Championing the Apollo Guidance Computer
With the 50th anniversary of the Apollo 11 moon landing fast approaching, there has been a surge in articles appearing in the press and online recently about the technology behind America’s pioneering space programme. Some of these have highlighted the computing power available to the astronauts through the on-board computer system, the Apollo Guidance Computer, comparing it unfavourably with today’s computer-based devices in statements such as:-
“An iPhone 5s has 1,300 times more processing power than the computer that landed Apollo 11 on the moon”
“… the on-board computers in the command and lunar modules had less capacity than some pocket calculators”
However, this is an unfair comparison. Let me explain why.
The Apollo Guidance Computer (AGC) was a special purpose real-time digital computer for flight navigation and control. Designed in 1961-62, the AGC embodied much of the leading edge technology developed over the previous decade for missile guidance and industrial process control. Only a few years earlier, most computers still relied on thermionic valves (or tubes) for their electronic circuitry. These devices were bulky, power-hungry and prone to failure, none of which made them ideal for use in spacecraft. The arrival of solid-state devices based on transistor technology in the mid 1950s improved matters considerably but reliability remained a major concern due to the number of wires and solder joints required to connect the many thousands of electronic components found in a typical computer. Fortunately, the introduction of integrated circuits in 1960 changed all that.
Integrated circuits do away with the need for wires by fabricating an entire circuit of miniaturised electronic components on a single piece of semiconductor material. An added benefit is that the physical size of the circuit is also substantially reduced. The AGC was one of the earliest computers to incorporate integrated circuits, allowing the MIT engineers who designed it to create an extremely compact, ultra-high reliability design.
The hardware specification of the AGC included a 16-bit word length, 1 MHz clock speed, 2,048 words (4 kilobytes) of read-write memory and 36,864 words (72 kilobytes) of read-only memory. This was equivalent to a high-end minicomputer of the period but squeezed into a case not much larger than a shoebox. As electrical power was at a premium, the AGC consumed only 70 Watts, a fraction of the power consumption of similarly-specified computers. It was more powerful than the early microcomputers of more than a decade later, all of which were based on an 8-bit word length. The AGC also featured a parallel architecture and sophisticated real-time operating system, necessary attributes to enable the precise timing of events for flight control and a capability that few computers today can boast. Furthermore, its magnetic core memory was non-volatile, which meant that the AGC retained vital navigation data when rebooted, which is exactly what happened during the moon landing stage of the Apollo 11 mission.
Each Apollo mission included two AGCs, one each in the Command Module and Lunar Module. Two additional computers were also installed to control the Saturn V launch vehicle and emergency abort system. The crew communicated with the AGC through a novel user interface unit known as the DSKY which incorporated a numeric display, indicator lights and a calculator-style keyboard.
The Apollo Guidance Computer really was state-of-the-art in the early 1960s and it remained fit for purpose for many years. It was influential in the design of later flight navigation and control computers and also played a role in demonstrating the practicality of fly-by-wire technology for flight control in military aircraft. Rather than compare it to much faster but less capable computers of recent years, we should recognise the AGC as a superb example of computer design and an important milestone in the history of the computer.