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4.5
"The Apollo Guidance Computer: Architecture and Operation" by Frank O'Brien is an extremely rare book: it actually says something new about Apollo while clarifying myriads of technical points that engineers, programmers, and pilots have been wondering about for decades. The book puts the history and development of the Apollo Guidance Computer (AGC) in historical perspective, and discusses the primary hardware, software, and programming features that made the system operate. From a modern perspective the AGC seems cumbersome and archaic, yet in the 1960's it was truly state of the art, featuring huge developments in integrated circuits, and amazing flexibility in a compact package that could fit in the Command Module (CM) and Lunar Module (LM.)The book really does start from the beginning, discussing everything from floating point numbers, binary and octal notation, instruction formats and the like. Not being a programmer, this was the most difficult part of the book for me, and I read it quite slowly to make sure I grasped the salient points before proceeding. The book then delves into the unique core memory architecture of the AGC and the structure of the various registers, timers, and basic logic. Following that O'Brien gets into concepts more familiar to me such as the Inertial Measurement Unit (IMU) and the various I/O devices involved with spacecraft navigation. He covers the basics of cislunar navigation, and discusses how the inputs were made (generally on the DSKY,) and explains the significance of all the annunciations on the DSKY and related areas. He has a particularly strong grasp of the perils of gimbal lock, and gives excellent explanations of the phenomena and the resulting problems (i.e. the progression from the "Gimbal Lock" light which is actually cautionary, to "No Att" which signifies the actually loss of orientation, p. 129.) Entwined in these discussions are explanations of higher level functionality of the AGC (e.g. the Executive and Interpreter,) DSKY entries, and limitations of the system.Chapter three, "The basics of guidance and navigation," was my favorite in the book because it elegantly took the theory of the AGC design and explained its practical use along with the use of the hardware in the CM and LM associated with navigating to the moon and back. O'Brien is very strong in his explanation of the IMU gimbals and the principles of the stable platform, and discusses the frequently-confusing axis-naming conventions used on Apollo (p. 201,) and the mechanics of gimbal operation and gimbal lock. He also gives a good explanation of the optics and sextant-based stellar navigation system (CM) and the Alignment Optical Telescope (AOT) in the LM. The discussion concludes with a great overview of coordinate systems ("Which way is up?") and introduces the crucial concept of the Reference to Stable Member Matrix (REFSMMAT,) which specifies the orientation of the platform.Chapter four, "Mission programs and operations," is the most fast-paced in the book. O'Brien rapidly progresses through the programs required for a typical mission, and explains them with reference to Apollo flightplans and excellent photographs of the actual hardware involved to make it easier to follow. DSKY entries are emphasized and the critical nature of the mission timeline is constantly reinforced. Rendezvous is extensively discussed, and the methods used throughout Apollo are discussed, and explanations of different methods, techniques, and requirements are well covered (e.g. Coelliptic Sequence Initiation, plane changes, etc.) I especially commend Mr. O'Brien's explanation of the Constant Delta Height (CDH) maneuver on p. 302. I had harbored some misunderstandings about this (specifically how the CDH burn could adjust the phasing between the CM and LM,) but O'Brien addressed the issue and made it easily grasped.The book winds down with an excellent discussion of the Digital Autopilot (DAP,) which was truly revolutionary. The DAP was enormously flexible, and allowed Apollo to pioneer "fly-by-wire" technologies and made the mission much more manageable for the astronauts. O'Brien then segues into the related discussion of RCS thrusters and coupling phenomena, followed by an explanation of earth atmosphere entry and the various entry profiles that might be selected and why (i.e. whether or not to use a "double dip" entry.) Finally (and of most interest to many readers,) the Apollo 11 executive overflow alarms (1202 and 1201) and the Apollo 14 abort discrete bit issue (likely caused by a loose floating ball of solder) are examined and explained in detail.The book also contains numerous appendices listing everything from AGC Interrupt Vectors to the CM and LM Programs (Major Modes) as well as complete Noun and Verb lists for both the CM and LM. The scope of this work is genuinely amazing.This book is not for the casual reader with a passing interest in the lunar landings, but for engineers, computer programmers, pilots, astronauts, and historians of technology this is a genre-defining work. I took a long time to read this book as there is much here to be learned, but don't regret a second of it: this is an amazing book and I recommend it highly.
