On March 10, 2009 a SpaceX press release announced successful testing of the Merlin Vacuum engine. A variant of the 1C engine, Merlin Vacuum features a larger exhaust section and a significantly larger expansion nozzle to maximize the engine's efficiency in the vacuum of space. Its combustion chamber is regeneratively cooled while the 2.7 metres (9 ft)-long[18] niobium alloy[1] expansion nozzle is radiatively cooled. The engine delivers a vacuum thrust of 411 kN (92,500 lbf) and a vacuum specific impulse of 342 seconds.[19] The first production Merlin Vacuum engine underwent a full duration orbital insertion firing (329 seconds) of the integrated Falcon 9 second stage on January 2, 2010.[20] It was flown on the second stage for the inaugural Falcon 9 flight on June 4, 2010. At full power the Merlin Vacuum engine operates with the greatest efficiency ever for an American-made hydrocarbon rocket engine.[21]
An interesting unplanned test of a modified Merlin Vacuum engine was made in December 2010. Shortly before the scheduled second flight of the Falcon 9, two cracks were discovered in the 2.7 metres (9 ft)-long Merlin Vacuum niobium alloy sheet nozzle. The engineering solution was to cut off the lower 1.2 metres (4 ft) of the nozzle, and launch two days later, as the extra performance that would have been gained from the longer nozzle was not necessary to meet the objectives of the mission. Even with the shortened nozzle, the engine placed the second-stage into an orbit of 11,000 kilometres (6,800 mi) altitude."
Tomoren a foldfelszini es alacsony magassagi valtozat helyett a magaslegkorre es bolygokoruli palyara optimalizalt merlin hajtomu. Alapvetoen nagyobb a fuvoka merete mint az alap tipusnal, ezert az urbeli vakumban hatekonyabb.