MIT's Role in Project Apollo, Vol. III: Computer Subsystem

298 Pages·2015·12.08 MB·English

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THE CHARLES STARK DRAPER LABORATORY
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R-700
MIT's ROLE IN PROJECT APOLLO
FINAL REPORT ON CONTRACTS
NAS 9-153 AND NAS 9_1065
VOLUME III
COMPUTER SUBSYSTEM
by
ELDON C. HALL
AUGUST 1972
CHARLES STARK DRAPER
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CAMBRIDGE,
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ACKNOWLEDGMENTS
This report was prepared under DSR Project 55-23890, sponsored by the Manned
Spacecraft Center of the National Aeronautics and Space Administration through
Contract NAS9-4065.
The author expresses appreciation to many individuals within Draper Laboratory
who contributed to the following sections of the report. In particular:
A. Hopkins Section 1, 2
A. Green, R. Filene Section 4
A. Harano Section 5
D. Bowler Section 6
D. Hanley, J. Partridge Section 7.
The publication of this document does not constitute approval by the National
Aeronautics and Space Administration of the findings or conclusions contained
herein. It is published for the exchange and stimulation of ideas.
O Copyright by the Massachusetts Institute of Technology
Published by the Charles Stark Draper Laboratory of the
Massachusetts Institute of Technology
Printed in Cambridge, Massachusetts, U.S.A., 1972
_ reportP"ermissiotanrfurtherreprodudN
, mustbeobtaineIrdotolhec_yr_htpr°lsketOr' _,
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ii
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FOREWORD
The title of these volumes, "MIT's Role in Project Apollo", provides but a
modest hint of the enormous range of accomplishments by the staff of this Laboratory
on behalf of the Apollo program. Man's rush into spaceflight during the 1960s
demanded fertile imagination, bold pragmatism, and creative extensions of existing
technologies in a myriad of fields. The achievements in guidance and control for
space navigation, however, are second to none for their critical importance in the
success of this nation's manned lunar-landing program, for while powerful space
vehicles and rockets provide the environment and thrust necessary for space flight,
they are intrinsically incapable of controlling or guiding themselves on a mission
as complicated and sophisticated as Apollo. The great achievement of this Laboratory
was to supply the design for the primary hardware and software necessary to solve
the Apollo guidance, navigation and control problem. It is to the credit of the entire
team that this hardware and software have performed so dependably throughout the
Apollo program.
The quantum leap in technology nurtured by the Apollo program has been and
should continue to be of immensely significant benefit to this country--s0cially,
economically and in terms of its national esteem. It is the responsibility of all
those who contributed to the proud achievements of Apollo to convince their
countrymen of the directions this nation ought to follow in implementing these newly
gained--and hard fought for--advances.
C. Stark Draper, President
Charles Stark Draper Laboratory
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R-700
MIT'S ROLE IN PROJECT APOLLO
Final Report on Contracts
NAS9-153 and NAS9-4065
VOLUME III
COMPUTER SUBSYSTEM
ABSTRACT
The Apollo guidance computer (AGC) is a real-time digital-control computer whose
conception and development took place in the early part of 1960. The computer
may be classified as a parallel, general-purpose or whole number binary computer.
This class of computer is representative of most of the ground-based digital
computers in existence in the late 1950s, when the precursers of the AGC were
being designed. However, at that time few computers of this class had been designed
for the aerospace environment, and those few embodied substantial compromises
in performance for the sake of conserving space, weight, and power. The design of
the AGC capitalized on advancements in digital computer technology in order to
provide significant improvements in computational performance and still conserve
space, weight and power.
The AGC is the control and processing center of the Apollo Guidance, Navigation
and Control system. It processes data and issues discrete output and control pulses
Io the guidance system and other spacecraft systems. An operational Apollo
spacecraft contains two guidance computers and three DSKY._ (keyboard and display
unit for operator interface), with one computer and two DSKYs in the command
module, andoneof each in the lunar module. The computers are electrically identical,
but differ in the use of computer software and interface control functions. As a
control computer, some of the major functions are: alignment of the inertial
measurement unit, processing of radar data, management of astronaut display and
PrecedinpgaEeblank
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controls and generation of commands for spacecraft engine control. As a general
purpose computer, the AGC solves the guidance and navigation equations required
for the lunar mission.
