Apollo 11: The computers that put man on the moon

It is hard to appreciate the technical challenges involved in putting a man on the moon, but 1960s computer technology played a fundamental role.

It is hard to appreciate the technical challenges involved in putting a man on the moon, but 1960s computer technology played a fundamental role.

 

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Full Apollo 11 coverage

 

By today's standards, the IT Nasa used in the Apollo manned lunar programme is pretty basic. But while they were no more powerful than a pocket calculator, these ingenious computer systems were able to guide astronauts across 356,000 km of space from the Earth to the Moon and return them safely.

The lunar programme led to the development of safety-critical systems and the practice of software engineering to program those systems. Much of this knowledge gleaned from the Apollo programme forms the basis of modern computing.

Apollo Guidance Computer

The lunar mission used a command module computer designed at MIT and built by Raytheon, which paved the way to "fly by wire" aircraft.

The so-called Apollo Guidance Computer (AGC) used a real time operating system, which enabled astronauts to enter simple commands by typing in pairs of nouns and verbs, to control the spacecraft. It was more basic than the electronics in modern toasters that have computer controlled stop/start/defrost buttons. It had approximately 64Kbyte of memory and operated at 0.043MHz.

The instruction manual for the AGC shows the computer had a small set of machine code instructions, which were used to program the hardware to run various tasks the astronauts needed.

The AGC program, called Luminary, was coded in a language called Mac, (MIT Algebraic Compiler), which was then converted by hand into assembler language that the computer could understand. The assembler code was fed into the AGC using punch cards.

Amazingly, the code listing for the AGC program can be downloaded as a PDF file. There is also an equivalent program for the lunar lander.

The AGC was designed to be fault-tolerant and was able to run several sub programs in priority order. Each of these sub programs was given a time slot to use the computer's sparse resources. During the mission the AGC became overloaded and issued a "1202" alarm code.

Neil Armstrong asked Mission Control for clarification on the 1202 error. Jack Garman, a computer engineer at Nasa (pictured below, left), who worked on the Apollo Guidance Program Section, told mission control that the error could be ignored in this instance, which meant the mission could continue. Apollo 11 landed a few seconds later.

Experts cite the AGC as fundamental to the evolution of the integrated circuit. It is regarded as the first embedded computer.

The importance of this computer was highlighted in a lecture by astronaut David Scott who said: "If you have a basket ball and a baseball 14 feet apart, where the baseball represents the moon and the basketball represents the Earth, and you take a piece of paper sideways, the thinness of the paper would be the corridor you have to hit when you come back."

While the astronauts would probably have preferred to fly the spacecraft manually, only the AGC could provide the accuracy in navigation and control required to send them to the Moon and return them safely home again, independent of any Earth-based navigation system.

IBM computers on Apollo 11

Along with the APG, mainframes were also heavily used in the Apollo programme. Over 3,500 IBM employees were involved, (pictured below). The Goddard Space Flight Center used IBM System/360 Model 75s for communications across Nasa and the spacecraft. IBM Huntsville designed and programmed the Saturn rocket instrument unit, while the Saturn launch computer at the Kennedy Space Center was operated by IBM.

An IBM System/360 Model 75 was also used at Nasa's Manned Spacecraft Center in Houston. This computer was used by Neil Armstrong and Buzz Aldrin to calculate lift-off data required to launch the Lunar Module off the Moon's surface and enable it to rendezvous with Command Module pilot Michael Collins for the flight back to Earth.

At the time, IBM described the 6Mbyte programs it developed, to monitor the spacecrafts' environmental and astronauts' biomedical data, as the most complex software ever written.

Even the simplest software today would far exceed the technical constraints the Apollo team worked under. The Apollo programme was pre-Moores's Law: in 1965 Intel co-founder Gordon Moore wrote his vision of how the performance of computer hardware would double every 18 months for the same price.

That a USB memory stick today is more powerful than the computers that put man on the moon is testimony to the relentless pace of technological development encompassed in Moore's Law. However, the Apollo programme proved that computers could be entrusted with human lives. Man and machine worked in unison to achieve something that 40 years on, has yet to be surpassed.

