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Light Years Ahead | The 1969 Apollo Guidance Computer

TNMoC・2 minutes read

Robert Wills discusses the Apollo 11 mission, highlighting the critical role of the flawless Apollo guidance computer in the lunar descent, showcasing innovative hardware and software design principles that ensured mission success. Despite facing program alarms and restarts during the descent, the computer's robust design, real-time operating system, job division, and checkpointing mechanisms allowed for recovery and effective control, revolutionizing software engineering practices for space missions.

Insights

Neil Armstrong's calm management of program alarms during the Apollo 11 lunar descent, facing five alarms and restarts just 2,000 feet above the moon's surface, showcases critical decision-making under pressure.
The Apollo guidance computer, despite limitations like a 15-bit word size and no floating-point numbers, functioned flawlessly during the Apollo missions, highlighting pioneering hardware and software design principles that ensured mission success.
Margaret Hamilton's innovative engineering approach and close collaboration among a small team of software engineers revolutionized the Apollo project, leading to reliable software and creative solutions to overcome limitations like the computer's 2,000 words of RAM.

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Recent questions

What was the Apollo guidance computer's role?
Guidance, navigation, and control in Apollo missions.
How did the Apollo guidance computer handle program alarms?
By restarting tasks and utilizing checkpoints.
What were the design principles of the Apollo guidance computer?
Starting on failure and dividing into jobs.
How did Margaret Hamilton contribute to the Apollo project?
Revolutionized software engineering for reliability.
What were the limitations of the Apollo guidance computer?
Limited memory, word size, and no floating-point numbers.

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Summary

00:00

"Apollo 11: Computer Crisis Averted"

Robert Wills, an engineer at Cisco and HAARP, specializes in software for Internet routers and has a keen interest in computing history, particularly the Apollo guidance computer.
He discusses the final stages of the lunar descent during the Apollo 11 mission on July 20th, 1969, highlighting a critical moment when the mission faced potential abort due to computer issues.
Neil Armstrong, with calm urgency, sought updates on program alarms during the descent, facing five alarms and restarts, the last occurring just 2,000 feet above the moon's surface.
Contrary to popular belief, the Apollo guidance computer functioned flawlessly, showcasing pioneering hardware and software design principles that ensured mission success.
The Apollo guidance computer, designed at MIT, was crucial for guidance, navigation, and control in Apollo missions, weighing only 32 kilograms, consuming 55 watts, and occupying one cubic foot.
Built from scratch using integrated circuits, the computer featured 2,000 words of RAM and 36,000 words of ROM, with software woven into the fixed memory using copper wires.
Despite limitations like a 15-bit word size and no floating-point numbers, the computer boasted real-time clock, interrupts, and extensive input/output connectivity.
Human interaction with the computer occurred through devices like the DSKY, using a verb-noun format for commands, and various other interface tools like the 8-ball indicator lamps and hand controllers.
The landing process on the moon involved utilizing a rocket engine for thrust, RCS thrusters for steering, an IMU for orientation and acceleration data, and lunar contact probes for landing accuracy, all managed by the Apollo guidance computer.
The descent to the moon involved executing maneuvers from the descent orbit's lowest point, including the challenging P63 braking phase to slow the spacecraft and descend gently onto the lunar surface.

18:03

"Spacecraft Landing: Precision and Control"

At the end of P 63, the spacecraft is only 8,000 feet above the surface, moving at 210 meters per second.
P 64 marks the approach phase where astronauts can see forward through a pitchover maneuver, fine-tuning the landing site with the computer.
In P 66, the final phase aims for a gentle vertical touchdown without lateral or backward movement.
The astronauts load the landing software by typing in verb 37 and program number 63, with the computer calculating the ignition time.
The computer signals the astronauts with verb 6 noun 61, indicating the time until ignition, and verb 50 noun 18, asking permission to maneuver the spacecraft.
Five seconds before ignition, the computer prompts the astronauts with verb 99 noun 62 to authorize the landing.
During P 63, the computer measures discrepancies between IMU and landing radar data, incorporating accurate radar data at lower altitudes.
In P 64, the computer uses the window axes to show astronauts the landing site, allowing adjustments with the hand controller.
Transitioning to P 66, the computer connects the hand controller directly to the steering software for final manual control.
Upon landing, the astronauts rely on cockpit instruments controlled by the computer to ensure vertical alignment and minimal lateral movement.

36:08

Apollo guidance computer: managing lunar module operations

The Apollo guidance computer managed the lunar module's landing, takeoff, and rendezvous with the command module, controlling the rocket engine, RCS thrusters, and various displays simultaneously.
A real-time operating system was developed for the Apollo guidance computer, using a high-level language to simplify complex guidance algorithms and provide more accurate calculations.
The system was divided into jobs, each performing specific tasks like reading data from the IMU or updating displays, with each job having a limited memory area for temporary storage.
Restarting on failure was crucial, with various levels of restart available to recover from rare software failures that could not be predicted during testing.
Checkpointing was implemented to save job states at sensible points, allowing for a restart from where the job left off if needed.
Hardware monitored the software to prevent system hang-ups, with cooperative multitasking requiring jobs to check for new tasks every 20 milliseconds.
Telemetry was sent back to Earth to provide detailed information about the computer's state, allowing experts to monitor and ensure proper functioning.
A hardware bug in the rendezvous radar caused an overload on the computer during the lunar landing, triggering a 1202 program alarm due to excessive load.
Buzz Aldrin's input of verb 16 noun 68 during the landing caused an additional 10% load on the computer, leading to a 1202 alarm and subsequent restart to recover.
Despite the program alarms, the system recovered by not restarting the tasks associated with Buzz's input, allowing the spaceship to continue flying successfully.

53:52

"Overcoming Program Alarms in Apollo Mission"

Buzz triggers the second program alarm by inputting verb 16 968 again, leading to the third alarm being triggered in the same manner.
Buzz realizes the connection causing the alarms and refrains from inputting verb 16 968 again, understanding its impact.
Five program alarms occur during the mission, with the last two happening 2,000 feet above the surface during P 64.
The complexity of P 64 causes the last two program alarms as the computer overloads itself without intervention.
Armstrong enters P 66 early to gain more direct steering control, reducing the computer load and preventing further alarms.
Design principles like starting on failure and dividing the system into jobs aid in the recovery process during program alarms.
Checkpoints ensure essential software functions continue running even if some parts are restarted.
Margaret Hamilton's engineering approach to software development revolutionized the Apollo project, leading to reliable software.
The limitations of the computer system, like 2,000 words of RAM, required creative solutions such as reusing memory locations for variables.
Close collaboration among the small team of software engineers ensured the system fit constraints and functioned effectively despite limitations.

01:11:22

Apollo 12 Computer Issues and Workarounds

The radar issue was discussed, focusing on telemetry and the computer's processing method, which involved pausing and restarting quickly, potentially causing issues that Houston might not have been able to detect.
The computer's unique way of feeding information led to confusion for Houston, as it could freeze and restart quickly, making it challenging to identify problems from the ground.
The computer's idle state was nicknamed "poo" due to being program zero, and a restart would clear all programs, leaving just the desktop, known as a "Pudi."
For Apollo 12, a workaround involved switching off the radar to avoid issues, and the computer was designed to withstand radiation with parity checking for memory.
The computer's power was not comparable to modern devices due to its specialized purpose for the mission, and during emergencies, a simpler spare computer could take over, albeit with limited functions.

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