I'm a big fan of Dissimilar Redundancies (but didn't know that was the term until today) for building system software.
Build for various Linux distros, and some of the BSDs. You'll encounter weird compile errors or edge cases that will pop up. Often times I've found that these will expose undefined behaviour or incorrect assumptions that you wouldn't notice if you were building for a single platform.
> 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.
Who sits down and determines that 8 is the correct number? Why not 4? Or 2? Or 16 or 32?
They probably set an acceptable total loss rate for the mission and worked backwards to determine how many replicas of each system they need to achieve that while minimizing total cost/weight.
So the answer is "some engineers sat down after talking to management".
Not just CPUs, they run a whole different (but also simpler) fallback program in case the main computers fail. I think they were more worried about programming errors but this should avoid all shared failures between the main computers (be it programming or hardware).
https://en.wikipedia.org/wiki/Lockstep_(computing)
Example: https://www.st.com/resource/en/datasheet/spc574k72e5.pdf
> each FCM consists of a self-checking pair of processors.
Build for various Linux distros, and some of the BSDs. You'll encounter weird compile errors or edge cases that will pop up. Often times I've found that these will expose undefined behaviour or incorrect assumptions that you wouldn't notice if you were building for a single platform.
Who sits down and determines that 8 is the correct number? Why not 4? Or 2? Or 16 or 32?
So the answer is "some engineers sat down after talking to management".
I other words, how over engineered is it.