A file photo shows a second stage being lowered into place atop the interstage of a Boeing Delta 2 rocket at the launch pad. The silver structure above is a work platform. Photo: NASA

Boeing and the U.S. Air Force will forego a targeted launch date later this month while engineers continue efforts to understand why sections of a Delta 2 rocket won't fit together atop a Cape Canaveral pad.

As Spaceflight Now first reported last week, technicians have been unable to attach the rocket's second stage to the awaiting first stage at launch pad 17A for an upcoming $100 million mission to place a NAVSTAR Global Positioning System satellite into orbit.

The two stages are joined together by the interstage structure -- a hollow cylinder that remains connected to the top of the first stage during launch and encloses the second stage engine.

See an illustration of a Delta 2 depicting the various parts of the rocket.

During the first attempt to "stack" the second stage two weeks ago, workers noted the stage and interstage did not join together evenly across the rocket's 8-foot-wide diameter.

The alignment was off by three-eighths of an inch or less around the circle, said Lt. Col. James Gazur, the Air Force's Delta 2 program manager.

The suspect interstage was removed and replaced but that did not fix the problem.

The mission patch for launch of GPS 2R-5. Photo: USAF

So this week Boeing is taking precision measurements of the stages to determine which section is creating the misalignment, company spokeswoman Beth Hill said Monday.

Once that work is completed and the cause of problem is identified, the rocket parts will be either modified or replaced so the vehicle can be assembled for launch.

For now, a new launch date has not been established. Officials say they will defer setting a date until after the trouble is cleared up.

The flight was first scheduled for June 15 but then slipped to June 21 to give Boeing more time to troubleshoot this unusual problem. On Monday, the reservation on the Eastern Range for launch on June 21 was recalled.

The GPS 2R-5 spacecraft to be carried aloft by the Delta 2 will join the Air Force's constellation of the navigation satellites. The GPS network provides military and civilians with highly accurate location, speed and timing information anywhere in the world.