Ares I
Following several design changes to the Ares I and Ares V rockets, the RS-25 was to be replaced with a single J-2X engine for the Ares I second stage and six modified RS-68 engines (which was based on both the SSME and Apollo-era J-2 engine) on the Ares V core stage; this meant that the RS-25 would be retired along with the space shuttle fleet. In 2010, however, NASA was directed to halt the Constellation program, and with it development of the Ares I and Ares V, instead of focusing on building a new heavy-lift launcher.