NASA could switch entirely to the Atlas V for future Cygnus flights. And Orbital ATK wants to build an Atlas V replacement

After the 2014 accident, Orbital scrambled to figure out what happened, eventually blaming the rocket's refurbished Soviet-era engines. In the meantime, they bought a United Launch Alliance-owned Atlas V to fly a Cygnus vehicle to the ISS. Because the Atlas V is more powerful, NASA and Orbital were able to squeeze more cargo into Cygnus, increasing its mass capacity from 3,200 kg to 3,513 kilograms. By using Atlas, Orbital realized it could fulfill its contractual obligation to fly 20 metric tons of cargo to the ISS in seven Cygnus flights instead of eight.

In 2015, NASA ordered three more Cygnus flights, bringing Orbital's total obligation up to 10. Orbital, in turn, booked two more Atlas rockets.

The company's final four CRS-1 cargo flights will use Antares, but all of this shuffling around has very much put the Atlas V in play for CRS-2, the second round of ISS cargo flights that will begin in 2019. Orbital ATK, SpaceX and Sierra Nevada are the winning CRS-2 providers, but no missions have yet been awarded.

"NASA hasn't actually told us which missions they'll want on which vehicles, and that's part of the contract proposal we made to them," said Orbital ATK space systems group president Frank Culbertson during an April 17 press conference. "So we're waiting to see which way they'd like to go, whether it's a mix or all one or the other."

Capping the Antares uncertainty is Orbital ATK's development of a larger rocket called the Next Generation Launch System, or NGL. NGL development has been supported by the U.S. Air Force. In 2016, the Air Force awarded contracts to Orbital ATK and SpaceX to spur development of possible replacements to the Atlas V, which uses Russian-built engines. Another round of Air Force contract awards are expected next year.

The competition, however, could be steep. And United Launch Alliance already has its own Atlas V exit plan: the Vulcan.