The Antares design has been upgraded with newly-built RD-181 first stage engines to provide greater payload performance and increased reliability. Zdroj Orbital ATK
https://www.orbitalatk.com/flight-systems/sp…
The RD-181 engine completed its certification program May 7 with the last of seven hotfire tests in Russia, according to NPO Energomash of Khimki, Russia, the engine’s manufacturer.