Light Years Ahead | The 1969 Apollo Guidance Computer

The speaker introduces the topic by welcoming Robert, an engineer with a keen interest in the history of computing and the Apollo 11 mission. Robert's fascination with the Apollo guidance computer is highlighted, and he provides an overview of the final stages of the lunar descent, focusing on the critical moments when program alarms occurred. He explains that the presentation will delve into the Apollo guidance computer's role, the revolutionary design principles behind it, and the misconceptions about the computer's performance during the mission.

Robert discusses the Apollo guidance computer's critical role in the Apollo 11 mission, detailing its responsibilities for guidance, navigation, and control. He emphasizes the computer's lightweight design, low power consumption, and compact size, which were remarkable for the era. The use of integrated circuits and the unique construction of its memory through copper wires woven into rings is also explained. Robert highlights the computer's real-time clock, interrupts, parity checking, and extensive input/output connectivity, while noting its quirks, such as the 15-bit word size and lack of floating-point numbers.

The presentation continues with an in-depth look at the Apollo guidance computer's technical features. Robert explains the computer's instruction set, including its limited use of logic gates and the special instructions for memory manipulation. The unique aspects of the computer's word size, integer representation, and memory limitations are discussed. He also describes the computer's input/output capabilities, including the 'disk e' interface used by astronauts and the various instruments and systems the computer could control or receive data from.

Robert sets the scene for the lunar descent, explaining the spacecraft's trajectory and the crucial phases involved in landing on the moon. He outlines the three main phases: the braking phase (P63), the approach phase (P64), and the final phase (P66). Each phase's objectives and the maneuvers performed are described, including the use of the rocket engine, RCS thrusters, IMU, and landing radar. The importance of the window in the spacecraft for determining the landing site is also highlighted.

This section delves into the real-time operations of the Apollo guidance computer during the lunar descent. Robert explains how the astronauts interacted with the computer using the 'disk key' and how the computer was programmed to initiate landing maneuvers. The role of the computer in calculating the spacecraft's position, speed, and trajectory is discussed, along with its ability to adjust the landing site based on the astronauts' input. The use of the computer's sophisticated guidance algorithms and the astronauts' role in fine-tuning the landing site are also covered.

Robert continues the narrative of the lunar descent, focusing on the dynamic interaction between the astronauts and the guidance computer. He describes how the astronauts used the hand controller to adjust the desired landing site and how the computer's guidance algorithms translated these inputs into spacecraft steering commands. The process of transitioning from automatic to manual control as the lunar module neared the moon's surface is detailed, along with the instruments the astronauts used to ensure a safe and precise landing.

In this section, Robert discusses the software that powered the Apollo guidance computer, known as Luminary. He outlines the six design principles that made the software reliable, including the use of a high-level language, job division, system restarts on failure, checkpointing, hardware monitoring, and telemetry. The importance of these principles in ensuring the computer's robust performance during the Apollo 11 mission is emphasized.

Robert addresses the 1202 program alarm that occurred during the Apollo 11 mission, explaining its cause and the team's response. He describes how the rendezvous radar's hardware bug and the additional load from extra calculations requested by Buzz Aldrin contributed to the computer's overload. The effectiveness of the computer's restart mechanism and the division of the system into jobs allowed for recovery from the alarm. The role of the guidance computer's telemetry in providing mission control with critical data is also discussed.

Robert concludes the presentation by highlighting the lasting impact of the Apollo guidance computer on modern software development. He acknowledges the contributions of key figures like Steve Bales and Margaret Hamilton and emphasizes the pioneering techniques used in the Apollo project that now form the basis of robust and reliable software. The talk ends with a reflection on the Apollo 11 mission's significance and the enduring relevance of the lessons learned from it.