Stratolaunch has officially signed Orbital Sciences to build its air-launched rocket, to be hauled to altitude by one of the largest aircraft in the world.

The contract confirms that Orbital will design and build the rocket, capable of launching 6,100kg (13,500lb) into low Earth orbit (LEO). Orbital was previously engaged to make conceptual studies, and was considered to be the odds-on favorite for design. Partner Scaled Composites will design and build the carrier aircraft, which has no formal name as of yet.

While few design characteristics have been formally released, Stratolaunch indicates that the launch vehicle (LV) will have both solid and liquid-fueled stages. A report from says the LV will have two solid-fuel core stages and a liquid-fuel upper stage, potentially powered by two Rocketdyne RL-10 engines.

Design models and concept artwork show horizontal stabilizers on the aft end of the LV, allowing the rocket to turn itself upwards after release, as well as a vertical stabilizer for course correction.

"We're not ready to give anybody any of the details of the rocket yet," says Orbital. "Today is really more about Stratolaunch saying, we've concluded the study contract with Orbitaland we're now moving ahead with the design and development of the rocket with details to follow."

SpaceX, initially favoured to build a rocket based on its Falcon 9, dropped out citing major design changes.

The carrier aircraft, currently under construction, will be built largely of composite materials, using subsystems and six engines scavenged from two ex-United Airlines 747-400s. The system requires a 12,500ft runway for takeoff, but has an operational radius of up to 1850km (1,000nm). The aircraft is scheduled for test flights in 2016.

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Orbital also builds the Pegasus, a smaller rocket air-launched by a modified Lockheed L-1011, currently the only operational air-launch rocket in the world.

"We have a lot of operational expertise, from a design/development standpoint to operational methodologies, everything from how to integrate it with the carrier aircraft and all the flight dynamics," says Orbital. "This is on a vastly different scale, but we have more experience than anybody in the world doing it, we've put together a good team and we're ready to move forward."

Orbital declined to provide additional details citing ongoing discussions. Stratolaunch did not respond to immediate questions.

Source: Flight International