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RS-68 rocket engine test.jpg
An RS-68 engine undergoing hot-fire testing at NASA's Stennis Space Center during its developmental phase.
Country of origin United States
Manufacturer Rocketdyne
Pratt & Whitney Rocketdyne
Aerojet Rocketdyne
Application First stage engine for Delta IV rocket
Liquid-fuel engine
Propellant LOX / LH2
Cycle Gas-generator cycle
Nozzle ratio 21.5
Thrust (SL) RS-68: 660,000 lbf (2,950 kN)
RS-68A: 705,000 lbf (3,137 kN)
Chamber pressure 1,488 psi (10.26 MPa)
Isp (vac.) RS-68: 410 s (4.0 km/s)
RS-68A: 412 s (4.04 km/s)[1]
Length 17.1 ft (5.20 m)
Diameter 8.0 ft (2.43 m)
Dry weight RS-68: 14,560 lb (6,600 kg)
RS-68A: 14,870 lb (6,740 kg)[2]
Used in
Delta IV

The Aerojet Rocketdyne (formerly Rocketdyne and later Pratt & Whitney Rocketdyne) RS-68 (Rocket System 68) is a liquid-fuel rocket engine that uses liquid hydrogen (LH2) and liquid oxygen (LOX) as propellants in a gas-generator power cycle. It is the largest hydrogen-fueled rocket engine.[3]

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 NASA's Ares V rocket that was later canceled.

Design and development

A leading goal 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 Space Shuttle Main Engine (SSME).[4] Simplicity came at the cost of lower thrust-efficiency versus the SSME: the RS-68's thrust-to-weight ratio is significantly lower and its specific impulse is 10% lower.[5] The benefit of the RS-68 is its reduced construction cost.[4] The RS-68 is larger, and more powerful than the SSME and it was designed to be a more cost-effective engine for an expendable launch vehicle.[citation needed]

The engine uses a gas generator cycle with two independent turbopumps. The combustion chamber uses a channel-wall design to reduce cost. This design, pioneered in the former Soviet Union, features inner and outer skins brazed to middle separators, forming cooling channels. Although this method is heavier, it is much simpler and less costly 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 lined with an ablative material. The nozzle's lining burns away as the engine runs, dissipating heat. This ablative coating is heavier than other engines' tube-wall nozzles but much easier and less expensive to manufacture. The presence of carbon in the exhaust from the ablative carbon-containing inner nozzle lining can be inferred by the yellow color of the engine exhaust, unlike the SSME's nearly-transparent flame of pure hydrogen burning. The combustion chamber burns liquid hydrogen and liquid oxygen at 1,486 lbf/in2 (10.25 MPa) at 102% power with a 1:6 engine mixture ratio.[citation needed]

The RS-68 was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Los Angeles, California, where the SSME is manufactured. It was designed to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The initial development engines were assembled at the nearby Santa Susana Field Laboratory where the Saturn V F-1 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 Stennis Space Center. The first successful test firing at AFRL was completed on September 11, 1998.[citation needed] The RS-68 was certified for use on Delta IV in December 2001.[6] 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.

At a maximum 102% thrust, the engine produces 758,000 pounds-force (3,370 kN) in a vacuum and 663,000 pounds-force (2,950 kN) at sea level. The engine's mass is 14,560 pounds (6,600 kg) at 96 inches (2.4 m). With this thrust, the engine has a thrust-to-weight ratio of 51.2, and a specific impulse of 410 s (4.0 km/s) in a vacuum and 365 s (3.58 km/s) at sea level.[7] The RS-68 is gimbaled hydraulically and is capable of throttling between 58% and 101% thrust.[8]

The RS-68A is an updated version of the RS-68, with changes to provide increased specific impulse and thrust (to over 700,000 pounds-force (3,100 kN) at sea level).[9] The first launch used three RS-68A engines mounted in a Delta IV Heavy. This first launch occurred June 29, 2012 from the Cape Canaveral Air Force Station.[10]

Proposed uses

In 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 included 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 would occur under a separate upgrade program for Delta IV.[11] Later the Ares V was changed to use six RS-68 engines, designated RS-68B.[12] Ares V was canceled along with Project Constellation; NASA's successor heavy-lift vehicle, the Space Launch System, uses Space Shuttle Main Engines (SSMEs).

The DIRECT alternative launch project included two or three RS-68 engines in "version 2.0" of the team's proposal, but switched to the SSME for "version 3.0".


It would reportedly require over 200 changes to the RS-68 to meet human-rating standards.[13] NASA states several changes are needed to human-rate the RS-68, including health monitoring, removal of fuel-rich environment at liftoff, and improved subsystems robustness.[14][15]


  • RS-68 is the initial engine version. It produces 663,000 pounds-force (2,950 kN) thrust at sea level.[16] Last flight was on 25th March 2015.
  • RS-68A is an improved engine version. It produces 705,000 lbf (3,140 kN) thrust at sea level and 800,000 lbf (3,560 kN) thrust in vacuum.[17] Vacuum specific impulse is 414 seconds (4.06 km/s). Certification testing was completed in November 2010. First flight was on a Delta IV Heavy launching NROL-15 on June 29, 2012.
  • RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program.[12] The Ares V was to use six RS-68B engines on a 10-meter 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.[18]

See also


  1. "Delta IV User's Guide" (PDF). ULA. Retrieved June 2013. Check date values in: |accessdate= (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. "DELTA IV". ULA. Retrieved July 2014. Check date values in: |accessdate= (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. "ATK Propulsion and Composite Technologies Help Launch National Reconnaissance Office Satellite" (Press release). Alliant Techsystems. January 19, 2009.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  4. 4.0 4.1 "AIAA 2002-4324, Propulsion for the 21st Century—RS-68". AIAA, July 8–10, 2002.
  5. The RS-68 produces 3370 kN of thrust and has a mass of 6,600 kg (T/W = 52) at a vacuum ISP of 410. The SSME produces 2,279 kN of thrust with a mass of 3,500 kg (T/W=66) at a vacuum ISP of 452.
  6. "Rocketdyne RS-68 Engine Certified for Boeing Delta IV" (Press release). Boeing. Dec 19, 2001.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  7. RS-68.
  8. Boeing white paper on RS-68 development
  9. "United Launch Alliance First RS-68A Hot-Fire Engine Test a Success" (Press release). United Launch Alliance. 2008-09-25. Retrieved 2008-09-30. 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.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  10. "United Launch Alliance Upgraded Delta IV Heavy rocket successfully Launches Second Payload in Nine Days for the National Reconnaissance Office" (Press release). United Launch Alliance. 2012-06-29.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  11. "NASA's Exploration Systems Progress Report" (Press release). NASA. 2006-05-18. Retrieved 2006-05-30.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  12. 12.0 12.1 "Overview: Ares V Cargo Launch Vehicle". NASA. Archived from the original on Sep 26, 2008. Retrieved 30 September 2008.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  13. "United Launch Alliance First RS-68A Hot-Fire Engine Test a Success". forum. 2008-09-27.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  14. "Frequently Asked Questions, question 3". NASA ESMD.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  15. Bearden, David A; Skratt, John P; Hart, Matthew J (June 1, 2009). "Human Rated Delta IV Heavy Study Constellation Impacts" (PDF). NASA. p. 8.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  16. "RS-68 Propulsion System" (PDF). Pratt & Whitney Rocketdyne. October 2005.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  18. "The engines that refused to retire – RS-25s prepare for SLS testing". NASA June 2013.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links