The Incredible Apollo Guidance Computer

Apollo Guidance Computer

History

In the 1960s, NASA engineers relied on the most advanced technology of the time to power their missions to space. That technology was the Apollo Guidance Computer (AGC). Used in every Apollo mission to date, this is how it all began . . .

In 1961, NASA established a task group within its Office of Advanced Research and Technology (OART) to manage Project AGC and design a guidance computer. The AGC was to monitor the instruments and control the spacecraft, while a separate computer (the Apollo Computer) was to be used for onboard calculations. The OART task group was headed by Dr. Charles Stark Draper, principal investigator of inertial navigation at Massachusetts Institute of Technology.

Design

To design and build AGC, the OART contracted with the MIT Instrumentation Laboratory (MITIL).

The AGC used lots of discrete solid-state microcircuits; it weighed 70 pounds (32 kg), and consumed 55 watts of power during usage.

The Apollo Guidance Computer (AGC) was the most complex and sophisticated electronic computer ever used in space until the launch of the first space shuttle in 1981. The AGC consisted of more than 4000 integrated circuits.

Gold in the AGC

They were mounted on a single printed circuit board. Each chip had a gold plated contact pad on the edge of the board to which a gold plated, coiled spring would allow contact from wire bundles attached to the chip. Some chips had circuits only on one side, while other integrated circuits were double sided. Each chip was wired and tested prior to being placed into the circuit board.

The heat generated by this computer was so intense that it would have melted an ordinary plastic housing. For this reason, the computer was housed in a metal box. The heat also generated so much heat that the computer had to be housed in the module.

The circuit board and chips were originally mounted on a perforated metal plate. This plate was attached to a case made of aluminum, which was mounted to a steel frame on an aluminium base plate. The frame and base plate were then mounted on top of a titanium base inside another metal box containing another circuit board containing the memory devices (ROM). The computer was stored in a vacuum chamber, so as to avoid the corrosive effects of humidity. The heat generated by the computer was so intense that all electrical contacts were fitted with gold plated connectors.

DSKY Interface

The Apollo Guidance Computer (AGC) displayed information from its memory and also from instrumentation devices using light emitting diode (LED) displays. These displays with their black-on-white symbols were intended for use at night, when the astronauts’ eyes had adapted to darkness.

The DSKY (Display System Control Board) was designed as a local display for the AGC. This board contained LEDs facing an alpha-numeric display. At the bottom of the interface were the keys: Verb, Noun, Enter Data, Clear Data, Proceed, Reset, Release, +, -, 0, 1, … .

AGC Usage

The Apollo Guidance Computer (AGC) computed the direction and altitude at which to point the spacecraft. This information was entered into the computer and used to control its flight. The AGC could also respond to the other onboard instruments automatically, or it could be commanded by an operator through a keyboard.

The AGC calculated its own speed and in doing so took into consideration Earth’s gravity field, spacecraft mass, thrust of the launch rocket, attitude of the spacecraft, phase of flight of the vehicle.

It was designed to provide full support for all phases of the flight, including preparation of flight plans, execution of those plans, and reentry and retrofire. This complex computer also provided automatic reentry guidance in the case that remote control from the ground had to be lost.

The AGC could also be used to control the other onboard systems in the spacecraft.

AGC Frequency

The AGC was designed to run at a clock speed of 2.048 MHz, although mission rules called for it to be slowed down slightly.

AGC Memory

The Apollo Guidance Computer (AGC) consisted of two electrical memories: a small random access memory (RAM) for storing instrument software and data in flight and a second very large contiguous memory where all spacecraft information was stored.

The AGC was designed to handle the following types of data: position, velocity, attitude, time and date. It could be programmed to read data from onboard sensors, and to store them in memory. This memory included the DSKY keyboard for entering commands during flight.

AGC Hardware and Software

The AGC was designed to run programs written in the assembly language “MOCAS” (Master Control Program for Apollo) and the machine language “DSKY” (Display System Control Board). The stack operating system used consisted of a hardware push down stack with 11 levels.

AGC Registers

The AGC had several registers, like: Accumlator, Program Counter, Address register, Buffer register, ROM bank register, RAM bank register, Input register, Output register, … .

AGC Instruction Set

The Apollo Guidance Computer Block I. had an instruction set of 11 instructions. The instruction set contained the following instructions: Transfer control, Count-compare-skip, Index, Resume, Exchange, Clear, Transfer, Add, Mask, Multiply, Divide and Subtract.

AGC Software Programs

The Apollo Guidance Computer (AGC) contained an operating system, a software development system, and a set of programs. The AGC contained a real-time operating system, which was called “Exec” and also a software interpreter.

Its software has been stored in core rope memory, that is a form of read-only memory.

Alarm Signals

An early form of an alarm signal was the “flashlight” which was sent by the vehicle’s instrumentation system. This signal was used for a variety of purposes including: locating the capsule, to assist in deciding which instrumentation system was malfunctioning, and telling the astronaut when alarms were activated.

Alarm 1201 and 1202

These alarms means executive overflow and the response was a soft restart in case of Apollo 11.

Electrical Power System (EPS) of Apollo

The power storage was provided by 3 entry and post landing battery (BAT A, BAT B and BAT C), 2 pyro battery (PYRO BAT A and PYRO BAT B) and a Cryogenic subsystem (2 O2 and 2 H2 tanks).

The power generation was provided by 3 fuel cell power plants (FC 1, FC 2 and FC 3) connected to the Cryogenic subsystem.

For power conversion they used 3 AC inverters and 1 battery charger.

For the power distribution 2 main powerlines were used: Main Bus A and Main Bus B.

There are several more interesting articles about computers at our PC Ocular website, please have a look at them too!

Benkő Attila is a Hungarian senior software developer, independent researcher and author of many computer science related papers.

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