they proved that with skill and the desire to succeed -- above all, with dedication -- we as a nation can indeed meet the most difficult tasks we set for ourselves.

George M. Low, Acting Administrator, National Aeronautics and Space Administration



Edwin D. Smally
December 1966

This report is in two main sections. The first section contains the operating procedures to be utilized by persons using the SELF-CHECK or SHOW-BANKSUM routines. It also has block diagram flow charts which should help explain how the operating procedures of SELF-CHECK may be used for diagnostic purposes. The procedures for SELF-CHECK are slightly different in BLOCK I and BLOCK II while the procedures for SHOW-BANKSUM are the same.

The second section of this report goes into an explanation of SELF-CHECK and SHOW-BANKSUM. The explanation of SELF-CHECK consists of an explanation of the computer internal selfcheck and an explanation of the check of the DSKY electroluminescents. There is a separate description of each subroutine in SELF-CHECK and SHOW-BANKSUM. There is also a separate flow chart, located in the appendix, for each subroutine. This section should prove helpful to field engineers in locating the cause of malfunctions in the computer.

All numbers in this report are octal unless specifically mentioned otherwise.

The two subroutines that check the multiply and divide arithmetic functions of the computer will be removed from flight ropes. Figure 1 shows that placing a ±6 or ±7 in the SMODE register will allow the computer to loop in either the arithmetic multiply or arithmetic divide subroutines if these subroutines are part of SELF-CHECK. Placing a ±6 or ±7 in the SMODE register will exercise the internal computer self-check when these two subroutines are removed from SELF-CHECK. Thus, ±6, ±7, or ±10 all perform the same function when the arithmetic multiply and arithmetic divide are removed from SELF-CHECK.

Digitally Controlled Pulse Torquing of Precision Inertial Instruments

Frank E. Gauntt
September 1957


   A test was performed to determine the feasibility of pulse torquing.  A small special purpose digital computer was coupled to the torque motor of a precise inertial gyro by a power switch in place of digital-to-analog conversion equipment.  The idea was to control the average current to the torque motor by holding its magnitude constant and reversing the direction of current flow in some prescribed cycling manner.  A brief sketch comparing a pulse torque system to an analog torque system is given in Appendix A.

Discussion of Several Problem Areas During the Apollo 13 Operation

Glynn S. Lunney, NASA Manned Spacecraft Center
AIAA 7th Annual Meeting and Technical Display
Houston, Texas, October 19-22, 1970
AIAA Paper No. 70-1260

The successful recovery of the Apollo 13 flight is shown to have been the direct result of the performance of the flightcrew and the application of the talents of many different organizations focused on the mission support task through the Mission Control Center.  This paper illustrates the treatment of the following phases: the time-critical activities in the hours after the oxygen tank rupture, the process of selecting a strategy for the return path to earth, the development of a technique for carbon dioxide removal, and the development and verification of the procedures used for entry.

A Review of the Apollo Lunar Program and Its Lessons for Future Space Missions

Thomas J. Kelly, Grumman Corporation
AIAA Space Programs and Conference
September 25-28, 1990
Huntsville, AL
AIAA Paper No. 90-3617

The paper describes the Lunar Module and its role in the Project Apollo lunar landing mission. LM and its design features, and the evolution of the design as the project progressed. are explained. Some of the technical problems encountered and the solutions are described. The Grumman LM team's "finest hour" occurred in April 1970, when LM was used as a "lifeboat" to rescue the Apollo 13 astronauts from space after a Service Module tank exploded while enroute to the Moon. The paper describes the manner in .which this emergency was handled, and the systems engineering and planning that resulted in the LM having this lifeboat capability. From this history of the Apollo LM, some key lessons are then drawn for the Space Station and international space projects. Lessons are discussed in the areas of program management. systems engineering and integration, and motivation and leadership. Grumman has transferred and adapted these lessons to its aircraft programs and its function as NASA's Space Station Systems Engineering and Integration Contractor. One overriding lesson from the Apollo LM project concerns the benefits and feasibility of international cooperation in future space endeavors, particularly as applied to Space Station Freedom.

Heroes in a Vacuum: The Apollo Astronaut as a Cultural Icon

Roger D. Launius
Smithsonian Institution
43rd AIAA Aerospace Sciences Meeting and Exhibit
January 10 - 13, 2005
Reno, Nevada
AIAA Paper No. 2005-702


Through this essay I shall explore the creation and sustaining of the iconographic mythology of the Apollo astronaut in American culture. No one could have predicted the public fascination with astronauts from the first unveiling of the Mercury seven in 1959 through Project Apollo. The astronaut as a celebrity and what that has meant in American life never dawned on anyone beforehand. To the surprise and ultimately consternation of some NASA leaders, they immediately became national heroes and the leading symbols of the fledgling space program. Even so, both NASA and the press contrived to present the astronauts as embodiments of the leading virtues of American culture and this has continued from the 1950s to the new millennium. Both NASA officials and the astronauts themselves carefully molded and controlled their public images every bit as successfully as those of movie idols or rock music stars.1 What follows is an exploration of the creation and sustaining of the iconographic mythology of the astronaut in American culture.

