Photos: Gallery of Delta 2 launch

United Launch Alliance is set to retire part of its rocket family Thursday with the final flight of the Delta 4 rocket’s “single stick” configuration, a mission set for liftoff from Cape Canaveral carrying a new GPS navigation satellite into orbit for the U.S. Air Force.

Nine Merlin engines on the first stage of SpaceX’s next Falcon 9 rocket briefly fired up on a Cape Canaveral launch pad Wednesday, and the launcher cleared a customary pre-flight checkout in preparation for liftoff early Sunday with a Canadian-owned communications satellite.

A SpaceX Dragon supply ship packed with nearly three tons of experiments, crew provisions and supplies will remain on the ground until at least Friday morning to allow more time for NASA flight controllers to troubleshoot a problem with an electrical distribution unit on the International Space Station.