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An RL10 at the U.S. Space & Rocket Center with cutaway showing tubing through the bell.
Country of origin United States of America
First flight 1962 (RL10A-1)
Designer Pratt & Whitney/MSFC
Manufacturer Pratt & Whitney Space Propulsion
Pratt & Whitney Rocketdyne
Aerojet Rocketdyne
Application Upper stage engine
Associated L/V Atlas
Delta IV
Saturn I
Status In production
Liquid-fuel engine
Propellant Liquid oxygen / Liquid hydrogen
Mixture ratio 5.5 or 5.85:1
Cycle Expander cycle
Nozzle ratio 84:1 or 280:1
Thrust (vac.) 110 kN (25,000 lbf)
Isp (vac.) 450 to 465.5 seconds (4.413 to 4.565 km/s)
Burn time 700 seconds
Length 4.14 m (13.6 ft) (nozzle extended)
Diameter 2.13 m (7 ft 0 in)
Dry weight 277 kg (611 lb)
Used in
References [1]
Notes Performance values and dimensions are for RL-10B-2.

The RL10 is a liquid-fuel cryogenic rocket engine used on the Centaur, S-IV and DCSS upper stages. Built in the United States by Aerojet Rocketdyne (formerly by Pratt & Whitney Rocketdyne), the RL10 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing 64.7 to 110 kN (14,545–24,729 lbf) of thrust in vacuum depending on the version in use. The RL10 was the first liquid hydrogen rocket engine to be built in the United States, and development of the engine by Marshall Space Flight Center and Pratt & Whitney began in the 1950s, with the first flight occurring in 1961. Several versions of the engine have been flown, with two, the RL10A-4-2 and the RL10B-2, still being produced and flown on the Atlas V and Delta IV.

The engine produces a specific impulse (Isp) of 373 to 470 s (3.66–4.61 km/s) in a vacuum and has a mass ranging from 131 to 317 kg (289–699 lb) (depending on version). Six RL10A-3 engines were used in the S-IV second stage of the Saturn I rocket, one or two RL10 engines are used in the Centaur upper stages of Atlas and Titan rockets and one RL10B-2 is used in the upper stage of Delta IV rockets.


  • History 1
    • Improvements 1.1
  • Applications for the RL10 2
    • Potential uses for the RL10 2.1
      • Common Extensible Cryogenic Engine 2.1.1
      • Advanced Common Evolved Stage 2.1.2
      • NextGen Propulsion Study 2.1.3
  • Variants 3
  • Specifications 4
    • Original RL10 4.1
    • Current design 4.2
  • Engines on display 5
  • See also 6
  • References 7
  • External links 8


The RL10 was first tested on the ground in 1959, at Pratt and Whitney's Florida Research and Development Center in West Palm Beach, Florida.[2] It was first flown in 1962 in an unsuccessful suborbital test;[3] the first successful flight took place on November 27, 1963.[4][5] For that launch, two RL10A-3 engines powered the Centaur upper stage of an Atlas launch vehicle. The launch was used to conduct a heavily instrumented performance and structural integrity test of the vehicle.[6] The RL-10 was designed for the USAF from the beginning as a throttleable motor for the Lunex lunar lander, finally putting this capability to use twenty years later in the DC-X VTOL vehicle.[7]


The RL10 has been upgraded over the years. One current model, the RL10B-2, powers the Delta IV second stage, as well as the Delta III second stage. It has been significantly modified from the original RL10 to improve performance. Some of the enhancements include an extendable nozzle and electro-mechanical gimbaling for reduced weight and increased reliability. Current specific impulse is 464 seconds (4.55 km/s).

A flaw in the brazing of an RL10B-2 combustion chamber was identified as the cause of failure for the May 4, 1999, Delta III launch carrying the Orion-3 communications satellite.[8]

Applications for the RL10

Four modified RL10A-5 engines, all of them with the ability to be throttled, were used in the McDonnell Douglas DC-X.

The DIRECT version 3.0 proposal to replace Ares I and Ares V with a family of rockets sharing a common core stage, recommends the RL10 for the second stage of their proposed J-246 and J-247 launch vehicles.[9] Up to seven (7) RL10 engines would be used in the proposed Jupiter Upper Stage, serving an equivalent role to the Ares V Earth Departure Stage.

Potential uses for the RL10

Common Extensible Cryogenic Engine

The CECE at partial throttle.

The Common Extensible Cryogenic Engine (CECE) is a testbed to develop RL10 engines that throttle well. NASA has contracted with Pratt & Whitney Rocketdyne to develop the CECE demonstrator engine.[10] In 2007 its operability (with some "chugging") was demonstrated at 11-to-1 throttle ratios.[11] In 2009 NASA reported successfully throttling from 104 percent thrust to eight percent thrust, a record for an engine of this type. Chugging was eliminated by injector and propellant feed system modifications that control the pressure, temperature and flow of propellants.[12]

Advanced Common Evolved Stage

As of 2009, an enhanced version of the RL10 rocket engine was proposed to power the upper-stage versions of the Advanced Common Evolved Stage (ACES), a long-duration, low-boiloff extension of existing ULA Centaur and Delta Cryogenic Second Stage (DCSS) technology.[13] Long-duration ACES technology is explicitly designed to support geosynchronous, cislunar, and interplanetary missions as well as provide in-space propellant depots in LEO or at L2 that could be used as way-stations for other rockets to stop and refuel on the way to beyond-LEO or interplanetary missions. Additional missions could include the provision of the high-energy technical capacity for the cleanup of space debris.[14]

NextGen Propulsion Study

NASA is partnering with the US Air Force (USAF) to study next-generation upper stage propulsion, formalizing the agencies joint interests in a new upper stage engine to replace the venerable Aerojet Rocketdyne RL10.

