In the vast expanse of space, where human presence is physically limited, computers have been our reliable allies, pushing the boundaries of our knowledge and capabilities.
Analog and Digital Computers: The Early Days of Spaceflight
As we embarked on our quest to conquer space, the reliance on computers was indispensable. Early space missions, including the Apollo missions, relied on a combination of analog and digital computers.
Analog computers, unlike their digital counterparts, used physical phenomena such as electrical voltage to model problems, offering real-time computational ability, albeit with less precision. In contrast, digital computers worked on discrete values, providing high accuracy.
The Apollo Guidance Computer (AGC) was one such digital computer that played a crucial role in the Apollo missions. It was a real-time, multi-tasking computer system, far ahead of its time. With a mere 64 kilobytes of memory and operating at 0.043MHz, the AGC successfully guided astronauts to the Moon and back.
The Space Shuttle Era: Ushering in Digital Dominance
With the advent of the Space Shuttle program in the 1980s, digital computers gained dominance. The Space Shuttle used five IBM AP-101 computers, which formed the core of the data processing system. These computers, each with a processing speed of about 1MHz and 424 kilobytes of memory, handled everything from navigation to controlling the shuttle’s systems.
The use of multiple computers provided redundancy. If one computer failed or made an error, the others could take over, ensuring the astronauts’ safety. This system was known as a fail-operational, fail-safe system.
The heart of the space shuttle’s control system is the five General Purpose Computers (GPCs). Despite their limited memory and slow speed compared to modern home computers, these machines were designed for reliability, not performance. They had to withstand the harsh environment of space and the intense vibrations of launching into orbit.
The GPCs operate in various formats to fly the shuttle, including the phases of on-orbit operations. The GPCs receive signals from the shuttle’s myriad sensors and utilize these data in complex mathematical algorithms to control various aspects of the shuttle’s flight. This includes swiveling the three main engines during launch, adjusting the elevons on the wings for landing, and firing the thrusters in space to set up a rendezvous with the International Space Station. This process is completed about 25 times every second, showcasing the immense computational demands of space navigation.
The space shuttle’s flight control system was the first computer-driven system for a production spacecraft. Known as a “fly-by-wire” design, it didn’t have any mechanical links from the pilot to the control surfaces and thrusters. Instead, the pilot moved the control stick in the cockpit, and the computers transmitted signals to the control mechanisms. This design made the shuttle highly dependent on computers; a fraction of a second without them could be catastrophic during critical parts of the flight.
To ensure reliability, the software changes went through about nine months of in-house simulator testing and then another six months of testing in a NASA lab before being accepted for flight. This rigorous testing regimen led to an impressive track record of reliability, with a software error never endangering the crew, shuttle, or a mission’s success.
The GPCs were networked, with four operational and one as a backup that could fly the launch and entry if the others failed. These computers received information from a host of sensors and actuators throughout the orbiter, external fuel tank, and solid rocket boosters.
The GPCs and their software were designed with failure adjustment in mind. For example, when one main engine shut down early during the STS-51F mission launch in 1985, the software steered the shuttle safely into a lower-than-planned orbit, ensuring mission success. Moreover, the software was organized into different sets to operate at various times on the computers, including pre-launch, launch, in-orbit operations, in-orbit checkout, and entry, each with their unique challenges.
Despite the anticipation of the importance of computers to the spacecraft, the GPC memory size limitations were a major hurdle before the first mission. To add new features or make adjustments to the software after the shuttle began flying, designers either had to remove something or code something more efficiently to stay within the memory limit. This limitation was partly addressed in a modernization effort in 1991, which increased the GPC’s capacity to 1 megabyte.
The Era of Modern Computers in Current Rocket Systems
Fast forward to today, the computational power used in space missions is exponentially higher. Modern rocket systems are equipped with cutting-edge computer systems that manage multiple complex tasks, including navigation, system health monitoring, communication, data collection, and payload management.
For instance, SpaceX’s Falcon 9 and Dragon spacecraft use a fault-tolerant design that includes three flight computers running in unison. If there’s any discrepancy, the system immediately isolates the disagreeing computer and continues the mission.
Moreover, NASA’s Mars 2020 Perseverance rover is powered by a RAD750 computer, a radiation-hardened device with 200MHz processing speed and 256MB of memory. It’s capable of withstanding the harsh environment of space while performing complex tasks, like analyzing Martian soil and climate.
Areas of Application
Computers in spaceflight are used in several key areas:
- Navigation and Control: They guide spacecraft trajectories and manage onboard systems.
- Health Monitoring: Computers monitor the health of the spacecraft, alerting operators to any potential issues.
- Communication: They handle the transmission and reception of data between the spacecraft and Earth.
- Data Collection and Analysis: Computers collect and process data from onboard instruments.
- Payload Management: They manage the operations of the payloads, such as scientific instruments or satellites.
From the humble beginnings of the Apollo Guidance Computer to the sophisticated systems of today, computers have been instrumental in our exploration of space. As we continue to push the boundaries of space exploration, the role of computers will only grow, becoming more complex and integral to our mission of understanding the universe.
- AGC restauration videos and press coverage: https://www.curiousmarc.com/space/apollo-guidance-computer
