Ракетный двигатель RS-68: разработка, модификации и применение.

A large hydrogen oxygen rocket engine that powers the Delta IV rocket
Its development started in the 1990s with the goal of producing a simpler, less costly, heavy lift engine for the Delta IV launch system. Two versions of the engine have been produced: the original RS 68 and the improved RS 68A. A third version, the RS 68B, was planned for the National Aeronautics and Space Administration's (NASA) Ares V rocket before cancellation of the rocket and the Constellation Program in 2010.

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 June 29, 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.

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.

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.

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. Its specific impulse in a vacuum is Certification was completed in April 2011. RS 68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program.