"We know the list price on an RL10. If you look at cost over time, a very large portion of the unit cost of the EELVs is attributable to the propulsion systems, and the RL10 is a very old engine, and there's a lot of craftwork associated with its manufacture," says Dale Thomas, associate director of technical issues at NASA Marshall. "That's what this study will figure out, is it worthwhile to build an RL10 replacement?"

USAF hopes to replace the Rocketdyne RL10 engines used on the upper stage of both the Lockheed Martin Atlas V and the Boeing Delta IV, known as evolved expendable launch vehicles (EELV) that are the primary method of putting US satellites into space. While NASA frequently uses EELVs to launch large scientific payloads, the programme's administration is largely run through other channels.[15]


Version Status First flight Dry mass Thrust Isp (vac) Length Diameter T:W O:F Expansion ratio Chamber pressure Burn time Associated stage Notes
RL10A-1 Retired 1962 131 kg (289 lb) 66.7 kN (15,000 lbf) 425 s (4.17 km/s) 1.73 m (5 ft 8 in) 1.53 m (5 ft 0 in) 52:1 40:1 430 s Centaur A Prototype
RL10A-3 Retired 1963 131 kg (289 lb) 65.6 kN (14,700 lbf) 444 s (4.35 km/s) 2.49 m (8 ft 2 in) 1.53 m (5 ft 0 in) 51:1 5:1 57:1 32.75 bar (3,275 kPa) 470 s Centaur B/C/D/E
RL10A-4 Retired 1992 168 kg (370 lb) 92.5 kN (20,800 lbf) 449 s (4.40 km/s) 2.29 m (7 ft 6 in) 1.17 m (3 ft 10 in) 56:1 5.5:1 84:1 392 s Centaur IIA [20]
RL10A-4-1 Retired 2000 167 kg (368 lb) 99.1 kN (22,300 lbf) 451 s (4.42 km/s) 1.53 m (5 ft 0 in) 61:1 84:1 740 s Centaur IIIA [21]
RL10A-4-2 In production 2002 167 kg (368 lb) 99.1 kN (22,300 lbf) 451 s (4.42 km/s) 1.53 m (5 ft 0 in) 61:1 84:1 740 s Centaur IIIB
Centaur V1
Centaur V2
RL10A-5 Retired 1993 143 kg (315 lb) 64.7 kN (14,500 lbf) 373 s (3.66 km/s) 1.07 m (3 ft 6 in) 1.02 m (3 ft 4 in) 46:1 6:1 4:1 127 s DC-X [23]
RL10B-2 In production 1998 277 kg (611 lb) 110 kN (25,000 lbf) 462 s (4.53 km/s) 4.14 m (13.6 ft) 2.13 m (7 ft 0 in) 40:1 5.85:1 280:1 44.12 bar (4,412 kPa) 700 s Delta Cryogenic Second Stage [1]
RL10B-X Cancelled 317 kg (699 lb) 93.4 kN (21,000 lbf) 470 s (4.6 km/s) 1.53 m (5 ft 0 in) 30:1 250:1 408 s Centaur B-X [24]
CECE In development 160 kg (350 lb) 66.7 kN (15,000 lbf) >445 s (4.36 km/s) 1.53 m (5 ft 0 in) Base demonstrator
RL10C-1 In testing 12/2014 191 kg (421 lb) 106.31 kN (23,900 lbf) 448.5 s (4.398 km/s) 2.22 m (7 ft 3 in) 1.44 m (4 ft 9 in) 57:1 5.88:1 130:1 2000 Centaur


Original RL10

  • Thrust (altitude): 15,000 lbf (66.7 kN)[17]
  • Burn Time: 470 s[17]
  • Design: Expander cycle
  • Specific impulse: 433 seconds (4.25 km/s)
  • Engine weight - dry: 298 lb (135 kg)
  • Height: 68 in (1.73 m)
  • Diameter: 39 in (0.99 m)
  • Nozzle expansion ratio: 40 to 1
  • Propellants: Liquid Oxygen & Liquid Hydrogen
  • Propellant flow: 35 lb/s (16 kg/s)
  • Contractor: Pratt & Whitney
  • Vehicle application: Saturn I / S-IV 2nd stage - 6-engines
  • Vehicle application: Centaur upper stage - 2-engines