The operational experience in the Apollo guidance systems includes 20 computers
which flew missions and another 25 flight type computers which are stillin various
phases of prelaunch activity including storage, system checkout, prelaunch spacecraft
checkout, etc. These computersweremanufaclured 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 monitored and documented. Since it has
demonstrated a unique reliability history, it is important to record the technique
and methods which have contributed to this history.
by Eldon C. Hall
August 1972
The operational experience includes missions through Apollo 15 which flew in
August of 1971. Three BI Iand one Bl II computers flew in unmanned mlssions.
Sixteen Bi II computers flew manned missions.
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CONTENTS
Section Page
Acknowledgments .......................... ii
Foreword ............................. iii
Abstract .............................. v
List of Illustrations ........................ • x-v
Preface ............................... I
1.0 SUBSYSTEM DEVELOPMENT ................. 3
1.1 Evolution of the Computer Subsystem ........... 3
1.2 Initial Decisions .................... 15
1.3 Expanding Requirements ................ 19
I. 3.1 Memory Size (1962 to 1969) ........... 19
1.3.2 Implementation Meetings - 1964 ......... 20
1.3.3 Weight Savings (1962 to 1965) .......... 20
1.3.4 Electromagnetic Interference Specifications - 1963. 21
1.3.5 Fire Proofing - 1967 .............. 21
2.0 COMPUTER SYSTEM DESCRIPTION .............. 23
2.1 Introduction ...................... 23
2.2 General Functional Description ............. 29
2.2.1 Computer ................... 29
2.2.2 Display and Keyboard .............. 29
2.3 Subsystem Characteristic ................ 32
2.3.1 Word Length .................. 32
2.3.2 Number Representation ............. 35
2.3.3 Multiple - Precision Arithmetic ......... 37
2.3.4 Instruction Set ................. 38
2.3.5 Timing and Priority Control ........... 46
2.3.6 Memory ..................... 46
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CONTENTS (Cont)
Section Page
2.3.7 Input/Output .................. 47
2.3.8 Standby Operation ................ 48
2.3.9 DSKY Characteristics .............. 48
2.4 Computer Mechanical Description ............. 51
2.5 DSKY Mechanical Description .............. 53
3.0 COMPUTER DESIGN DESCRIPTION ............... 55
3.I Introduction ....................... 55
3.2 Logic Design ...................... 61
3.2. l Logic Circuit Element .............. 61
3.2.2 Logic Timing and Sequencing ........... 63
3.2.3 Central Processor ................ 67
3.2.4 Address and Instruction Decoding ......... 71
3.2.5 Interrupt Structure ............... 71
3.2.5.1 Counter Interrupts ........... 77
3.2.5.2 Program Interrupts .......... 80
3.3 Oscillator Circuit .................... 82
3.4 Power Supplies ..................... 83
3.5 Memory Circuits .................... 85
3.5.1 Erasable Memory ................ 87
3.5,2 Fixed Memory ................. 91
3.6 Alarms and Fault Detection ............... 93
3.7 Interface Methods .................... 97
3.7.1 Circuits .................... 101
3.7.2 Data Format Conversion ............. 101
3.7.3 Interface Example ................ 103
3.8 DSKY Description .................... 107
3.9 Computer Mechanical Description ............. 115
3.9.1 Detailed Description ............... 117
3.9.1.1 Tray A Wired Assembly ........ 117
3.9.1.2 Tray B Wired Assembly ........ 121
3.9.1.3 Interconnection Header Assembly .... 123
3.9.1,4 Mid-Spacer and Tray Covers ...... 123
3.9.1.5 Module Construction Features ...... 123
3.9.1,6 Assembly Features ........... 125
3.9.1.7 Thermal Design ............ 130
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