One of the main goals of the RS-68 program was to produce a simple engine that would be cost-effective when used for a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch
RS-25 Space Shuttle Main Engine (SSME). The adverse consequences of this simplicity were a significantly lower thrust-to-weight ratio and a 10% lower specific impulse compared to the SSME. The benefit of this simplicity is the RS-68's reduced construction cost. The RS-68 was certified in December 2001 for use on Delta IV rockets. An RS-68 was part of each Delta IV
Common Booster Core. The largest of the launch vehicles, the
Delta IV Heavy, used three CBCs mounted together. The engine produced in a vacuum and at sea level. The engine's mass was . With this thrust, the engine had a thrust-to-weight ratio of 51.2 and a
specific impulse of in a vacuum and at sea level. The RS-68 was
gimbaled hydraulically and was capable of throttling between 58% and 102% thrust. The RS-68A is an updated version of the RS-68, with increased specific impulse and thrust (to over at sea level). The first launch on 29 June 2012, from the
Cape Canaveral Air Force Station used three RS-68A engines mounted in a
Delta IV Heavy rocket. The RS-68 was retired as of the last Delta IV Heavy flight in April 2024.
Proposed uses In 2006, NASA announced an intention to use five RS-68 engines instead of SSMEs on the planned
Ares V. NASA chose the RS-68 because of its lower cost, about $20 million per engine including the cost of NASA's upgrades. The upgrades included a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and a reduction in the amount of helium used during countdown and flight. 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 needed 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 ==