Slowdown

NASA is droping plans to modify the shuttle main engine for the job, in exchange for long term cost saving on the way to Mars. A decision to use a modern version of the Apollo-era J-2 upper stage engine to launch the planned Crew exploration Vehicle will eliminate the need to develop a Space Shuttle Main Engine (SSME) that can be started at altitude. The low power of the J-2 230,000 lb vs 512,300 lb in vacuum for the SSME will mean the 'single stick' Crew Launch Vehicle will require five solid fuel segments in its first stage instead of four. That matches the solid fuel requirement for the shuttle derived heavy lifter to be used on lunar missions, so NASA won't have to maintain two different solid fuel boosters as it works its way to moon. One possibility that could change the ultimate decision to use methane could be discovery of water ice at the lunar poles, which would make a liquid oxygen/liquid hydrogen engine more attractive for lunar ascent.