The primary Guidance Navigation Computer did this perfectly and Buzz thought the AGS was almost as good. (There is a nice compact technical description of the AGS in Wikipedia: ).īuzz Aldrin got a doctorate from MIT with a thesis about manual orbital rendezvous of two spacecraft. The entire system occupied only 3 cubic feet and consisted of three major components: (a) an Abort Electronic Assembly (AEA), which was the computer (b) an Abort Sensor Assembly (ASA), which was the inertial sensor and (c) a Data Entry and Display Assembly (DEDA), which was a simple version of the data entry keyboard on the primary system. The system used sensors fixed to the LEM to determine motion, rather than a stable platform as in conventional inertial guidance systems. The AGS was a pioneer in that it was the first “strapped-down” guidance system. (Apollo 9 was an Earth orbit mission, and with Apollo 7 the most forgotten of the Apollo flights.) Here is as an obscure fact: A Descent Propulsion System firing with the LM docked to the Command Service Module was first performed in low Earth orbit during the Apollo 9 mission (March 1969) to test the Descent Propulsion System backup capability for the Service Propulsion System. This was necessary because the Primary Guidance and Navigation System used too much water - it used gyros that needed cooling - and the LM’s supply of water was tight. Once the LM Descent Engine was fired to put the LM on a trajectory that made an Earth re-entry, the AGS was used for most of the return to Earth, which included two mid-course burns to correct the trajectory. Apollo 13 was flying the first non-free-return trajectory of the Apollo missions, so Lunar Orbit Insertion became Trans Earth Injection, so to speak. The primary computer in the LM was used, though the procedure had to be created on the spot. On Apollo 13 a nominal translunar abort could not be done because the Service Module Motor was gone. Then the AGS would fly a nominal ascent and program insertion into a rendezvous orbit with the CM. The other possibility was failure of the LM primary before nominal ascent. The tricky abort situations were during descent: If the LM primary failed, then the AGS had to do an ‘ascent’ into a rendezvous orbit with the CM. When the LM and CM separated, if the LM primary failed before or during powered descent, the AGS could be used for establishing a rendezvous. The Abort Guidance System came into play when orbiting the Moon. Once inserted into Earth orbit, the Command Module (CM) Primary Guidance, Navigation, and Control System was programmed to do aborts from everywhere: from Earth orbit and after insertion onto the transfer orbit to the Moon. If the Instrument Unit failed, the crew could switch control of the Saturn to the Command Module’s computer both these system had abort modes. During the Saturn V launch the trajectory was controlled, for a while, by a computer with the exotic name of Instrument Unit. The AGS existed due to a NASA requirement that at the Moon there could not be a single-point failure in the Guidance, Navigation and Control (GN&C) system for the LM. As it happened the Abort Guidance System was never used for a landing abort on any mission, even though it was on and in backup mode if it was needed. There is, justifiably, a fair amount about the Primary Guidance, Navigation, and Control System (PGNCS). ![]() ![]() The Apollo Spacecraft News Reference, compiled for the press, is 330 pages long and contains not a single note about the Lunar Module (LM) Abort Guidance System (AGS). Al was a subsystem expert on the AGS as part of his role as instructor on the Lunar Module Simulator. Usefully, the AGS could verify navigation data when the Lunar Module was behind the Moon. Thus a loss of either the primary guidance and navigation control system or the AGS would have caused an abort. But NASA’s abort policy made its presence mandatory - an abort would be ordered if one additional system failure could potentially cause the loss of the crew. The very invisibility of the system is in its way a tribute to the primary guidance and navigation systems, for while the AGS could abort a landing, it was never needed for that purpose. A NASA historical document on computers aboard the Apollo spacecraft refers to the Abort Guidance System as “…probably the most obscure computing machine in the manned spaceflight program to date.” The AGS was a backup computer system offering the capability of aborting the mission if the Lunar Module’s primary guidance system failed during descent to the lunar surface, ascent or rendezvous. Al Jackson shares more memories of Apollo this morning in his account of a little known spacecraft component, the Abort Guidance System.
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