The Fascinating Story of the Computer That Got Us on the Moon
And the thousands of work-hours it took to sew it all together.
Man landed on the Moon on 20 July 1969. What was science-fiction only a decade earlier had become a reality in a matter of years thanks to the ingenuity, the dedication, and the sacrifices of so many brave and brilliant men and women.
The story of the space programme and its crowning achievement has always fascinated me. Seeing the Apollo 11 module land on the Moon, and then seeing Neil Armstrong take his “giant leap” always brings tears to my eyes. If you have never seen the original footage, please do yourself and favour and watch it:
Space exploration is, without a doubt, the greatest adventure of our history. It brought a new perspective on the fragility and the tenuousness of our existence. It ushered in a new era of technological development and scientific advancement. By exploring a new world, albeit so briefly, it completely changed ours forever.
“They did it with less computing power than a phone”
The top comment on the second video posted above is probably one of the most mentioned trivia facts about the Apollo missions:
We went to the Moon with less computing power than a modern USB-C charger.
I’m going to copy the table that Heller published on his website because I find the comparison incredible:
On his article, Heller later calculates that taking into accounts the limitations of the Apollo 11 AGC, the CPU of the Anker USB-C charger is actually 563 times faster than the AGC.
With roughly twice the storage and twice the memory, the charger beats the Apollo computer handily.
If you look a bit closer at the table, you will notice that the memory (RAM) and the storage are not initially displayed in the same units.
While the chargers use our common “bytes” and “KB”, the Apollo AGC uses “15-bit words”. You might already know what this means, but I didn’t, so almighty Wikipedia came to my rescue :
In computing, a word is the natural unit of data used by a particular processor design. A word is a fixed-sized piece of data handled as a unit by the instruction set or the hardware of the processor. The number of bits in a word (the word size, word width, or word length) is an important characteristic of any specific processor design or computer architecture.
Now that this was cleared, I had to dig deeper and learn more about this less-powerful-than-a-charger computer that took men to the Moon.
And boy, was I not disappointed by the journey!
The Handmade Computer
What do a needle and threads have to do with the Apollo 11 computer, you might ask? As it turns out, much more than you can imagine.
At the time, computers were using what’s called Core Rope Memory. And it is exactly what it sounds like: copper threads on which tiny metallic doughnuts were woven. The way the weaving was done, and the magnetic field of the doughnuts, would decide whether the thread would give out a 0 or a 1.
Here is a view of a rope memory plane from the Apollo AGC:
Let’s take a closer look at the interwoven threads and doughnuts:
Here are two videos showing you how they were doing it:
Now imagine that each AGC Rope Memory plane had 8'192 of these intersections and that each memory module contained 14 planes. This equals to a total of 114'688 intersections to weave to produce a single memory module capable of holding around 4608 15-bit words.
The Apollo Guidance computer used 8 of these memory modules for the total mentioned above of 36'864 words or roughly 72KB of storage.
That’s 917'504 intersections that had to be properly woven over months for the rocket to follow its course to the Moon.
The computer might have had less memory than USB-C charger, but I don’t think any charger ever required that much time, dedication, and efforts to produce.