While dwindling engine supplies meant that Orbital Sciences Corp already had a plan to replace its elderly AJ-26 (refurbished Russian NK-33) engines used in pairs on the first stage of its Antares launch vehicle, the need to do this has been accelerated by the recent Antares 130 launch failure. In the nearer term, given the hiatus in Antares launches, the company is also urgently looking for replacement launches on an alternative vehicle in order to fulfil its NASA Commercial Resupply Services contract commitments by the end of 2016.
The Antares 130 failure, which lost the Cygnus Orb-3 cargo mission during a launch to the International Space Station, is now thought to have been caused by the failure of an AJ-26 first stage engine turbo-pump. This, and two previous engine test stand failures, one in May and one in 2011, has led to the company giving up on the AJ-26 (NK-33) engine and to urgently search for a new engine.
One replacement possibility remains replacing the pair of AJ-26 engines on Antares with a single Russian-built RD-180, an option initially blocked by ULA (United Launch Alliance) but then later agreed to, partly under legal pressure, and partly after Russian opposition. The RD-180 being flown on ULA-operated Atlas V rockets is used for US military missions. Other sources report that the LOx (liquid oxygen)/kerosene-burning RD-193 engine, built by NPO Energomash for the Soyuz 2-1v, has already been selected as a direct one-for-one replacement for the AJ-26.
However, whatever the final engine choice is, changing all the Antares pipework and tankage for a new Russian engine is likely to take several months if not years. As it is, there may still be significant political difficulty given that the Antares first stage tanks are made in Ukraine.
All this leaves Orbital Sciences Corp with the near term problem of how to find launchers to complete its contract to NASA to send Cygnus resupply vessels to the International Space Station. Orbital has so far refused to fully identify the contenders for outsourced launches but it is thought that the Falcon 9 of its competitor SpaceX is one possibility, while ULA could also offer a version of its Atlas V. One other outsourcing alternative is the Russian/Arianespace operated Soyuz U/2/ST rocket family. Orbital Sciences Corp hopes to fulfil its NASA cargo commitments using four extended Cygnus cargo craft launches instead of its originally planned five normal ones.