Is It Time to Reconsider Kennedy's Space Policy?  A Post-Cold War, Post-Modern Perspective

Roger D. Launius
Smithsonian Institution
41st Aerospace Sciences Meeting and Exhibit
January 6-9, 2003
Reno, Nevada
AIAA Paper No. 2003-656

The answer to this question is a resounding yes. Because of the demise of the Soviet Union and the end of the Cold War there are opportunities not present before to reconsider the Kennedy space policy for Project Apollo. The major contours of that story have been told and retold many times, but the revisionist perspectives expressed about Kennedy in recent years, and especially the availability of new documentary materials—many of which had been highly classified as part of Cold War security concerns—offer an opportunity to reconsider the major themes of his centerpiece space policy announcement, the decision to land Americans on the Moon before the end of the decade. This paper explores the parameters of reinterpretation that forty years of perspective offer.

Atmospheric Entry of Nuclear-Powered Vehicles Due to Accidental/Inadvertent Termination of Operations

Gene P. Menees, Chul Part, and Michael E. Tauber
NASA Ames Research Center
AIAA Paper No. 92-3279
AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference and Exhibit
July 6-8, 1992, Nashville, TN


The entries of the radioactive components into Earth's atmosphere resulting from an accident or inadvertent abort of a space vehicle powered by nuclear-thermal-rockets are investigated. The study is made for a typical piloted Mars mission vehicle incapacitated by an accident or malfunction during the trans-Mars-injection maneuver due to simultaneous multiple failures of its component systems. The three different accident/abort modes considered are the following: i) a constant-rate angular pitching motion of the vehicle; ii) a constant-acceleration angular pitching motion of the vehicle; iii) the rocket engine breaks away from the rest of the vehicle with a finite relative (dispersion) velocity. The speeds and angles of the atmospheric entries are calculated for each mode for different values of the time of the accident, pitching rate, acceleration, and dispersion velocity. For the most severe entry speeds and flight-path angles, the stagnation-point pressures, heat transfer rates, thickness and mass per unit area of the heat shields necessary to protect the radioactive components from disintegrating, deceleration g-loads, and ground-impact velocities are calculated. The study points out that the high g-loads and high ground-impact velocities are the most serious problems that must be ad- dressed.

A Program for Translation
Mathematical Equations for Whirlwind I

Engineering Memorandum E-364
J.H. Laning Jr. and N. Zierler
January, 1954

An interpretive program for Whirlwind I is described that will accept algebraic equations, differential equations, etc. expressed on Flexowriter punched paper tape in ordinary mathematical notation (within certain limits imposed principally by the Flexowriter) as input and automatically provide the desired solutions.

Users Guide to the Block II AGC/LGC Interpreter

Charles A. Muntz
April 1965

A description is given of the AGC/LGC (Apollo and LEM Guidance Computers) algebraic interpreter, a language in which Apollo Mission computer programs may be conveniently prepared.

Logical Description for the Apollo Guidance Computer (AGC 4)

Albert Hopkins, Ramon Alonso, and Hugh Blair-Smith
March 5, 1963


This report describes the logical structure of the APOLLO guidance and navigation computer. A previous computer, AGC 3, designed for the APOLLO mission, was predominately composed of core -transistor logic. The computer design described here employs miniature integrated NOR logic, whose use will result in the next APOLLO computer (AGC 4) being just over half the size of AGC 3.

The decision to change over to integrated circuitry was made in October, 1962. About a year ago, it was deemed inadvisable to commit the APOLLO Guidance Computer (AGC) to integrated circuitry. Its desirable attributes of small size, high speed, and universality were then offset by its high cost, the difficulty in regulating power consumption as a function of speed of computation, and the absence of operational experience in large scale systems. Because of its potential, however, a computer-design investigation was conducted with integrated circuits at the Instrumentation Laboratory during the development of AGC 3.

Now, a year later, the price of integrated circuit elements has changed from high to moderate; and enough experience has been gained in their use, by MlT and by others, to permit extrapolation of their reliability data with substantial confidence. The adoption of the new technology, with the consequent redesign of the computer, is being undertaken at a time when it is felt that it can still be effected without causing undue delays in the program.

Since the first design of AGC 3 of about a year ago, much has been learned about the capabilities demanded of the APOLLO computer; enough programming experience has been gained to warrant the inclusion of programming features not present in AGC 3 and the exclusion of others that were. Consequently, AGC 4 is sufficiently different from AGC 3 to make existing AGC documents inadequate for use in further developing the guidance system and its production and support facilities. The prime purpose of this report is to furnish necessary information to members of the Laboratory and its contract and industrial support associates. Fine detail and internal consistency have been underemphasized for the sake of promptness so that this report could be written within a few weeks of the inception of the design.

Apollo Guidance Computer and Other Computer History

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