 

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Full Apollo 11 coverage

 

All images courtesy Nasa

This was last published in July 2009

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Are there really over 3,500 IBM employees in that picture? Amazing.
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Wow, given that I wasn't even alive back then, that is pretty amazing to think how far computer technology has advanced in mere decades. 64KB on a computer that helped guide people to the moon. 

It may have been one of the first times that human lives were trusted to a computer, but today there are many lives which that is true of, every day. 
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Just saying this is great information and this helped me write my History Fair paper.
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It's always been this way. We make astonishing use of the technology at hand, then look back amazed that we did so much with so little. And do it again and again anyway. The wheel, the automobile, the airplane.
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Hi Cliff;

I recently stumbled upon your article on the Apollo Guidance Computer, and frankly, I was horrified about the level of misinformation you present.

As an introduction, I’m Frank O’Brien, author of “The Apollo Guidance Computer, Architecture and Operation” (Springer, 2010).

Your comments are wrong on so very levels, I’m compelled to address them in detail. In particular, I’m curious mostly about where you obtained your information from? References, please.

I’d like to address some of the more egregious issues here:

“But while they were no more powerful than a pocket calculator…”

Hardly. The Apollo Guidance Computer (AGC) was a fully multiprogrammed, priority scheduled, fully fault-tolerant computer that was years ahead of its time. I can’t think of many calculators that have a similar capability. Citations, please.

“The lunar mission used a command module computer designed at MIT and built by Raytheon, which paved the way to "fly by wire" aircraft.”

The AGC did not pave the way for “fly by wire” aircraft. It was the *original* “fly by wire” vehicle. Indeed, the very first fly by wire aircraft, an F-8 years later, used an Apollo Guidance Computer to control it.

"It was more basic than the electronics in modern toasters that have computer controlled stop/start/defrost buttons"

Oh really? Since when did a toaster run several programs at once, each with an interface with the user? The AGC provided guidance, navigation and a *digital* autopilot (not to mention the recently invented Kalman filter). Sorry, my toaster knows nothing of these. I’m lucky to have my bagel toasted correctly.

“It had approximately 64Kbyte of memory and operated at 0.043MHz”

These are interesting numbers. Where did you get them? The AGC had 32K of 15 bit (16 with parity) words of ROM, and 2K of RAM. That doesn’t add up to 64K of anything. And the clock speed is interesting. Where was that obtained? Clock speed on the AGC was 2 MHz. Depending on the processing, it might have been 1 MHz (issues with microcode). 43KHz - Where does that come from?

“The AGC program, called Luminary, was coded in a language called Mac, (MIT Algebraic Compiler)…”

What is the source of this? All coding was in hand-coded Assembly – high level languages were never an option. Oh, someone might have used a higher level language for initial studies, but memory was so terribly tight (typically only a dozen or so words were available when they were done) that anything other than hand-crafted machine language (assembler) would work.

Oh, and Luminary was only the Lunar Module software. The Command module code was called "Collossus"

"Neil Armstrong asked Mission Control for clarification on the 1202 error. Jack Garman, a computer engineer at Nasa (pictured below, left), who worked on the Apollo Guidance Program Section, told mission control that the error could be ignored in this instance"

I know Jack Garman. Jack had nothing to do with the call on Apollo 11. It was Steve Bayles, a back room engineer who made the call.

In summary, it troubles me that your research is so lacking. I wonder if your other coverage of computing is so lacking.

