Everyone has to accept that simple fact from the beginning. You can build in all sorts of expensive and heavy redundancy to help survive the failures. However, you also need to design all the systems so they can be repaired in flight. At least one of the crewmembers on a manned Mars mission needs to be an engineer when good mechanical ability, a "Junkyard Wars" type. The systems will have to be build with accessability and maintainability in mind. We don't need any of the super optimized but damned near impossible to maintain systems like Kim has described on the Shuttle.

It has been said that Air Force planes are designed by people with PhDs, flown by people with bachelor's degrees and maintained by people with high school degrees. There's a lot of truth to that. Over the years, aircraft designers have finally began learning lessons about how to design planes that can be maintained. Back in the early 1990s, I read an article about how aerospace manufacturers sent their design engineers out to Air Force bases and had them work side by side with the maintainers. The engineers got an earful of complaints, like how difficult it was to put safety wire in a tight spot while wearing gloves on a Minot, ND flight ramp at 30 degrees below zero. The engineers have finally started taking these lessons to heart and the planes are said to be a lot easier to repair. Maybe we'll need to send a few engineers up to the ISS for a few weeks and let them see how difficult it is to work in weightlessness before we let them design a Mars vehicle.