9 flight on June 4, 2010. At full power and as of March 10, 2009, the Merlin Vacuum engine operates with the greatest efficiency of any American-made hydrocarbon-fueled rocket engine. An 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 niobium-alloy-sheet nozzle of the Merlin Vacuum engine. The engineering solution was to cut off the lower 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. The modified engine successfully placed the second stage into an orbit of altitude. Merlin 1D The Merlin 1D engine was developed by SpaceX between 2011 and 2012, with first flight in 2013. The design goals for the new engine included increased reliability, improved performance, and improved manufacturability. In 2011, performance goals for the engine were a vacuum thrust of , a vacuum specific impulse (Isp) of , an expansion ratio of 16 (as opposed to the previous 14.5 of the Merlin 1C) and chamber pressure in the "sweet spot" of . Merlin 1D was originally designed to throttle between 100% and 70% of maximal thrust; however, further refinements since 2013 now allow the engine to throttle to 40%. The basic Merlin fuel/oxidizer mixture ratio is controlled by the sizing of the propellant supply tubes to each engine, with only a small amount of the total flow trimmed out by a "servo-motor-controlled butterfly valve" to provide fine control of the mixture ratio. On November 24, 2013, Elon Musk stated that the engine was actually operating at 85% of its potential, and they anticipated to be able to increase the sea-level thrust to about and a thrust-to-weight ratio of 180. This version of the Merlin 1D was used on Falcon 9 Full Thrust and first flew on Flight 20. In May 2016, SpaceX announced plans to further upgrade the Merlin 1D by increasing vacuum thrust to and sea-level thrust to ; according to SpaceX, the additional thrust will increase the Falcon 9 LEO payload capability to about 22 metric tons on a fully expendable mission. SpaceX also noted that unlike the previous Full Thrust iteration of the Falcon 9 vehicle, the increase in performance is solely due to upgraded engines, and no other significant changes