California's debut launch of the Boeing Delta 4 rocket is facing an extended delay -- perhaps six weeks -- while engineers try to reconcile differing predictions of sloshing fuel inside the booster during flight.

Launch dates earlier this week were scrapped to perform repairs on the cork insulator material that serves as a thermal barrier around the rocket's main engine. Preparations resumed for a Wednesday liftoff from Vandenberg Air Force Base only to be halted Tuesday because officials were worried about conflicting analytical models about the launch.

Sophisticated computer models that indicate how the rocket will operate during the flight disagree on the potential severity of propellant sloshing in the second stage during an orbital coast period mid-way through the launch.

After separating from the Delta 4's first stage four minutes into flight, the second stage performs a 10-minute firing of its RL10 engine to reach an initial parking orbit around Earth that is egg-shaped and stretches from 104 nautical miles at its lowest point to 1,196 miles at its highest.

The stage and the attached payload enter the 25-minute coast phase, flying southward over the Pacific and skirting the southern tip of South America to reach the orbit's high point. The stage is placed in a slow rolling motion to keep thermal heating evenly distributed across the vehicle during the coast. Small attitude control jets will be active during this time, eventually performing the maneuver to stop the rolling and to re-orient the stage for its next engine firing.

The second stage re-ignites at about T+plus 40 minutes above the South Atlantic to accelerate into a highly elliptical orbit of 601 by 20,308 nautical miles for deployment of the national security spy satellite payload.

An earlier analytical model did not reveal any sloshing problems based on the launch sequence and the rocket's trajectory. However, a newer simulation predicted the unwanted sloshing condition.

The time needed to understand and resolve the differences, including a full technical review and analysis of the mission, will bump this launch out of Vandenberg's rocket lineup for the next month or more.

Officials are devising plans to remove the classified National Reconnaissance Office satellite from atop the Delta 4 rocket's Space Launch Complex-6 pad and place it in safe storage. The pad is located on the southern edge of Vandenberg, while a few miles to the north is the Space Launch Complex-4 where a massive Titan 4 rocket stands ready to loft another NRO cargo on October 19, sometime between 9 a.m. and 1 p.m. PDT (12-4 p.m. EDT; 1600-2000 GMT).

Overflight concerns dictate that the costly Delta 4 payload be hauled off the rocket before the Titan 4 can thunder overhead on its southerly course to orbit. Military officials want to protect the satellite from the possible threat, however minuscule, that the Titan could rain debris on the Delta pad in a launch failure.

Meanwhile, NASA intends to ship its CloudSat and CALIPSO environmental satellites to the Space Launch Complex-2 pad on North Vandenberg around October 10 for mating atop a Delta 2 rocket. That dual-payload launch is scheduled for 3:01 a.m. PDT (6:01 a.m. EDT; 1001 GMT) on October 26.

The NRO satellite for the Delta 4 mission will be brought back to SLC-6 and reunited with its rocket for liftoff no earlier than mid-November, the Air Force said.

How the much-delayed GOES-N civilian weather satellite fits into this evolving plan was not clear Wednesday. That Delta 4 rocket at Florida's Cape Canaveral has been waiting to launch for months, but a variety of satellite and booster issues have postponed the commercial mission. Boeing had envisioned flying the mission around November 5 -- one month after the Vandenberg Delta 4. The company has said it needs about 30 days between Delta 4's for post-flight data review.