The U.S. Air Force has further delayed boosting its third Orbital Test Vehicle mission by two weeks to allow more time to investigate an anomaly in the RL10 upper-stage performance in the last launch.

The launch of the reusable X-37B spacecraft on the OTV-3 mission has now been slipped to Nov. 27, according to the United Launch Alliance, which manages the Atlas and Delta rocket families.

During an Oct. 8 launch of the Boeing GPS IIF spacecraft, the Delta IV was observed to have “lower than normal upper-stage engine chamber pressure.” The satellite made it to its intended orbit, but the Air Force convened an accident investigation board to ensure the problem is not systemic. Pratt & Whitney Rocketdyne manufactures the RL10.

The additional two weeks before the OTV-3 launch is needed in part to conduct a “thorough crossover assessment,” as the OTV-3 will be launching on an Atlas V. The Delta IV uses the ARL10B-2, while the Atlas V uses the RL10A. But, operators want to ensure there is no common problem between the two.