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. Current and proposed uses RSC Energia is proposing an "Aurora-L.SK" launch vehicle, which would use an NK-33 to power the first stage and a Blok DM-SL for the second stage. Soyuz-2-1v In the early 2010s the Soyuz launch vehicle family was retrofitted with the NK-33 engine – using the lower weight and greater efficiency to increase payload; the simpler design and use of surplus hardware might actually reduce cost. TsSKB-Progress uses the NK-33 as the first-stage engine of the lightweight version of the Soyuz rocket family, the Soyuz-2-1v. The NK-33A intended for the Soyuz-2-1v was successfully hot-fired on 15 January 2013, following a series of cold-fire and systems tests of the fully assembled Soyuz-1 in 2011–2012. The NK-33 powered rocket was finally designated Soyuz-2-1v, with its maiden flight having taken place on 28 December 2013. One NK-33 engine replaces the Soyuz's central RD-108, with the four boosters of the first stage omitted. A version of the Soyuz rocket with four boosters powered by NK-33 engines (with one engine per booster) has not been built, which results in a reduced payload compared to the Soyuz-2 launch vehicle. Versions During the years there have been many versions of this engine: NK-15 (GRAU index 11D51): Initial version for the N1 first stage. NK-15V (GRAU index 11D52): Modified NK-15 optimized for vacuum operation, used on the N1 second stage. NK-33 (GRAU index 11D111): Improved version for the N1F first stage, never flown. NK-43 (GRAU index 11D112): Vacuum-optimized NK-33 for the N1F second stage, never flown. AJ26-58 and AJ26-59: Modified NK-33 by Aerojet Rocketdyne. Planned used on the Kistler K-1. AJ26-62: Modified NK-33 with additional gimbal mechanism by Aerojet Rocketdyne. Used on the Antares 100-series first stage. NK-33A (GRAU index 14D15): Refurbished NK-33. Used on the Soyuz-2-1v first stage. NK-33-1: Uprated NK-33 with gimbal mechanism. Planned used on the Soyuz-2.3 core stage. Gallery See also Comparison of