WASHINGTON — The U.S. Air Force says it will launch an unmanned X-37B spaceplane for a fourth mission no earlier than May 20.

Built by Boeing Space & Intelligence Systems of El Segundo, California, the X-37Bs are reusable unmanned orbital maneuvering vehicles that launch atop an expendable rocket and return to Earth much like NASA’s now-retired space shuttle and glides in for a runway landing.

The Air Force declined to say which of the two X-37B spaceplanes the service will use for the mission.

“The program selects the orbital test vehicle for each activity based upon the experiment objectives,”  Air Force spokesman Capt. Chris Hoyler said.

The spaceplane is the primary payload for the AFSPC-5 mission and will launch aboard United Launch Alliance’s Atlas 5 rocket from Cape Canaveral Air Force Station in Florida. All three previous X-37B missions launched from the Cape and landed at Vandenberg Air Force Base, California.

The Air Force did not say where the upcoming X-37B mission would land, or how long it would remain in orbit. The last X-37B launched into space logged 674 days in orbit before landing last October.

“We are excited about our fourth X-37B mission,” Randy Walden, the director of the Air Force’s Rapid Capabilities Office, said in an April 24 statement.  “With the demonstrated success of the first three missions, we’re able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads.”

This flight is aimed at evaluating improvements to the spaceplane’s performance and will also include an experimental propulsion system developed by the Air Force Research Laboratory and the Air Force Space and Missile Systems Center, the release said.

“We’re honored to host these collaborative experiments that will help advance the state-of-the-art for space technology,” Walden said.

The Air Force has been tight-lipped about the X-37B program’s purpose, only saying the flights are used for “risk reduction, experimentation and concept of operations development for reusable space vehicle technologies.”

Mike Gruss covers military space issues, including the U.S. Air Force and Missile Defense Agency, for SpaceNews. He is a graduate of Miami University in Oxford, Ohio.