It has proved so difficult and expensive to upgrade the F-22 Raptor, whose stealthy body contains sensors and electronic brains, that the U.S. Air Force may take the unprecedented step of threading what amounts to a second central nervous system into a fighter jet.

By introducing an open architecture to one of the world's most tightly knit proprietary systems, service officials hope to make it much cheaper and easier to insert new technology - even gear developed for the F-35 Lightning II - into the stealthy air-superiority fighter.

"This jet has a very highly integrated avionics system. Because of that tight coupling and that highly integrated nature, it makes it very difficult, and we are highly reliant upon [Raptor makers] Lockheed Martin and Boeing to do any kinds of modifications to the jet," said David Weber, deputy director of the F-22 System Program Office (SPO) at Wright-Patterson Air Force Base, Ohio.