There is much speculation by some, as to how the flight computer aboard the Apollo missions managed to get men to the moon when it had just a tiny fraction of the computing power of something like a modern smartphone.
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But this is quite misleading as there was not one solitary computer controlling the Apollo craft, there were 4 computers and no fancy touch screens, GUI’s or other things in a typical computer of today to waste resources on.
The first of the 4 computers was the Saturn Launch Vehicle Digital Computer or LVDC, this got the rocket from the launch pad to earth orbit.
Then there was the Apollo Guidance Computer or AGC, this is the one which most people think of. There were in fact, two of them, one in the Command Module to get from the earth orbit to the moon and back again. The second was in the lunar module that would control the landing and then the ascent back to the command module and docking.
The fourth computer was one which was never used on any mission because it would control an emergency abort and ascent should something happen during the descent to the moons surface like the landing computer failing or they ran out of fuel.
The Apollo Guidance Computer wasn’t as dumb as many make it out to be. As time when by in Apollo’s development, the tasks that it was meant to do increased in both number and sophistication, this in turn created ever more issues with the limited resources available .
One of the biggest problems was the limited amount of memory due to the technological limitations of the time, this meant that the programmers had to make use of every single byte available.
The AGC also had a unique operating system. Systems like UNIX, Linex, Windows and Apple iOS are in control and share time out to the programs. In the AGC, the programs controlled how much time they got depending on how important they were. So that in the case of an emergency, the highest priority programs would get most of the time and non-essential operations were dropped to free up resources which became the basis of mission critical system for all manned mission afterwards.
The computer had a performance somewhere around that to that of the first generation of personal computers like the Apple II, commodore 64, ZX Spectrum that would arrive 10 years later in the late 70’s.
It had 2k of RAM and 36k of fixed storage magnetic rope core memory, which was woven by hand and took months to assemble, so any software bugs were literally woven into the system.
A comparison between the Apollo Guidance Computer and say an iPhone 6 is tricky because the AGC was not a general purpose computer. It was built for a very specific task, had a unique operating system and with the 48-year gap in the technologies used, we can only really get very rough estimates.
The Apple iPhone 6 uses the ARM A8 processor which has about 1.6 billion transistors in it, the AGC had just 12,300. The iPhone 6 has 1Gb of RAM, about 488,000 times the AGC and in this one, 128Gb of non-volatile storage or about 3.5 million times the AGC.
As for performance, the iPhone 6 is somewhere between maybe 4 and 30 Million times faster than the AGC depending on what type of calculations are being done and if you include the iPhone’s GPU it would be even more.
So, if you had to fly back to the moon in an Apollo craft and given the choice, would you trust your life to a couple of iPhones in place of the AGC’s?, because you would actually have more computing power in just one of them than the whole of NASA had during all the Apollo missions.......
Title: Adam Are You Free?
Author: P C III
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