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The RS-68 (Rocket System 68) is the largest existing liquid hydrogen / liquid oxygen engine, producing a thrust of 650,000 lbf (2.9 MN) at sea level and 745,000 lbf (3.3 MN) in a vacuum (compare 1.8 MN for a Space shuttle main engine).
It was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Ca. to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The RS-68 was the first new American booster engine developed in over 25 years. The combustion chamber burns liquid hydrogen and liquid oxygen at 1410 lbf/in² (9.7 MPa). The specific impulse is 4000 m/s or 408 seconds at maximum power level.
Simplicity of design and cost effectiveness were the primary design goals of this engine, which resulted in 80% fewer parts than the SSME (and a 10% reduction in specific impulse).
The engine itself is a gas generator cycle engine with two independent turbopumps. The nozzle has an expansion ratio of 21.5 and is made from an ablative material. While the design was done at the Canoga Park facility, where the Space Shuttle Main Engine (SSME) is manufactured, the initial development engines were assembled at the Santa Susana facility where the Saturn V engines were developed and tested for the Apollo missions to The Moon. The RS-68 had initial testing done at Edwards Air Force Base and later at the John C. Stennis Space Center. The first successful test firing was completed on September 11 2000 and the first successful launch using the new engine and launch vehicle occurred on November 20 2002.
The RS-68 is part of the Common Booster Core (CBC) used to create the many variants of the Delta IV family of launch vehicles. The largest of the launch vehicles includes three CBC's mounted together.
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