WASHINGTON — ATK Aerospace will develop and produce “large-diameter” solid-rocket motors for the first and second stages of Stratolaunch Inc.’s planned three-stage air-launched rocket, the Magna, Utah, propulsion provider announced Aug. 13.

Stratolaunch, established in 2011 by Microsoft co-founder Paul Allen, is building what would be the world’s largest air-launched rocket, capable of carrying around 6,800 kilograms of payload to low Earth orbit. The launcher, to be built by Orbital Sciences Corp., requires a massive carrier aircraft, which is being built by Scaled Composites of Mojave, Calif.

ATK’s subcontract with Orbital was signed in late July, Scott Lehr, vice president and general manager of ATK’s Defense and Commercial Division, said in a phone interview. He declined to give the value of the contract.

Stratolaunch of Huntsville, Ala., has said it wants to launch six to 10 missions per year once it is ready to begin routine operations. Gary Wentz, the company’s chief executive, said in an Aug. 20 email that Stratolaunch is targeting its first rocket test for 2018, followed by certification flights for NASA and the U.S. Air Force in 2019.

“We have tentatively asked Orbital to plan for an instrumented payload to fly onboard the demo launch to validate the design and performance of the system,” Wentz wrote. “We have not defined the [payloads for] certification flights [but] we have looked at the NASA Launch Services and U.S. Air Force criteria for certification and are looking at what is required to be positioned to meet their requirements.”

The order Orbital placed with ATK is enough to cover a small number of these initial launches.

The third stage of the Stratolaunch rocket is to be liquid fueled, and Stratolaunch has not yet said which engine it will use. With the exception of Space Exploration Technologies Corp. — which does not sell its Merlin rocket engines to outsiders — and a few of boutique manufacturers, only Aerojet Rocketdyne builds liquid-fueled rocket engines in the United States.

Wentz said Orbital is “working alternative [third-stage] engines and will definitize the approach in the coming months.”

Meanwhile, Orbital’s order will result in some new design and development work at ATK, bringing with it the potential for some new hires in Utah, Lehr said. “We may have to hire some engineering folks, at least in the development phase,” he said. Lehr did not say how many new employees might be necessary, but he did say that their work would be spread across ATK’s Bacchus, Utah, campus, which is near Magna, and the company’s primary rocket test facility in nearby Promontory, Utah.

The motors ATK is building for Orbital will be “large diameter,” Lehr said — along the lines of the 3.7-meter-diameter solid boosters the company built for NASA’s space shuttle program or the 3.2-meter-diameter solids it made for the U.S. Air Force’s Titan 4 rocket. The casings for these motors would be made out of composite materials, similar to what ATK uses for its GEM series of solid boosters that are occasionally used on launches of United Launch Alliance’s Delta 4 rocket.

ATK is the largest U.S. provider of solid-rocket motors. The company also has a long history with Orbital, providing the solid motors used on the latter’s Pegasus, Taurus and Antares launch vehicles. ATK solids also power the interceptor boost vehicle Orbital builds under a subcontract for Boeing Co. for the Missile Defense Agency’s Ground-based Midcourse Defense program.

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...