COLORADO SPRINGS, Colo. — Aspiring launch services provider Stratolaunch Inc. is set to award Orbital Sciences, Dulles, Va., a contract to build what would be the world’s largest air-launched rocket.

“We have requested a proposal from Orbital and are in the process of negotiating a design and manufacturing contract,” Gary Wentz, Stratolaunch’s chief executive, told SpaceNews in an April 10 email. “We anticipate an award in the May timeframe.”

He declined to discuss the terms of the contract.

Stratolaunch, unveiled to the public in December 2011, is working on a colossal air-launch system that would launch a rocket from underneath the world’s largest airplane — a twin-fuselage carrier ship powered by six jet engines salvaged from decommissioned Boeing 747s. Scaled Composites, Mojave, Calif., is building the carrier aircraft and is targeting a 2017 test flight.

The massive mothership, which has a wingspan of 117 meters, would be capable of flying 2,400 kilometers from a launch site before deploying a rocket.

Orbital has been working on booster concepts since November, and is expected to recommend a final configuration to Stratolaunch at the end of this month, Wentz said.

The details of Orbital’s design for Stratolaunch have not been made public, but models and videos displayed at Orbital’s booth here at the National Space Symposium offer hints.

For example, a computer-generated video playing at Orbital’s booth shows Stratolaunch’s carrier ship deploying a three-stage rocket that appeared to comprise both solid- and liquid-fueled stages. The core stage of the rocket also had aerodynamic chines — which join a wing’s leading edge to a craft’s fuselage to help generate lift at supersonic speeds — and tail fins.

Orbital spokesman Barron Beneski referred questions about the rocket’s design to Wentz, who said only that Stratolaunch has not ruled out a booster with both solid- and liquid-fueled stages.

Stratolaunch is funded by Paul Allen, the Microsoft co-founder who also bankrolled the hybrid rocket-powered SpaceShipOne that in 2004 claimed the $10 million Ansari X Prize by staging two flights to an altitude of 100 kilometers within a two-week period.

Stratolaunch once planned to buy its air-launched booster from Space Exploration Technologies Corp. (SpaceX), and even signed a contract with the Hawthorne, Calif.-based company for work on the rocket.

However, that relationship was dissolved last year after SpaceX decided that designing a four- or five-engine variant of its nine-engine Falcon 9 rocket would be too cumbersome a task, considering the Falcon 9 orders the company is already fulfilling for NASA and a slough of commercial satellite customers.

The original Stratolaunch configuration with the SpaceX booster would have been able to launch 6,100 kilograms to low Earth orbit or 2,300 kilograms to geosynchronous orbit.

Dan Leone is the NASA reporter for SpaceNews, where he also covers other civilian-run U.S. government space programs and a growing number of entrepreneurial space companies. He joined SpaceNews in 2011.Dan earned a bachelor's degree in public communications...