Thrust and specific impulse increases would occur under a separate upgrade program for the Delta IV rocket. Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B. Ares V was dropped as part of the cancellation of the Constellation program in 2010. NASA's current successor heavy-lift vehicle, the Space Launch System, uses four RS-25 engines instead. Human-rating In 2008, it was reported that the RS-68 needs over 200 changes to receive a human-rating certification. NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems. Variants RS-68 is the original version. It produces thrust at sea level. RS-68A is an improved version. It produces thrust at sea level and thrust in a vacuum. RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program. The Ares V was to use six RS-68B engines on a diameter core stage, along with two 5.5-segment solid rocket boosters. It was later determined that the ablative nozzle of the RS-68 was poorly suited to this multi-engine environment, causing reduced engine efficiency and extreme heating at the base of the vehicle. See also Comparison of orbital rocket engines M-1 (rocket engine) National Launch System Rocketdyne J-2 RS-83 Space Launch Initiative TR-106 References External links Aerojet Rocketdyne's RS-68 page RS-68 page on Astronautix.com Rocket engines using hydrogen propellant Rocketdyne engines Rocket engines using the gas-generator cycle Rocket engines of the United States