I'll start out by mentioning the role of haste and cost-cutting in all three disasters. The Apollo 1 capsule was considered a lemon by crew and engineers. It had a lot of problems, but NASA management didn't want to go to the expense of having it redone, or take the time to do so. Back then we were in a race to the Moon. The o-rings that failed in the Challenger disaster were known to be out-of-spec, but were not replaced because Morton Thiokol would have had to eat the cost, and the launch would have been pushed back. Now we're finding out that the main fuel tank used for the STS-107 flight was considered obsolete...