{"id":51558,"date":"2026-04-13T14:19:33","date_gmt":"2026-04-13T18:19:33","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=51558"},"modified":"2026-05-04T10:55:17","modified_gmt":"2026-05-04T14:55:17","slug":"artemis-iis-fault-tolerant-computer","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2026\/04\/13\/artemis-iis-fault-tolerant-computer\/","title":{"rendered":"Artemis II’s Fault-Tolerant Computer"},"content":{"rendered":"

Logan Kugler<\/a> (via Hacker News<\/a>):<\/p>\n

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To ensure those wrong answers never reach the spacecraft’s thrusters, NASA moved beyond the triple redundancy of traditional systems. Orion utilizes two Vehicle Management Computers, each containing two Flight Control Modules, for a total of four FCMs. But the redundancy goes even deeper: each FCM consists of a self-checking pair of processors.<\/p>\n

Effectively, eight CPUs run the flight software in parallel. The engineering philosophy hinges on a “fail-silent” design. The self-checking pairs ensure that if a CPU performs an erroneous calculation due to a radiation event, the error is detected immediately and the system responds.<\/p>\n

“A faulty computer will fail silent, rather than transmit the ‘wrong answer,’” Uitenbroek explained. This approach simplifies the complex task of the triplex “voting” mechanism that compares results. Instead of comparing three answers to find a majority, the system uses a priority-ordered source selection algorithm among healthy channels that haven’t failed-silent. It picks the output from the first available FCM in the priority list; if that module has gone silent due to a fault, it moves to the second, third, or fourth.<\/p>\n

[…]<\/p>\n

Orion carries a completely independent Backup Flight Software (BFS) system. This is a prime example of dissimilar redundancy. It is implemented on different hardware, runs a different operating system, and utilizes independently developed, simplified flight software.<\/p>\n<\/blockquote>\n\n

Jim Hillhouse<\/a>:<\/p>\n

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There are two main flight computers that use two radiation hardened IBM PowerPC 750FX single-core processors, a CPU introduced in 2002 and used in Apple computers such as the iBook G3 until 2005.<\/p>\n<\/blockquote>\n\n

Previously:<\/p>\n