The Air Force is hard at work on a super-secret plane with a very not-public mission, and the service just flew the X-37B unmanned aircraft for the fifth time this past week. The X-37B will be flying in low orbit around the Earth using the second stage of a SpaceX Falcon launch vehicle, as the first stage returned to Earth and successfully landed on a launch pad in Florida just a few minutes after the launch happened.

It’s been more than seven years since the Air Force first launched the X-37B after spending more than a decade in a development. On that first flight, it was in orbit for an astonishing 718 days. The Air Force says its purpose is to perform risk reduction, experimentation, and development of space vehicles that are reusable, but many experts believe the X-37B will also be used to test military space technologies and improve spying capabilities.

Because of the Air Force’s focus on reusable spacecraft technologies, they chose SpaceX as a partner, as Elon Musk’s company has demonstrated the successful landing of first-stage rockets after launches, both on land and on a barge at sea.

“The X-37B Orbital Test Vehicle, or OTV, is an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force,” the Air Force says on its website. “The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth.”