NPO Energomash
Aerojet agreed to recondition sufficient NK-33s to serve Orbital's 16-flight NASA Commercial Resupply Services contract. Beyond that, it had a stockpile of 23 1960s- and 1970s-era engines. Kuznetsov no longer manufactures the engines, so Orbital sought to buy RD-180 engines. Because NPO Energomash's contract with United Launch Alliance prevented this, Orbital sued ULA, alleging anti-trust violations. Aerojet offered to work with Kuznetsov to restart production of new NK-33 engines, to assure Orbital of an ongoing supply. However, manufacturing defects in the engine's liquid-oxygen turbopump and design flaws in the hydraulic balance assembly and thrust bearings were proposed as two possible causes of the 2014 Antares launch failure. As announced on 5 November 2014, Orbital decided to drop the AJ-26 first stage from the Antares and source an alternative engine. On 17 December 2014, Orbital Sciences announced that it would use the NPO Energomash RD-181 on second-generation Antares launch vehicles and had contracted directly with NPO Energomash for up to 60 RD-181 engines. Two engines are used on the first stage of the Antares 100-series.