Current design

Second stage of a Delta IV Medium rocket featuring an RL10B-2 engine.
RL10B-2 Specifications
  • Thrust (altitude): 24,750 lbf (110.1 kN)[29]
  • Design: Expander cycle[30]
  • Specific impulse: 464 seconds (4.55 km/s)[29]
  • Engine weight - dry: 610 lb (277 kg)[29]
  • Height: 163 in (4.14 m)[29]
  • Diameter: 87 in (2.21 m)[29]
  • Expansion ratio: 250 to 1
  • Mixture ratio: 5.85 to 1 [29]
  • Propellants: Liquid oxygen & liquid hydrogen[29]
  • Propellant flow: Oxidizer 41.42 lb/s (20.6 kg/s), fuel 7.72 lb/s (3.5 kg/s)[29]
  • Contractor: Pratt & Whitney
  • Vehicle application: Delta III, Delta IV second stage (1 engine)

The other current model, the RL10A-4-2, is the engine used on Centaur upper stage for Atlas V.[29]

Engines on display

See also


  1. ^ a b Mark Wade (17 November 2011). "RL-10B-2". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  2. ^ Connors, p 319
  3. ^ "Centaur". Gunter's Space Pages. 
  4. ^ Sutton, George (2005). History of liquid propellant rocket engines. American Institute of Aeronautics and Astronautics.  
  5. ^ "Renowned Rocket Engine Celebrates 40 Years of Flight". Pratt & Whitney. November 24, 2003. 
  6. ^ "Atlas Centaur 2". NASA NSSDC. 
  7. ^ "Encyclopedia Astronautica - Lunex Project page". Mark Wade. 
  8. ^ "Delta 269 (Delta III) Investigation Report" (PDF).  
  9. ^ "Jupiter Launch Vehicle – Technical Performance Summaries". Archived from the original on 2009-06-08. Retrieved 2009-07-18. 
  10. ^ "Common Extensible Cryogenic Engine (CECE)". United Technologies Corporation. 
  11. ^ "Throttling Back to the Moon". NASA. 2007-07-16. 
  12. ^ "NASA Tests Engine Technology for Landing Astronauts on the Moon". NASA. Jan 14, 2009. 
  13. ^ Kutter, Bernard F.; Frank Zegler; Jon Barr; Tim Bulk; Brian Pitchford (2009). "Robust Lunar Exploration Using an Efficient Lunar Lander Derived from Existing Upper Stages" (PDF). AIAA. 
  14. ^ Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture" (PDF). AIAA SPACE 2010 Conference & Exposition. AIAA. Retrieved 2011-01-25. ACES design conceptualization has been underway at ULA for many years. It leverages design features of both the Centaur and Delta Cryogenic Second Stage (DCSS) upper stages and intends to supplement and perhaps replace these stages in the future. ... 
  15. ^ Roseberg, Zach (April 12, 2012). "NASA, US Air Force to study joint rocket engine". Flight Global. Retrieved June 1, 2012. 
  16. ^ Mark Wade (17 November 2011). "RL-10A-1". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  17. ^ a b c Bilstein, Roger E. (1996), "Unconventional Cryogenics: RL-10 and J-2", Stages to Saturn; A Technological History of the Apollo/Saturn Launch Vehicles, Washington, D.C.: National Aeronautics and Space Administration, NASA History Office, retrieved 2011-12-02 
  18. ^ "Atlas Centaur". Gunter's Space Page. Retrieved 29 February 2012. 
  19. ^ Mark Wade (17 November 2011). "RL-10A-3". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  20. ^ Mark Wade (17 November 2011). "RL-10A-4". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  21. ^ Mark Wade (17 November 2011). "RL-10A-4-1". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  22. ^ Mark Wade (17 November 2011). "RL-10A-4-2". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  23. ^ Mark Wade (17 November 2011). "RL-10A-5". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  24. ^ Mark Wade (17 November 2011). "RL-10B-X". Encyclopedia Astronautica. Retrieved 27 February 2012. 
  25. ^ "Commons Extensible Cryogenic Engine". Pratt & Whitney Rocketdyne. Retrieved 28 February 2012. 
  26. ^ [3]
  27. ^ "Cryogenic Propulsion Stage" (PDF). NASA. Retrieved 11 October 2014. 
  28. ^ [4]
  29. ^ a b c d e f g h i "RL10B-2" (PDF).  
  30. ^ Sutton, A M; Peery, S D; Minick, A B (January 1998). "50K expander cycle engine demonstration". AIP Conference Proceedings 420: pp. 1062–1065.  
  31. ^ "Pratt & Whitney RL10A-1 Rocket Engine". New England Air Museum. Retrieved April 26, 2014. 
  32. ^ a b "Photos of Rocket Engines". Historic Spacecraft. Retrieved April 26, 2014. 
  33. ^ Colaguori, Nancy; Kidder, Bryan (November 3, 2006). "Pratt & Whitney Rocketdyne Donates Model of Legendary Rl10 Rocket Engine to Southern University". PR Newswire (Press release). Pratt & Whitney Rocketdyne. Retrieved April 26, 2014. 
  34. ^
  • Connors, Jack (2010). The Engines of Pratt & Whitney: A Technical History. Reston. Virginia:  

External links

  • RL10B-2 at Astronautix
  • Spaceflight Now article
  • Spaceflight Now article
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