Frank O’Brien
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i wish hal would stop playing dum for one minute
Wed June 15, 6:42:59pm
anon4292: not guide the computer hal....not input commands............JUST BASIC BALANSE OF THE LAM HAL, CAUSE ONCE IT GOES SIDEWAYS IT'S OVER COWBOY!!!
anon4292: it's a very very complex task hal
Wed June 15, 6:46:03pm
anon4292: and with only one hundred pound thurst force,,,,,,,,,the window of any angle that might throw it of balance is very very small hal
Wed June 15, 6:47:22pm
anon4292: u have the lem going sideways, the only thing holding it up is one single thruster on the bottom, and little ones all the way on top of the lem to keep it all balanced out, do u have any fking idea how hard that is? they made same things on earth 20 years later, and all faled due to it being to complicated hal
anon4292: stop playing stupid hal
Wed June 15, 6:50:04pm
anon4292: i cant believe hal still dont get it........in order for the lem to stop moving sideways u cant just fire side thrusters and it stopps......no, the TOP part stops, and the bottom would go out of balance, and after it does it would just go way out of controll.......so in order to do that, the lem going sideways would have to GET TILTED WHILE FIRING THE TOP THRUSTERS SO THE BOTTOM DONT FISH TAIL HAL..........and only then with an accurate and very casious exact top thruster burns it would stop hal!!! and then it has to decent to it's final destination hal
Wed June 15, 6:51:07pm
anon4292: and im just guessing that the angle of no recovery is like 10 degrees hal.......if not less
anon4292: because if the lem even without going sideways would tilt lets say 11 degrees then the 100 pound side thrust would just NOT BE ENOUGH to reocover the massive 12 or so thousand pound lem hal
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While I'm impressed by how quickly we've come so far, I'm stunned that we were able to do so much with (what now seems like) so little. Then again, it was about 14 years after the moon landing that I bought my first Kaypro with a massive 64Kb RAM and two 191Kb 5" floppy drives. It seemed utterly amazing, did everything I could imagine and seemed incredibly advanced. It's now available for about $150 on eBay. Photos taken on the moon can be had for $7. Yeah, progress arrives quickly.
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Apollo 11 to N reached a maximum only in Earth's orbit!
The data analysis of the NASA document "Selected Mission Weights (lbs)" (http://history.nasa.gov/SP-4029/Apolloo_18_37_Selected_Mission_Weights.htm.) Was clearly seen that Apollo 11 to N only got into Earth orbit. Because on the top line and the first position of this document has a mass of 45.7 t of command service module including Lunar Module (CSM / LM) is declared. After the alleged transportation and docking maneuvers, the mass of this "space configuration" reduced to 43.6 t. This would, however, to reach the second cosmic speed 11.31 km / s (8 km / s * √2 = 11.28 km / s) from the Earth's orbit a speed difference .DELTA.v of 3.31 km / s (11, 31- 8 = 3.31) energetically overcome with rocket fuel. With the stated by NASA fuel combination of hydrazine / dimethylhydrazine as fuel and nitrogen tetroxide (N2O4) as the oxidizer is an effective exhaust velocity ve of about 2.6 km / s achieved! This would be a fuel composition of
MTr = [1- (1: 2.72 3.31: 2.6] * 43,7 t ≈ 31.5 t (1)
have been necessary in order to achieve the second Cosmic speed. According to NASA -indications but only 18.5 t rocket fuel in CSM were abgebunkert. With the amount of fuel the Luna module were a total of only 29 t rocket fuel available! So NASA has impressively refutes itself. In other words: A moon landing never tired found!
Siegfried Marquardt, Koenigs Wusterhausen
Apollo 11 to N reached a maximum only in Earth's orbit!
The data analysis of the NASA document "Selected Mission Weights (lbs)" (http://history.nasa.gov/SP-4029/Apolloo_18_37_Selected_Mission_Weights.htm.) Was clearly seen that Apollo 11 to N only got into Earth orbit. Because on the top line and the first position of this document has a mass of 45.7 t of command service module including Lunar Module (CSM / LM) is declared. After the alleged transportation and docking maneuvers, the mass of this "space configuration" reduced to 43.6 t. This would, however, to reach the second cosmic speed 11.31 km / s (8 km / s * √2 = 11.28 km / s) from the Earth's orbit a speed difference .DELTA.v of 3.31 km / s (11, 31- 8 = 3.31) energetically overcome with rocket fuel. With the stated by NASA fuel combination of hydrazine / dimethylhydrazine as fuel and nitrogen tetroxide (N2O4) as the oxidizer is an effective exhaust velocity ve of about 2.6 km / s achieved! This would be a fuel composition of
MTr = [1- (1: 2.72 3.31: 2.6] * 43,7 t ≈ 31.5 t (1)
have been necessary in order to achieve the second Cosmic speed. According to NASA -indications but only 18.5 t rocket fuel in CSM were abgebunkert. With the amount of fuel the Luna module were a total of only 29 t rocket fuel available! So NASA has impressively refutes itself. In other words: A moon landing never tired found!
Siegfried Marquardt, Koenigs Wusterhausen
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