The X-37B is 9 meters long and 4.5 meter wide (29 X 15 ft) and its payload bay is 2.1 by 1.2 meters (7 by 4 feet). The vehicle was built at Boeing Phantom Works, based on an orbital and re-entry demonstrator design initially developed by NASA, then handed over to the Pentagon.

Rumors of an X-37B launch have been circulating since 2008.

The robotic space plane, or X-37B, lifted off from Cape Canaveral atop an Atlas V rocket at 7:52 pm local time on Thursday, according to a video released by the military.

"The launch is a go," said Air Force spokeswoman Major Angie Blair.

The lift-off appeared to proceed as planned without major problems, judging by the commentary in the Air Force webcast.

Resembling a miniature space shuttle, the plane is 8.9m long and has a wing-span of 4.5m.

The reusable space vehicle has been years in the making and the military has offered only vague explanations as to its purpose or role in the American military's arsenal.

The vehicle is designed to "provide an 'on-orbit laboratory' test environment to prove new technology and components before those technologies are committed to operational satellite programs", the Air Force said in a recent release.

Originally the vehicle was scheduled for launch in from the payload bay of the Space Shuttle, but that plan was axed following the Columbia accident.

It's cute. It's little. It's also top secret. The X-37B orbital test vehicle is at Cape Canaveral in Florida, and the word is that it will be launched on board an Atlas V rocket on Monday April 19, 2010 at around 10 pm EDT. Other than that, the Air Force isn't saying much about this mini-space shuttle look-alike. The reusable unmanned vehicle is capable of staying in orbit for 270 days, but the mission duration hasn't been announced. Additionally, the ship has a payload bay for experiments and deployable satellites, but no word if any payloads will be included on the inaugural flight of this mini space plane.

The original X-37 was to feature an AR-2/3 engine for maneuvering and deorbiting, which uses Hydrogen Peroxide and JP-8 as oxidizer and fuel. Later the propusion system has been changed to a hypergolic nitrogen tetroxide / hydrazine version with a robust propellant load for maneuvering in space and for the deorbit burn.

The X-37B is a prototype of a reusable unmanned satellite. The first prototype is for atmospheric testing, while the second has a stengthened structure to support an unshrouded launch, which was originally planned to be launched in the Shuttle payload bay, the transfered after the Columbia accident to a Delta-7920 launch vehicle. Due to problems with the aerodynamics of an unshrouded launch, the first space test was be launched shrouded on a Atlas-5(501) or on a Delta-4M in 2006. Multiple flights with mission durations up to 21 days are planned. NASA asked Boeing to design the X-37 orbital vehicle for a 270-day stay on orbit.

The X-37’s shape is a 120 percent scale derivative of the Air Force’s X-40A, also designed and built by Boeing, which was released from a helicopter and glide-tested in 1998. The X-40A, which lacks the X-37’s advanced thermal protection materials, rocket engine, experiment bay and other spacecraft systems, iwas released from a U.S. Army Chinook helicopter in a series of free flight tests in 2001 to reduce technical risk before flight testing the X-37.

NASA has directed Boeing to throttle back on development of the orbital variant of the X-37 prototype space plane until more money is found for the program, an action likely to delay a re-entry and landing demonstration that was planned for 2006.

Tags: X-37B