By MARCIA DUNN

AP Aerospace Writer

CAPE CANAVERAL, Fla. (AP) _ A mini military space plane is poised for liftoff Wednesday on another long orbital test flight. But as usual, the Air Force isn’t saying much about the unmanned mission.

This will be the fourth flight of an X-37B space plane, a secretive, experimental program run by the Air Force. The three previous missions also began with rocket launches from Cape Canaveral Air Force Station.

The mystery test vehicle _ essentially a technology test bed _ is designed to orbit the Earth and then land like one of NASA’s old shuttles. It is operated robotically, without anyone on board, and is reusable. It is 29 feet long _ about one-fourth the size of a NASA shuttle.

The longest X-37B flight lasted about 675 days; touchdown was last October. There’s no official word on how long this one will stay up. All three previous missions ended in California.

NASA has a materials experiment aboard, while the Planetary Society is tagging along with a solar-sail demo.

Although largely mum about this X-37B flight, the Air Force has acknowledged a thruster experiment involving electric propulsion. Air Force researchers want to check design modifications to ion thrusters already flying on some advanced military communication satellites.

Wednesday’s liftoff of the Atlas V rocket is scheduled for 11:05 a.m.