The space launch vehicle and spacecraft manufacturer Orbital Sciences Corp has a reported shortlist of three engines as it searches for a replacement engine for its Antares launch vehicle. The rocket currently uses two first stage engines of the NK-33 design which were originally used in large numbers on the N-1 rocket booster – the Soviet heavy lift equivalent of the Saturn V. These NK-33 rocket engines, which have been in long term storage for over 40 years, are currently refurbished to AJ-26 standard by the Aerojet Rocketdyne company. However the supplies of these engines are running out and a new engine needs to be found. As reported by Space News, One of the proposals is that these NK-33 engines could be newly manufactured in Russia on a rebuilt production line.
While the Liquid Oxygen (LOx)/kerosene NK-33/AJ-26 is highly rated for its efficient pre-burner rocket cycle, it only has about half the thrust compared to its competitors. One of these is the LOx/kerosene burning Russian-designed RDAmross RD-180 rocket engine which is currently used under licence by the Lockheed Martin-built Atlas V rocket series. While it has two nozzles it only has a single combustion chamber and only one would be needed in place of two NK-33s. Orbital Sciences Corp and the Boeing/Lockheed Martin joint company ULA (United Launch Alliance) have now settled their dispute over the right to use this engine.
The third alternative for Antares is to use an unspecified US rocket design. This latter option maybe favoured for political reasons in the light of the Ukrainian annexation dispute with Russia. Aerojet-Rocketdyne is known to be working on a 1 million pound thrust engine broadly equivalent to the RD-180 called the AJ-1-E6. This uses a similar pre-burner cycle to the Russian engine and would fit the bill though it may be more expensive in the near term to develop.
Less likely would be that Orbital Sciences would be supplied by its competitor Space Exploration Technologies (SpaceX). SpaceX is also working on a large pre-burner cycle engine dubbed Raptor to replace its nine less efficient gas-generator cycle Merlin first stage engines. This new engine would not be LOx/kerosene powered but would use LOx/methane as propellants instead.
Update on 30/04/2014: Russian-sourced engine options may soon be unavailable however as there are political moves in response to the Ukrainian annexation crisis to curtail their use. Aviation Week reports that in the short term the US Congress may ban US military payloads flying on launch vehicles using any Russian-built engines. The US House of Representatives later moved to finance a new build engine to replace the RD-180 which may involve one of the options listed above.