No doubt modern smartphones are far more powerful than the computers used on Apollo missions, yet such comparisons underplay the amazing technology employed by NASA during these missions to get us there.
MIT-designed Apollo Guidance Computer was crashproof, operating according to a hierarchical system which allocated energy based on program importance; this allowed energy distribution while avoiding any single program dominating the system.
1. The Guidance Computer
The Guidance Computer was the centerpiece of Apollo spacecraft and lunar module. Responsible for safely piloting its crew from Earth to Moon and back again, its job required being both powerful and reliable – no margin for error was allowed for. A feat of engineering designed by top minds at MIT’s Instrumentation Laboratory.
At a time when computers were room sized and filled with numerous vacuum tubes, engineers found it challenging to fit an electronic brain into spacecraft the size of a minivan. Yet they managed to do so through knowledge of space flight physics as well as hard work.
The AGC was the first flight computer to use integrated circuits. Furthermore, it featured the world’s first display keyboard system designed specifically to interact with astronauts during space flight: DSKY was its name; modern laptops and tablets use similar systems today.
AGC was an impressive machine, but not without flaws. On its first moon landing mission, AGC encountered a problem which would have rendered any other computer inoperable: its rendezvous radar accidentally left on, overloading it with meaningless data that normally would cause system crash; however, AGC quickly and intelligently shut off every program except that which had top priority and provided Mission Commander Neil Armstrong with the GO command for descent into the Sea of Tranquility.
Future generations may look back with wonder at the AGC as Moore’s Law accelerates computing power; yet to criticise its power as being less impressive than modern smartphones would be disrespectful and demonstrate an ignorance of just what an amazing technical achievement the Apollo mission was.
Springer/Praxis’ book The Apollo Guidance Computer provides a thorough, in-depth, yet easy-to-read examination of the technology behind one of humanity’s greatest engineering feats: visiting the Moon. Techies seeking an in-depth view into how this computer system was constructed will find much to enjoy here; while space enthusiasts looking to delve into its intricacies will also find this read interesting and educational.
2. The Executive
Astronauts referred to the computer on board their Command Module as an essential fourth crew member, because without its functioning the mission would fail completely. Early computers filled entire rooms and were too big for spaceflight; engineers created something much smaller and more compact instead. Their aim was a portable digital spaceflight computer capable of processing data from all sensors aboard and controlling its flight dynamics.
Milt Trageser, Hal Laning, Richard Battin, and Dave Hoag of MIT Instrumentation Lab were the driving forces behind what would become known as the Apollo Guidance Computer (AGC). Utilizing transistors instead of vacuum tubes allowed for smaller and faster machines; their special operating system would enable multiple programs to run concurrently; plus they devised methods of using AGC for performing Boolean functions such as AND, OR, and XOR essential to navigation.
Software development on the AGC took 1,400 man-years, with 350 engineers being directly involved during peak development time. Reliability was of great concern given that its components needed to operate across a quarter million miles without service outlets; its components were hermetically sealed against dust and moisture infiltration and rigorous tests conducted against its entire AGC configuration.
The AGC used a microprocessor known as the Executive as its central unit, breaking instructions down into subinstructions and executing them sequentially. For instance, when computing multiplication, the Executive would start by using the Accumulator Register before switching over to using program Counter Register and Arithmetic Unit Register before finally loading Memory Address Register with results before writing them back out onto display keyboard.
The AGC used erasable core memory, consisting of eight blocks storing two kilowords of information in tiny ferrite cores stored under each memory block. High-current pulses could then read this memory through switching magnetization of cores; each memory cycle represented by this series of bits was known as a memory cycle. Furthermore, AGC utilized several registers such as program counter register, arithmetic unit registers and buffer registers as temporary storage options.
3. The Display Keyboard
The Apollo Guidance Computer (AGC) may not be one of the more famous Apollo computers, but it remains a favorite among historians of ancient computing. Designed at MIT and manufactured by Raytheon (a weapons and defense contractor), its integrated circuit chips were precursors of today’s semiconductors; and it featured 72 kilobytes of read-only memory.
The AGC was the sole system on board the Command Module and Lunar Module which carried Neil Armstrong and Buzz Aldrin to the moon. It could execute roughly 40,000 instructions per second – much lower than your typical laptop which typically can perform 10 billion per second!
The AGC had an intriguing feature that enabled astronauts to view memory locations of its computer on its display screen using interrupt-driven user interface routines – the first ever attempt of its kind! Ultimately, this led to how we interact with computers today: keyboards, mices and GUIs. Moore’s Law states that computer chip density doubles every 18 months – this has proven true throughout history; your smart phone likely boasts millions more processing power than AGC!
4. The DSKY
As part of their efforts to send men to the moon, over 400,000 space engineers, programmers and designers nationwide worked on every facet of this epic endeavor – from rockets that would launch them into outer space to computer systems to help guide them there. One key piece was the Apollo Guidance Computer created at MIT Instrumentation Lab; designed and manufactured there specifically so astronauts could fly their spacecraft all the way from Earth orbit back onto lunar soil without issue.
As with other computers of this era, the DSKY had to be programmed for each mission it faced. Programmers wrote their codes using Hal Laning’s revolutionary MAC language developed at MIT and stored on read-only rope memory inside its computers. It allowed for easier programming while scheduling tasks more effectively and correcting errors on-the-fly – something no other operating system at that time could achieve.
The DSKY was equipped with lights, numerical displays and a calculator-style keyboard designed to allow astronauts to communicate with the computer using an intuitive number language that was easy for them to comprehend. Commands were entered numerically using two-digit numbers that corresponded with verbs and nouns: verbs indicated what needed to be done by the computer while nouns outlined which data had been affected by each command; there were over 100 verb/noun code pairs to keep track of on board but its reliability made life much simpler for all mission participants involved!
Many outside the MIT Instrumentation Lab questioned if this computer system would work; after all, space is an inherently dangerous environment with plenty of opportunities for error. But Doc Draper and his I L crew knew this new digital computer would do something no other computer ever had before: send men safely into space.