Candidly, while I understand the need for some amount of redundancy, I'm curious what this level of redundancy adds in terms of complexity to the system of a whole and whether or not that complexity-add almost outweighs the higher redundancy. I'm sure NASA has calculated the trade off, but I'd be curious to see the thoughts behind that.<p>I feel in a similar vein when learning of certain aircraft accidents over the years, where it feels like the redundancy of certain systems and the complexity it adds has been the indirect cause of accidents instead of preventing them. I suppose there's not really a way to quantify the accidents that it's prevent to be able to compare them directly.
> 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>Who sits down and determines that 8 is the correct number? Why not 4? Or 2? Or 16 or 32?
Eight shall be the number thou shalt count, and the number of the counting shall be eight. Nine shalt thou not count, neither count thou seven, excepting that thou then proceed to eight.
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.<p>So the answer is "some engineers sat down after talking to management".
Given a list of estimates of failure probabilities, finding the right mix of redundancy becomes a very tractable problem, maybe even freshman-level.
Interesting. In safety components we are using Lockstep Microcontrollers which are doing something similar in a much smaller scale.<p><a href="https://en.wikipedia.org/wiki/Lockstep_(computing)" rel="nofollow">https://en.wikipedia.org/wiki/Lockstep_(computing)</a><p>Example: <a href="https://www.st.com/resource/en/datasheet/spc574k72e5.pdf" rel="nofollow">https://www.st.com/resource/en/datasheet/spc574k72e5.pdf</a>
For the Airbus they used different CPUs because CPUs have bugs too...
> 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>How does a pair determine which of the pair did the calculation correctly?
It doesn't have to. It raises an error that the system can detect and take action on. Usually that'll be some combination of interrupt/reset and an external pin to let the rest of the system know what's happened.
In simple terms, this works by doing an XOR on the outputs and if they disagree, performing a fault recovery.<p>There's also space systems that use 3 processors and a majority vote for the correct output, but that's different.
I'm a big fan of Dissimilar Redundancies (but didn't know that was the term until today) for building system software.<p>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.
What I would like to see is the fault data. Also a graph of the # of in sync FMCs over time and how well did it correlate with predictions.<p>I other words, how over engineered is it.
The training the astronauts need must be a lot
When the Apollo astronauts learned that they might need to repair the computer if it breaks they joked they might as well learn brain surgery if they end up needing that too.<p>(This was when they planned on sending a modular computer with them. In the end they settled for sending up a fully assembled spare computer instead, which made replacement easier.)