After troubles with Falcon-9 FT, which caused delay with Dragon spacecraft cargo missions, Orbital ATK seemed to be Deus ex machina with their Antares 230. But due the same reason it seems, that again Orbital ATK will have to rely on Atlas V launch vehicle.
According to latest news Orbital ATK is going to resume cooperation with United Launch Alliance. ULA was contracted as launch service provider during period after disaster of Antares rocket on October 28 2014 until latest mission of Antares 230. As far as launch performed on October 17, 2016 with Cygnus cargo spacecraft on atop was full success, Orbital ATK declared, that would like to launch their next Cygnus OA-7 using Atlas V rocket instead Antares. It is quite unusual, especially if we take under consideration previous announcements of Orbital ATK that Company is not going to use Atlas V in following flights of their cargo spacecraft. Here is the explanation: OA-7 mission is scheduled for March 2017 and will cover delivering 3500 kg of payload in pressurized section of Cygnus additional external Nanoracks deployment device weighing with Cubesats around 100 kg. Total cargo inside Cygnus will exceed 3200 kg, which is limit of payload capacity for Antares during missions to International Space Station. This fact was also mentioned in official statement of Orbital ATK – Company claims, that decision is result of joint discussion with NASA and necessity of delivering more payload in March 2017. NASA is forced provide resources for scientific experiments conducted by astronauts inside ISS and keep research schedule. After number of problems which have appeared in 2016 with all four spacecrafts delivering cargo to ISS, delay should be reduced in following year. To remind: Japanese HTV was postponed for September 30 to December 9; Russian Progress was postponed after problems with onboard computer from April 22 to December 1. Cygnus also was delayed due the prolonged time devoted for testing Antares 230 for three months (from June to October). Now probably in 2017 we can expect that space of cargo vehicles flying to ISS will be extremely well used, so it should not be surprising that NASA would like to stretch OA-5 a little bit more, even if this decision will not make management of Orbital ATK happy.