By Irene Klotz
(Reuters) - An unmanned Atlas 5 rocket carrying a small robotic space shuttle lifted off from Cape Canaveral Air Force Station in Florida on Tuesday for the third flight in a classified military test program.
The 196-foot (60-meter) rocket blasted off at 1:03 p.m. ET (1603 GMT) carrying the military's original X-37B experimental space plane, also known as an Orbital Test Vehicle, or OTV.
The unmanned, reusable space shuttle, one of two operated by the U.S. Air Force, spent 224 days circling Earth during its debut mission in 2010. A sister ship blasted off in 2011 and landed itself after 469 days in space, completing the second orbital test flight.
The military is not saying how long the third X-37B mission will last, nor what the vehicle will be doing in orbit.
"The focus of the program remains on testing vehicle capabilities and proving the utility and cost-effectiveness of a reusable spacecraft," Air Force spokeswoman Tracy Bunko wrote in an email to Reuters.
While launching from Florida, the military has been landing the robotic space planes at Vandenberg Air Force Base in California. The agency is considering landing and refurbishing its X-37B spaceships at NASA and Air Force bases in Florida, which has been courting new customers since the retirement of NASA's space shuttles last year.
"We are investigating the possibility of using the former shuttle infrastructure for X-37B OTV landing operations and are looking into consolidating landing, refurbishment and launch operations at Kennedy Space Center or Cape Canaveral Air Force Station in an effort to save money," Bunko wrote.
"Those investigations are in an early state, and any specifics will not be known for some time," she added.
The vehicles, which were built by Boeing, are about one-fourth the size of a NASA shuttle and use solar panels to generate power, rather than chemical fuel cells that limited the space shuttles' time in orbit.
Neither NASA nor the Air Force has plans to upgrade the X-37B to carry people.
The OTV-3 flight had been delayed several months pending the results of an investigation into an upper-stage engine problem during an October 4 Delta 4 flight to put a Global Positioning System satellite into orbit.
The Delta 4 upper-stage is similar to one used on Atlas rockets. Both vehicles are built and flown by United Launch Alliance, a partnership of Boeing and Lockheed Martin.
(Editing by Jane Sutton and Philip Barbara)