The Rocketdyne RS-68 (Rocket System 68) is a liquid hydrogen / liquid oxygen engine developed starting in the 1990s with the goal of producing a simpler, less-costly heavy-lift rocket engine for the Delta IV rocket. The RS-68 produces a thrust of 663,000 lbf (2.9 MN) at sea level, while the RS-68A variant has produced 700,000 lbf (3.1 MN) in testing.cite press release | publisher=PRNewswire| date=2008-09-25 | title=United Launch Alliance First RS-68A Hot-Fire Engine Test a Success | url= |quote=Currently, the RS-68 engine can deliver more than 660,000 pounds of sea level thrust and the upgraded RS-68A will increase this to more than 700,000 pounds. The RS-68A also improves on the specific impulse, or fuel efficiency, of the RS-68. | accessdate=2008-09-30] The RS-68B variant is the proposed main engine for NASA's Project Constellation and has 80% fewer parts than the Space Shuttle Main Engine.


The RS-68 was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Los Angeles, California, to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The combustion chamber burns liquid hydrogen and liquid oxygen at 1486 lbf/in² (9.7 MPa) at 102% with a 1:6 engine mixture ratio.

At a maximum 102% thrust, the engine produces 758,000 lbf (3.3 MN) in a vacuum and 663,000 lbf (2.9 MN) at sea level. The engine's mass is 14,560 lb (6,600 kg) at 96 Inches(2.4384 m). With this thrust, the engine has a thrust-to-weight ratio of 51.2, and a specific impulse of 410 s (4 kN·s/kg) in a vacuum and 365 s (3.58 kN·s/kg) at sea level. The RS-68 is gimbaled hydraulically and is capable of throttling from 57% to 102% thrust.

A leading goal of the RS-68 program was to produce a simple engine that would be cost-effective when jettisoned after a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch Space Shuttle main engine (SSME). Simplicity came at the cost of lower thrust-efficiency versus the SSME: the RS-68's thrust-to-weight ratio is significantly lower and the RS-68's specific impulse is 10% lower. The benefit of the RS-68 is its reduced construction cost: To build an RS-68 for the Boeing Delta IV program costs about $14 million, compared to $50 million for the SSME. While the SSME's higher costs were designed to be spread across multiple launches, the larger, less-costly, more powerful (50% more thrust) RS-68 was a more cost-effective engine for an expendable launch vehicle.

The engine itself is a gas generator cycle engine with two independent turbopumps. The combustion chamber uses a channel-wall design to reduce cost. This design, pioneered in the Soviet Union, features inner and outer skins brazed to middle separators, forming cooling channels. This method is heavier, but much simpler and cheaper than the tube-wall design (using hundreds of tubes, bent into the shape of the combustion chamber and brazed together) used in other engines. The lower nozzle has an expansion ratio of 21.5 and is made from an ablative material. The nozzle's lining is designed to burn away as the engine runs, dissipating heat. This is also heavier than the tube-wall nozzles used in other engines, but is also much easier and cheaper to manufacture. While the design was done at the Canoga Park, California facility, where the SSME is manufactured, the initial development engines were assembled at the nearby Santa Susana Field Laboratory where the Saturn V engines were developed and tested for the Apollo missions to the Moon. The RS-68 had initial testing done at Air Force Research Lab, Edwards AFB and later at NASA's John C. Stennis Space Center. The first successful test firing at AFRL was completed on September 11 1998, at Stennis on September 22 1999, 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 five variants of the Delta IV family of launch vehicles. The largest of the launch vehicles includes three CBCs mounted together for the Heavy.

Future use

On May 18, 2006, NASA announced that five RS-68 engines would be used instead of SSMEs on the planned Ares V (CaLV). NASA chose the RS-68 because of its lower cost, about $20 million per engine after NASA upgrades. The modifications to the RS-68 for the Ares V include a different ablative nozzle to accommodate a longer burn, a shorter start sequence, hardware changes to limit free hydrogen at ignition, and changes to reduce helium use during countdown and flight. Thrust and specific impulse increases will occur under a separate upgrade program for Delta IV. [cite press release | publisher=NASA | date=2006-05-18 | title=NASA's Exploration Systems Progress Report | url= | accessdate=2006-05-30] The Ares V is now said to use six RS-68 engines. The engine version for NASA's Ares V is designated RS-68B. [ [ Overview: Ares V Cargo Launch Vehicle] , NASA. Retrieved 30 September 2008.] Another project which should use RS-68 engines (two or three) is the DIRECT proposal.

On April 4, 2008, the Air Force awarded Boeing Launch Services of Huntington Beach, Calif., a modified contract for $20 million. This contract modification will authorize Boeing to perform demonstration testing on a rebuilt RS-68 engine, labeled 10009. The government has authorized work under the Assured Access to Space initiative to develop hardware that will reduce or element these risks and increase the reliability of the RS-68 engine. [cite web || date=2008-09-25 | title=Boeing Launch Services Contract to the Air Force given on 4/4/2008 in support of the Military-Industrial Complex | url= |quote=Boeing Launch Services of Huntington Beach, Calif., is being awarded a modified contract for $20,000,000. This contract modification will authorize Boeing to perform demonstration testing on a rebuilt RS-68 engine, labeled 10009...

On September 25, 2008, the upgraded RS-68A successfully completed its first test firing. The RS-68A is an updated version of the RS-68, with changes to provide increased specific impulse and thrust (to over 700,000 lbf at sea level). The RS-68A is planned to be certified in 2010, with initial launch capability in early 2011.

See also

* RS-83
* RS-84
* J-2 (rocket engine)


External links

* [ Rocketdyne's RS-68 page.]
* [ RS-68 at Astronautix]
* [ RS-68 2002 AIAA paper]
* [ "First RS-68A hot-fire engine test a success",]

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