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RS-68
	An RS-68 engine undergoing hot-fire testing at NASA's Stennis Space Center during its developmental phase.
Country of origin	United States
First flight	20 November 2002
Last flight	9 April 2024
Designer	Rocketdyne
Manufacturer	Rocketdyne (2002–2005); Pratt & Whitney Rocketdyne (2005–2013); Aerojet Rocketdyne (2013–2024);
Application	First stage engine
Associated LV	Delta IV · Delta IV Heavy
Status	Retired
Liquid-fuel engine
Propellant	LOX / LH2
Cycle	Gas-generator
Configuration
Nozzle ratio	21.5:1
Performance
Thrust, sea-level	RS-68: 2,950 kN (660,000 lbf); RS-68A: 3,137 kN (705,000 lbf)
Thrust-to-weight ratio	RS-68: 45.3:1; RS-68A: 47.4:1
Chamber pressure	1,488 psi (10.26 MPa)
Specific impulse, vacuum	RS-68: 410 s (4.0 km/s); RS-68A: 411.9 s (4.039 km/s)
Dimensions
Length	5.20 m (17.1 ft)
Diameter	2.43 m (8 ft 0 in)
Dry mass	RS-68: 6,600 kg (14,560 lb); RS-68A: 6,740 kg (14,870 lb)

The RS-68 (Rocket System-68) was a liquid-fuel rocket engine that used liquid hydrogen (LH₂) and liquid oxygen (LOX) as propellants in a gas-generator cycle. It was the largest hydrogen-fueled rocket engine ever flown.^[3]

Designed and manufactured in the United States by Rocketdyne (later Pratt & Whitney Rocketdyne and Aerojet Rocketdyne). 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 the cancellation of the rocket and the Constellation Program in 2010.

Design and development

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).^[4] 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.^[4]

The RS-68 was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Los Angeles, California, where the SSME was 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's Rocketdyne F-1 engines were developed and tested for the Apollo missions to the Moon. The initial testing of the RS-68 occurred at the Air Force Research Laboratory (AFRL), Edwards Air Force Base, California, and later at NASA's Stennis Space Center.^[5]^[6] The RS-68 was certified in December 2001 for use on Delta IV rockets.^[7]

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.^[8]

The engine produced 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 was 14,560 pounds (6,600 kg). With this thrust, the engine had a thrust-to-weight ratio of 51.2 and a specific impulse of 410 seconds (4.0 km/s) in a vacuum and 365 seconds (3.58 km/s) at sea level.^[9] The RS-68 was gimbaled hydraulically and was capable of throttling between 58% and 102% thrust.^[10]

The RS-68A is an updated version of the RS-68, with increased specific impulse and thrust (to over 700,000 pounds-force (3,100 kN) at sea level).^[11] 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.^[12]

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.^[13] Later, the Ares V was changed to use six RS-68 engines, designated the RS-68B.^[14] Ares V was dropped as part of the cancellation of the Constellation program in 2010.^[15] NASA's current successor heavy-lift vehicle, the Space Launch System, uses four RS-25 engines instead.^[16]

Human-rating

In 2008, it was reported that the RS-68 needed over 200 changes to receive a human-rating certification.^[17] NASA has stated that those changes include health monitoring, removal of the fuel-rich environment at liftoff, and improving the robustness of its subsystems.^[18]^[19]

Variants

RS-68 is the original version. It produces 663,000 pounds-force (2,950 kN) thrust at sea level.^[20]
RS-68A is an improved version. It produces 705,000 lbf (3,140 kN) thrust at sea level and 800,000 lbf (3,560 kN) thrust in a vacuum.^[21] Its specific impulse in a vacuum is 411.9 seconds (4.039 km/s).^[22] Certification was completed in April 2011.^[23]
RS-68B was a proposed upgrade to be used in the Ares V launch vehicle for NASA's Constellation program.^[14] The Ares V was to use six RS-68B engines on a 10 metres (33 ft) diameter 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.^[24]

References

^ ^a ^b "DELTA IV". United Launch Alliance. Archived from the original on 20 July 2014. Retrieved 13 July 2014.
^ "Delta IV User's Guide" (PDF). United Launch Alliance. June 2013. Archived (PDF) from the original on 10 July 2014. Retrieved 13 July 2014.
^ "ATK Propulsion and Composite Technologies Help Launch National Reconnaissance Office Satellite" (Press release). Alliant Techsystems. 19 January 2009. Archived from the original on 22 February 2014.
^ ^a ^b "AIAA 2002-4324, Propulsion for the 21st Century—RS-68" Archived 19 March 2009 at the Wayback Machine. AIAA, 8–10 July 2002.
^ "Boeing Tests RS-68 Rocket Engine (Apr. 24)". Defense Aerospace. 23 April 2001. Retrieved 26 June 2023.
^ "Boeing Delta IV CBC/RS-68 Engine Successfully Completes Test Program" (Press release). Boeing. 9 May 2001. Retrieved 26 June 2023.
^ "Rocketdyne RS-68 Engine Certified for Boeing Delta IV" (Press release). Boeing. 19 December 2001. Archived from the original on 10 October 2012.
^ "Atlas V and Delta IV Technical Summary" (PDF).
^ "United Launch Alliance Delta IV Heavy".
^ Boeing white paper on RS-68 development Archived 15 April 2007 at the Wayback Machine
^ "United Launch Alliance First RS-68A Hot-Fire Engine Test a Success" (Press release). United Launch Alliance. 25 September 2008. Retrieved 10 April 2023. 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.
^ "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. 29 June 2012. Archived from the original on 20 August 2016.
^ "NASA's Exploration Systems Progress Report" (Press release). National Aeronautics and Space Administration. 18 May 2006. Archived from the original on 5 April 2012. Retrieved 30 May 2006.
^ ^a ^b "Overview: Ares V Cargo Launch Vehicle". National Aeronautics and Space Administration. Archived from the original on 26 September 2008. Retrieved 30 September 2008.
^ Amos, Jonathan (11 October 2010). "Obama signs Nasa up to new future". BBC news. Retrieved 7 June 2019.
^ "Space Launch System Factsheet" (PDF). National Aeronautics and Space Administration. Archived from the original (PDF) on 13 May 2019. Retrieved 7 June 2019.
^ "United Launch Alliance First RS-68A Hot-Fire Engine Test a Success". NASAspaceflight.com. 27 September 2008. Archived from the original on 28 March 2018.
^ "Frequently Asked Questions, question 3". National Aeronautics and Space Administration ESMD. Archived from the original on 12 January 2010.
^ Bearden, David A.; Skratt, John P.; Hart, Matthew J. (1 June 2009). "Human Rated Delta IV Heavy Study Constellation Impacts" (PDF). National Aeronautics and Space Administration. p. 8. Archived (PDF) from the original on 28 February 2017.
^ "RS-68 Propulsion System" (PDF). Pratt & Whitney Rocketdyne. October 2005. Archived from the original (PDF) on 14 July 2018. Retrieved 7 April 2019.
^ "P&W Successfully Completes Hot-Fire Test on 2nd RS-68A Certi". asdnews.com. Archived from the original on 25 July 2011. Retrieved 25 April 2018.
^ "Delta IV User's Guide" (PDF). United Launch Alliance. June 2013. Archived (PDF) from the original on 10 July 2014. Retrieved 13 July 2014.
^ "RS-68A | L3Harris® Fast. Forward". www.l3harris.com. Retrieved 6 February 2024.
^ "The engines that refused to retire – RS-25s prepare for SLS testing". nasaspaceflight.com. June 2013. Archived from the original on 30 August 2017.