WASHINGTON — An unmanned military spaceplane was returned to orbit Dec. 11 atop a United Launch Alliance (ULA) Atlas 5 rocket.

The launch of the U.S. Air Force’s X-37B spaceplane was ULA’s first flight since early October, when a performance issue with the upper stage of the company’s Delta 4 rocket marred the otherwise successful launch of an Air Force GPS satellite. X-37B, a reusable spaceplane that resembles a miniature space shuttle, had been scheduled to launch Oct. 25.

Because the Atlas 5 uses a variant of the Pratt& Whitney Rocketdyne RL-10 engine that powers the Delta 4 upper stage, both vehicles were grounded while ULA and the Air Force investigated the Oct. 4 anomaly, which involved a fuel leak in the interior of the engine’s thrust chamber.

ULA said Dec. 7 that although the investigation continues, the company had assured itself and the Air Force that Atlas 5 was not at risk of experiencing the same problem.

Liftoff occurred at 1:03 p.m. EST from Cape Canaveral Air Force Station, Fla. ULA and X-37 prime contractor Boeing announced about an hour later that the spaceplane had been successfully returned to orbit. The launch was ULA’s 10th and final mission of 2012.

“The ULA team is proud to have played a critical role in successfully launching these three [X-37B] missions for the Air Force [Rapid Capabilities Office],” Jim Sponnick, ULA vice president of mission operations, said in a statement. “This is a unique spacecraft since it is the first to launch on an Atlas 5, return to Earth landing at Vandenberg Air Force Base in California, then fly again on this mission.”

The reusable X-37B spaceplane that returned to orbit Dec. 11 was first launched in April 2010 and returned to Earth that December. A second X-37B, dubbed Orbital Test Vehicle-2, launched in March 2011 and completed its 496-day classified mission in June.

ULA’s next launch, an Atlas 5 launch from Florida of NASA’s Tracking and Data Relay Satellite-K, is scheduled for Jan. 29. The next spacecraft slated to fly on a Delta 4 is the Wideband Global Satcom 5, a military communications satellite that will launch from Cape Canaveral. No date has been set for that mission, which had been scheduled for early 2013.

Brian Berger is editor in chief of SpaceNews.com and the SpaceNews magazine. He joined SpaceNews.com in 1998, spending his first decade with the publication covering NASA. He was named senior staff writer in 2004, a position he held...