On Earth, navigation is, at its simplest, about finding one’s way from one fixed point on the globe to another. For a trip to the Moon, it’s like standing with a rifle on a turntable that’s spinning at the center of a much larger turntable on which is a third turntable sitting on the rim, with all the tables spinning at different and varying speeds, and trying to hit the target by aiming at where it will be three days from now.
When Apollo 11 touched down in the Sea of Tranquility on July 20, 1969, it was more than a triumph of the human spirit, it was also the story of a cybernetic wonder called the Apollo Guidance Computer (AGC), which helped the Apollo astronauts safely navigate to the Moon and back. It was a computer so advanced for its time that the engineers who created it said they probably wouldn’t have tried to do so if they’d known what they were getting themselves into.
The Apollo Guidance Computer is one of the unsung successes of the Space Race. This is probably because it was so phenomenally successful, having had very few in-flight problems – and most of those were due to human error. Carried aboard both the Command Service Module (CSM) and the Lunar Module (LM), it flew on 15 manned missions, including nine Moon flights, six lunar landings, three Skylab missions, and the Apollo-Soyuz Test Mission in 1975.
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At the time it was the latest and most advanced fly-by-wire and inertial guidance system, the first digital flight computer, the most advanced miniature computer to date, the first computer to use silicon chips, and the first onboard computer where the lives of crew depended on it functioning as advertised.
Not that the Apollo Guidance Computer was much to look at. At first glance, it appeared like a brass suitcase in two parts, measuring a total of 24 × 12.5 × 6.5 in (61 × 32 × 17 cm) and weighing in at 70 lb (14 kg). Inside, it isn’t even very impressive by modern computer standards, having about as much oomph as a smart bulb with a total of about 72 K of memory and a 12-microsecond clock speed.
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It’s also hard to make an accurate comparison with modern devices because the AGC wasn’t a general purpose computer, but one that was literally hardwired for a particular task, which allowed it to perform at the level of the Commodore 64 or ZX Spectrum of the early 1980s – try to imagine getting to the Moon using a Commodore 64 to handle the navigation and not break into a cold sweat.
A job for a computer
The reason why all the Apollo missions carried at least one of these computers is that the Moon missions involved navigation problems that would have made Captain Cook go bug eyed. On Earth, navigation is, at its simplest, about finding one’s way from one fixed point on the globe to another. For a trip to the Moon, it’s like standing with a rifle on a turntable that’s spinning at the center of a much larger turntable on which is a third turntable sitting on the rim, with all the tables spinning at different and varying speeds, and trying to hit the target by aiming at where it will be three days from now.
Given the enormous number of variables, the above analogy only gives a small taste of the complexity of the equations that need to be solved for such a journey – and even then, the result will be a series of increasingly accurate estimates rather than a certain path. But what was certain from very early in the Apollo program was that space navigation is too complex and too counterintuitive for the astronauts to handle. In private, the engineers preferred that they not be allowed to have anything to do with it at all.Continue reading about Apollo’s brain: The computer that guided man to the Moon.
Source: New Atlas