|Country of origin||Soviet Union|
|Designer||Kuznetsov Design Bureau|
|Application||1st stage engine|
|Successor||AJ26-58, AJ26-59, AJ26-62|
|Propellant||LOX / kerosene|
|Thrust (vac.)||1,680 kN (380,000 lbf)|
|Thrust (SL)||1,510 kN (340,000 lbf)|
|Chamber pressure||14.83 MPa (2,151 psi)|
|Isp (vac.)||331 seconds (3.25 km/s)|
|Isp (SL)||297 seconds (2.91 km/s)|
|Length||3.7 m (12 ft)|
|Diameter||2 m (6 ft 7 in)|
|Dry weight||1,240 kg (2,730 lb)|
The NK-33 and NK-43 are rocket engines designed and built in the late 1960s and early 1970s by the Kuznetsov Design Bureau. The NK designation is derived from the initials of chief designer Nikolay Kuznetsov. They were intended for the ill-fated Soviet N-1 rocket moon shot. The NK-33 engine is among the highest thrust-to-weight ratio of any Earth-launchable rocket engine, only NPO Energomash RD-253 and SpaceX Merlin 1D engine achieve a higher ratio. The specific impulse of the NK-33 is significantly higher than both these engines.
The NK-43 is similar to the NK-33, but is designed for an upper stage, not a first stage. It has a longer nozzle, optimized for operation at altitude, where there is little to no ambient air pressure. This gives it a higher thrust and specific impulse, but makes it longer and heavier.
Modified versions of these engines by Aerojet are known as the AJ26-58, AJ26-59 and AJ26-62.
In 2010 stockpiled NK-33 engines were successfully tested for use by the Orbital Sciences Antares light-to-medium-lift launcher. 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. This failure led Orbital Sciences to replace the AJ-26 engines with newly built RD-181 engines for future launches of the Antares rocket.
NK-33 and NK-43 are derived from the earlier NK-15 and NK-15V engines, respectively.
The engines are high-pressure, regeneratively cooled staged combustion cycle bipropellant rocket engines, and use oxygen-rich preburners to drive the turbopumps. The turbopumps require subcooled liquid oxygen (LOX) to cool the bearings. These kinds of burners are highly unusual, since their hot, oxygen-rich exhaust tends to attack metal, causing burn-through failures. The United States had not much investigated oxygen-rich combustion technologies until the Integrated Powerhead Demonstrator project in the early 2000s. The Soviets, however, perfected the metallurgy behind this method. The nozzle was constructed from corrugated metal, brazed to an outer and inner lining, giving a simple, light, but strong structure. In addition, since the NK-33 uses subcooled LOX and kerosene, which have similar densities, a single rotating shaft could be used for both turbopumps. Given its longer, heavier nozzle, the NK-43 ratio in vacuum is slightly heavier, with a thrust-to-weight ratio of about 120:1.
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. 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.
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 America. Nearly thirty years after they were built, disbelieving rocket engineers were led to the warehouse. One of the engines was later taken to America, and the precise specification of the engine was demonstrated on a test stand.
About 150 engines survived, and in the mid-1990s, Russia sold 36 engines to Aerojet General for $1.1 million each. This company also acquired a license for the production of new engines. Aerojet has modified and renamed the updated NK-33 to AJ26-58 and AJ26-62, and NK-43 to AJ26-59. 
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, 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.
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. The NK-33 powered rocket was 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) is as of 2015 not realized, which results in a reduced payload compared to the Soyuz-2 launch vehicle.
On May 13, 2014, Russian Deputy Prime Minister Dmitry Rogozin announced that Russia would no longer supply rocket engines for U.S. military launches, amid tensions arising from the 2014 Russian military intervention in Ukraine.
Current and proposed uses
The Orbital Sciences Antares light-to-medium-lift launcher has two modified NK-33 in its first stage, a solid Castor 30-based second stage and an optional solid or hypergolic third stage. The NK-33s are imported from Russia to the United States and modified into Aerojet AJ26s, which involves removing some electrical harnessing, adding U.S. electronics, qualifying it for U.S. propellants, and modifying the steering system.
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.
Aerojet has agreed to recondition sufficient NK-33s to serve Orbital's 16-flight NASA Commercial Resupply Services contract. Beyond that, it has a stockpile of 23 1960s and 1970s era engines. Kuznetsov no longer manufactures the engine, and the lack of a continuing supplier brings into question the long-term viability of Antares. To address this, Orbital has sought to buy RD-180 engines, but maker NPO Energomash's contract with United Launch Alliance prevents this. Orbital has sued ULA for this, alleging anti-trust violations. Aerojet has offered to work with Kuznetsov to restart production of new NK-33 engines, to assure Orbital of an ongoing supply. As announced on 5 November 2014, and due to the recent October 28 launch failure, Orbital will drop the AJ-26 first stage from the Antares and source an alternative engine.
Aurora-L.SK and Soyuz-2-1v
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 Soyuz-2-1v was launched successfully on its maiden flight with Aist 1 on 28 December 2013.
An NK-33, renamed to Aerojet AJ26, in a Test Stand at John C. Stennis Space Center.
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- The Engines That Came In From The Cold!, Equinox, Channel Four Television Corporation, 2000. Documentary video on Russian rocket engine development of the NK-33 and its predecessors for the N1 rocket. (NK-33 story starts at 24:15–26:00 (program shuttered in 1974); the 1990s resurgence and eventual sale of the remaining engines from storage starts at 27:25; first use on a US rocket launch in May 2000.)
- NK-33's specifications
- NK-33 specifications & key components design (in Russian)
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