You document everything and peer review all the time. NASA has an incredible amount of documentation on anything they do. There are controlled processes for eve

You document everything and peer review all the time. NASA has an incredible amount of documentation on anything they do. There are controlled processes for everything so whenever you need to go back and see how someone made something, there is an old document for it. The only thing that can't really be passed down is familiarity. You can have the old engineers write down every process, method, or quirk of a system but the new guy still needs to read all of it and get familiar with the system. Long term projects like these involve a lot of learning from other people so that there is a continuous chain of knowledgeable people running the project, like a master and apprentice kind of situation. And if the chain is ever broken, you come back using the documents and information passed on to get up and running as smooth as possible.