A secretive United States Air Force space plane is on a fourth trip into orbit.

As with the previous flights, the Air Force revealed few details about what the unmanned X-37B spacecraft, which resembles a smaller version of NASA’s retired space shuttles, will be doing.

“The test mission furthers the development of the concept of operations for reusable space vehicles, and fine-tunes technical parameters for an affordable, reusable space vehicle,” said Capt. Christopher M. Hoyler, a spokesman for the Air Force.

Boeing has built two X-37B spacecraft for the Air Force. The first launched in April 2010, setting off speculation over the vessel’s purposes. Some suggested that the craft had something to do with space weapons, which the Pentagon denied. It landed in December that year. The second orbiter was sent up from March 2011 to June 2012. The first craft made a second flight, launching in December 2012 and returning 674 days later, in October 2014.

The Air Force was not saying which of the two was packed inside an Atlas 5 rocket on the launchpad at Cape Canaveral, Fla.