WASHINGTON — The U.S. Air Force was fortunate not to have lost a navigation satellite after the rocket on which it was launched Oct. 4 experienced engine trouble, a senior service official said.

Gen. William Shelton, commander of Air Force Space Command, said the service is not even close to finishing its investigation of the anomaly that occurred during the ultimately successful launch of the GPS 2F-3 satellite aboard a United Launch Alliance (ULA) Delta 4 rocket from Cape Canaveral Air Force Station, Fla. The launcher’s RL-10 upper-stage engine underperformed during the mission but still managed to place the satellite into its intended orbit using reserve fuel.

Speaking with reporters Nov. 7 at an Air Force Association Breakfast in Washington, Shelton characterized the mission’s ultimate success as a “diving save.” Had the satellite been heavier or the mission profile more constrained — GPS missions do not require the full lift capability of a Delta 4 rocket — the result could have been a failure to reach orbit, he said.

As part of its investigation, the Air Force is examining all possible anomaly scenarios and trying to find the root cause through a process of elimination, Shelton said. A separate investigation is being conducted by Delta 4 builder ULA of Denver and RL-10 supplier Pratt & Whitney Rocketdyne of Canoga Park, Calif.

“It is incumbent on us to make sure that we get this investigation to root cause, that we understand exactly what happened and we don’t have a reoccurrence,” Shelton said. The Air Force cannot afford to lose an expensive satellite in a launch failure, he said.

At least two upcoming missions have been delayed by the investigation. The Air Force has postponed by at least a month the scheduled late-October launch of an experimental spaceplane aboard a ULA-built Atlas 5 rocket, whose upper-stage engine is an RL-10 variant. The launch of a NASA Tracking and Data Relay Satellite, also aboard an Atlas 5, was pushed from December to January.