ATK [ATK] will develop the largest diameter composite-cased solid rocket motors the company has ever assembled for Stratolaunch Systems’ Air Launch Vehicle (ALV), according to a company executive.

ATK said it was awarded a contract from Orbital Sciences [ORB], the integrator of Stratolaunch’s ALV, to provide first and second stage propulsion as part of a revolutionary air-launched space transportation system. The contract includes the design, development and flight hardware for the initial Stratolaunch missions.

ATK Vice President and General Manager of Defense and Commercial Scott Lehr told Defense Daily yesterday the company wasn’t sure if the third stage motor would be liquid or solid, but he believed it would be liquid and that it would be Orbital’s decision. An email to Orbital was not returned by press time. ATK, along with GenCorp’s [GY] Aerojet Rocketdyne division, is one of two solid rocket motor manufacturers in the United States.

ATK has experience developing large diameter solid rocket motors, including those used on the Space Shuttle and for the Titan IVB launch vehicle. Lehr said in a statement the stages will use high-strength, low-weight graphite composite cases, advanced propellants and heritage materials from ATK’s line of commercial solid rocket motors.

Orbital and ATK have worked together in the past on Pegasus, Orbital’s air launched rocket launched from its Stargazer L-1011 carrier aircraft. Stargazer is an L-1011 commercial transport aircraft modified to launch Pegasus as well as a serve as a platform for airborne research projects. The aircraft has been used to successfully launch 35 Pegasus rockets, according to Orbital.

Lehr said ATK’s biggest challenge would be its fitting motors into the ALV’s size and weight constraints due to the dimensions of the aircraft and the length and weight of the rocket.

“Balancing all that out is by far the biggest challenge here,” Lehr said. “In an ALV, weight is really a premium.”

Lehr deferred questions on when work was to begin and complete to Orbital. Lehr also declined to comment on the value of the contract.

According to a video posted on Stratolaunch’s website, the ALV will have capability to deliver unmanned payloads and manned spacecraft into orbit. Stratolaunch’s Scaled Composites-developed carrier aircraft will be powered by six 747 engines with the wingspan of 385 feet and a gross weight of over 1.3 million pounds. The carrier vehicle will also have a mission range of 1,000 nautical miles and the goal of delivering 13,500 pounds of payload to low earth orbit (LEO).

Lehr said the biggest advantage of an ALV over a traditional ground launch is you aren’t constrained by weather as you are with a pad launch.

“All weather, any orbit, based on the aircraft being able to fly to a specific location before it drops a vehicle,” Lehr said.

Stratolaunch is on track for carrier aircraft flight testing by 2016, according to a company statement released in June.