Orbital ATK signed agreement with Vulcan Aerospace and declared that it will provide number of Pegasus XL rockets to be used with Stratolauncher HTHL launch vehicle. It is positive end of long history of relations between both companies and sign, that Stratolauncher is reaching end of its development phase with Pegasus XL as second stage.
Orbital ATK announced on October 6, 2016, about signing contract, which covers delivering for Vulcan Aerospace, company managing Stratolaunch Systems designer and manufacturer of Stratolauncher, unspecified quantity of their Pegasus XL rockets. They will be used during commercial missions of Stratolauncher vehicle as its second stage. Plane will take off from regular airfields, reach specified altitude and deploy Pegasus XL with up to 450 kg of payload on atop. Rocket will gradually increase altitude and change angle of attack to reach LEO orbit. Cooperation with Orbital ATK is nothing new for Vulcan Aerospace: both companies were cooperating under Stratolauncher, also Pegasus XL was considered earlier as potential second stage for Stratolauncher, but Companies did not manage to reach any final agreement.
There is one very interesting matter in this contract: Orbital ATK in fact has own motherplane for Pegasus XL, Lockheed L-1011, and already contracted one launch for 2016 and second mission for 2017. It was not unveiled when Stratolauncher will start to operate with Pegasus XL, but on June 2016, Stratolaunch Systems announced that vehicle is ready in 76% and first flight is more than possible before end of 2016. It means that after first test flight, first commercial mission could be possible in the second half of 2017, what probably will not create competition for market plans of Orbital ATK (it would not be good for Orbital to cooperate with launch service provide with similar launch system). Probably Orbital ATK would like to abandon using Pegasus rocket, which was operated by Orbital ATK since 1990 and finally switch to ordinary rocket launch vehicles like Antares. It is worth to remind that Company put effort and money to upgrade their Antares to 230 version and is planning to use it extensively in following years. Contract with Vulcan Aerospace is probably is the way to get rid of these Pegasus rockets, which were already partially assembled or parts necessary for manufacturing process were already contracted and delivered. From the other side, for Vulcan Aerospace it is great way for entering commercial market - until now Company has not announced about significant progress in development of their own project of second stage for Stratolauncher. After signing contract with Orbital ATK and acquiring Pegasus XL rockets, Stratolaunch vehicle will be fully operational sooner and will start to provide income for Vulcan Aerospace.
Stratolaunch showed in their press release picture of Stratolauncher with three Pegasus XL rockets attached to the center of the wing - with planned 250t capacity of the Stratolauncher (it will be the biggest airplane ever created) it is possible, because Pegasus XL weight is 23 t with payload and propellant. Using three rockets during single flight will surely more economic comparing to flight with one rocket (like Orbital ATK) what makes possible fact reducing role of Orbital ATK only to rocket manufacturer.