NK-33

The N-1 launcher originally used NK-15 engines for its first stage and a high-altitude modification (NK-15V) in its second stage. After four consecutive launch failures and no successes, the project was cancelled. While other aspects of the vehicle were being modified or redesigned, Kuznetsov improved his contributions into the NK-33 and NK-43 respectively.[6] The 2nd-generation vehicle was to be called the N-1F. By this point the Moon race was long lost, and the Soviet space program was looking to the Energia as its heavy launcher. No N-1F ever reached the launch pad.[7]

When the N-1 program was shut down, all work on the project was ordered destroyed. A bureaucrat instead took the engines, worth millions of dollars each, and stored them in a warehouse. Word of the engines eventually spread to the US. Nearly 30 years after they were built, rocket engineers were led to the warehouse. One of the engines was later taken to the US, and the precise specification of the engine was demonstrated on a test stand.[7]

The NK-33 closed-cycle technology works by sending the auxiliary engines' exhaust into the main combustion chamber. This made the engine design unique. This technology was believed to be impossible by Western rocket engineers.[8] The fully heated liquid O₂ flows through the pre-burner and into the main chamber in this design. The extremely hot oxygen-rich mixture made the engine dangerous: it was known to melt 3-inch (76 mm) thick castings "like candle wax. One of the controversies in the Kremlin over supplying the engine to the US was that the design of the engine was similar to Russian ICBM engine design. The NK-33's design was used in the later RD-180 engine, which had twice the size of the NK-33. The RD-180 engines were used (as of 2016) to power the Atlas V rocket. This company also acquired a license for the production of new engines.[9][10][11]

About 60 engines survived in the "Forest of Engines", as described by engineers on a trip to the warehouse. In the mid-1990s, Russia sold 36 engines to Aerojet General for $1.1 million each, shipping them to the company facility in Sacramento CA.[12] During the engine test in Sacramento, the engine hit its specifications.

Aerojet has modified and renamed the updated NK-33 to AJ26-58, AJ-26-59 and AJ26-62, and NK-43 to AJ26-60.[9][10][11][13]

Kistler Aerospace, later called Rocketplane Kistler (RpK), designed their K-1 rocket around three NK-33s and an NK-43. On August 18, 2006, NASA announced that RpK had been chosen to develop Commercial Orbital Transportation Services for the International Space Station. The plan called for demonstration flights between 2008 and 2010. RpK would have received up to $207 million if they met all NASA milestones,[14][15][16] but on September 7, 2007, NASA issued a default letter, warning that it would terminate the COTS agreement with Rocketplane Kistler in 30 days because RpK had not met several contract milestones.[17]

An Antares rocket being rolled out for testing, showing the two NK-33 engines

The initial version of the Orbital Sciences Antares light-to-medium-lift launcher had two modified NK-33 in the first stage, a solid Castor 30-based second stage and an optional solid or hypergolic third stage.[18] The NK-33s were imported from Russia to the United States, modified, and re-designated as Aerojet AJ26s. This involved removing some electrical harnessing, adding U.S. electronics, qualifying it for U.S. propellants, and modifying the steering system.[19]

In 2010 stockpiled NK-33 engines were successfully tested for use by the Orbital Sciences Antares light-to-medium-lift launcher.[19] The Antares rocket was successfully launched from NASA's Wallops Flight Facility on April 21, 2013. This marked the first successful launch of the NK-33 heritage engines built in early 1970s.[20]

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.[21] Aerojet offered to work with Kuznetsov to restart production of new NK-33 engines, to assure Orbital of an ongoing supply.[22] 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.[23] 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.[24]

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.[26] TsSKB-Progress uses the NK-33 as the first-stage engine of the lightweight version of the Soyuz rocket family, the Soyuz-2-1v.[27] The NK-33A intended for the Soyuz-2-1v was successfully hot-fired on 15 January 2013,[28] following a series of cold-fire and systems tests of the fully assembled Soyuz-1 in 2011–2012.[29] 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.