Space Shuttle

For making fine adjustments, the spacecraft depends on six small rockets termed vernier (VUR-nee-ur) jets, two in the nose and four in the OMS pods. These allow small changes in the shuttle's flight path and orientation.

The computer system used aboard the shuttle, which governs all events during takeoff and on which the shuttle's pilots are completely dependent for interacting with its complex control surfaces during the glide back to Earth, is highly redundant. Five identical computers are used, four networked with each other using one computer program, and a fifth operating independently. The four linked computers constantly communicate with each other, testing each other's decisions and deciding when any one (or two or three) are not performing properly and eliminating that computer or computers from the decision-making process. In case all four of the interlinked computers malfunction, decision-making would be turned over automatically to the fifth computer.

This kind of redundancy is built into many essential features of the shuttle. For example, three independent hydraulic systems are available, each with an independent power systems. The failure of one or even two systems does not, therefore, place the shuttle in what its engineers would call a "critical failure mode"—that is, cause its destruction. Many other components, of course, simply cannot be built redundantly. The failure of a solid-fuel rocket booster during liftoff (as occurred during the Challenger mission of 1981) or of the delicate tiles that protect the shuttle from the high temperatures of atmospheric reentry (as occurred during the Columbia mission of 2003) can lead to loss of the spacecraft.