Stratolaunch is to complete the systems design review (SDR) of its new launch system "in the next couple of months".

That is the timeframe set out by Jim Halsell, director of Stratolaunch systems at Dynetics, which has been contracted to design the technical integration and to mate and demate procedures and systems.

"We are on the cusp of doing the systems design review, and we're moving toward a preliminary design review [PDR]," said Halsell. "Between those two, the SDR and the PDR, we will lock down the details of the technical approach, the outer mold lines of all the systems. It's the grunt early work of designing a complex system."

Major system trades and exact specifications, including information crucial to operation such as maximum gross take-off weight and required runway length, will not be finalised until the PDR.

Disclosed in December 2011, the ambitious Stratolaunch system involves a massive Scaled Composites-built aircraft with a SpaceX-built rocket suspended between twin fuselages. The system will launch payloads of up to 6,100kg (13,500lb) in weight and 5m (16.4ft) in diameter into low Earth orbit (LEO). Although Stratolaunch eventually hopes to launch people into orbit and will build to strict human spaceflight standards, design efforts are on hold while the focus is on building and testing the launch system.

Preliminary construction has begun on the assembly facility in Mojave, California, where the aircraft will be built and tested. Construction of a wing spar and wing box for test purposes has also begun, with actual operational examples scheduled for completion in the summer.

Scaled Composites has selected two ex-United Airlines Boeing 747-400s, from which the company will take the Pratt & Whitney 4056 engines, hydraulic system, electrical systems, landing gear and windshields, among other major components.

"While the 747-400 wasn't the only airplane [available], it quickly became apparent that it was a good choice, and that a lot of the systems were designed for the take-off and landing weights in the family of what we're talking about here," said Halsell. "The hydraulic systems, the electrical systems, all of them had the kind of capacity or greater than what we would need for our application."

The first rocket launch is scheduled for 2016; no customers have yet stepped forward, but Stratolaunch hopes to be competitive in the light-to-medium satellite market, a growing market in a niche inhabited by the SpaceX Falcon 1, Boeing Delta II and Orbital Sciences Antares launch vehicles. Production of both the Falcon 1 and Delta II have ceased, although options remain for restarting production, and the Antares has yet to complete its first launch, scheduled for June 2012.

Although Stratolaunch officials have repeatedly mentioned plans to operate from the NASA Kennedy Space Center (KSC) runway, one of the longest and widest runways in the world, there has as yet been no formal agreement between Stratolaunch and facility operator Space Florida.

Operating from the KSC runway would enable Stratolaunch to fly south, closer to the equator, allowing greater payload and launch azimuth flexibility. Launching to the east over the Atlantic Ocean would take advantage of the Earth's rotation, allowing additional advantages.

Only a single aircraft will be produced, but Stratolaunch is open to building more aircraft. "Certainly our technical focus right now is making it work for a launch platform," said Halsell. "However, it is not beyond a stretch of the imagination, if a customer were to come to us and say, 'I need an externally carried large payload of significant mass and also volume requirements,' we would certainly value the opportunity to take a swing at satisfying those requirements."

According to Stratolaunch chief executive Gary Wentz, a larger version of the aircraft is feasible for launching larger rockets or carrying outsize cargo. "Based on physics and aerodynamics, scaling up is feasible," he said. "Material selection and design of the wing structure will have a great effect. Also, growing the wing to be much longer presents operational issues with runway selection."

Source: Flight International