In the early 1960s, however, the tools and techniques for creating an interactive, “real-time” computing environment were in a primitive state. Your laptop runs dozens of programs at once. Such multitasking is routine in computer systems today. These guidance and control tasks were particularly challenging because the module’s mass and center of gravity changed as fuel was consumed and because a spacecraft sitting atop a plume of rocket exhaust is fundamentally unstable-like a broomstick balanced upright on the palm of your hand.Īlong with the primary tasks of navigation, guidance, and control, the AGC also had to update instrument displays in the cockpit, respond to commands from the astronauts, and manage data communications with ground stations. At the same time it had to adjust the magnitude of the thrust to maintain the proper descent velocity. Data came from the gyroscopes and accelerometers of an inertial guidance system, supplemented in the later stages of the descent by readings from a radar altimeter that bounced signals off the Moon’s surface.Īfter calculating the desired trajectory, the AGC had to swivel the nozzle of the rocket engine to keep the capsule on course. The first task was navigation: measuring the craft’s position, velocity, and orientation, then plotting a trajectory to the target landing site. In their hardware the two machines were nearly identical software tailored them to their distinctive functions.įor a taste of what the computers were asked to accomplish, consider the workload of the lunar module’s AGC during a critical phase of the flight-the powered descent to the Moon’s surface. Furthermore, although the documents are technical, they have a powerful human resonance, offering glimpses of the cultural milieu of a high-profile, high-risk, high-stress engineering project.Įach Apollo mission to the Moon carried two AGCs, one in the command module and the other in the lunar module. The reward is seeing firsthand how the designers worked through some tricky problems that even today remain a challenge in software engineering. Deciphering even small fragments of the programs can be quite an arduous task. You can even run those programs on a “virtual AGC.”Īdmittedly, long lists of machine instructions, written in an esoteric and antiquated programming language, do not make easy reading. You can read through the programs that guided Apollo 11 to its lunar touchdown. Working from rare surviving printouts, volunteer enthusiasts have transcribed several versions of the AGC software and published them online. Among all the available resources, one trove of historical documents offers a particularly direct and intimate look inside this novel computer. At least five books tell the story, and there is more information on the web. And it had to be utterly reliable a malfunction could put lives in jeopardy.Īlthough the AGC is not as famous as the astronauts, its role in the Apollo project has been thoroughly documented. The Apollo Guidance Computer (AGC) had to fit in a compartment smaller than a carry-on bag and could draw no more power than a light bulb. Computers then were bulky, balky, and power hungry. Looking back from the 21st century, when everything is computer controlled, it’s hard to appreciate the audacity of NASA’s decision to put a computer aboard the Apollo spacecraft.
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