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For navigation, the Apollo craft wouldn’t have to rely solely on their onboard systems: Earth-based radar would track them, and mission control would send up course corrections as long as it could maintain radio contact. But during what were probably the most critical stages of a lunar mission, radio contact would be impossible. The long, curving trajectory of the spacecraft would bring it closest to the moon on the side facing away from Earth, so that’s where it had to enter lunar orbit and deploy the landing module–but of course, there would be no line of sight with Earth-based tracking stations. And when the returning command module entered Earth’s atmosphere, friction from its descent would heat the air around it and create a cloud of ions that would jam any radio transmissions.

The heart of the navigation and control system was Doc ­Draper’s brainchild, the inertial measurement unit, or IMU. The IMU was basically a disc surrounded by two concentric rings inside a spherical housing about a foot and a half across. The outer ring was attached to the housing by two hinges, so it could spin around one axis; the second ring was attached to the first one and spun around a perpendicular axis; and the disc spun around an axis perpendicular to the second ring’s, so it had perfect freedom of motion in three dimensions. On the disc–the inertial platform–were three accelerometers and three gyroscopes, also aligned in three different directions. If the IMU housing rotated, the gyros would register the motion, and motors would turn the rings to maintain the platform’s orientation: imagine a waiter who holds a tray of glasses parallel to the ground–even as he runs up and down walls and across the ceiling. If the inertial platform’s orientation remained perfectly stable, data from the accelerometers could locate the IMU anywhere in space by reference to its original position.

But the platform wasn’t perfectly stable. To allow for midflight course corrections, the Instrumentation Lab also designed a telescope and sextant that together could help locate the craft in space. Using an eyepiece on the command module’s console, an astronaut could find a trio of landmarks–say, Earth’s horizon, the moon’s, and Alpha Centauri–and press a button. The onboard computer would calculate the craft’s position from the angles between the sightings.

The IMU and the sighting optics had to provide virtually error-free information, and their design had to take into account the eccentricities of operation in space; hundreds of Instrumentation Lab engineers worked on them. Nonetheless, they were largely elaborations on existing technologies. The design of the Apollo guidance computer, however, took the Instrumentation Lab into uncharted waters.

Silicon Dawn

In the annals of technology, the most important event of 1961 may not have been JFK’s unveiling of the lunar program but, rather, an announcement a few months earlier from a four-year-old company called Fairchild Semiconductor: the first commercial release of a computer chip. An early example of an integrated circuit, it combined multiple electronic components in a single piece of silicon.

Today, when Intel can cram a billion transistors onto a chip, the advantages of integrated circuits seem obvious. But that wasn’t the case in 1961. For one thing, the new chips didn’t hold a billion transistors each; they held three. Integrated circuits would, in principle, take up about 40 percent less space than so-called core transistors, which consisted of wires wound around magnets. But they also demanded more electricity, a serious drawback in spacecraft with limited resources. What’s more, it was not at all clear that integrated circuits could be mass-produced with the reliability that spaceflight required. NASA administrators originally specified that the Apollo flight computer would use the larger core transistors.

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Credit: Courtesy of the MIT Museum
Video by Aero/Astro Department, courtesy of MIT TechTV

Tagged: Computing

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