The RL10 is a liquid-fuel cryogenic rocket engine used on the Centaur, S-IV, and Delta Cryogenic Second Stage 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 three, the RL10A-4-2, the RL10B-2, and the RL10C-1 still being produced and flown on the Atlas V and Delta IV.
An RL10A-4 engine in London's Science Museum
|Country of origin||United States of America|
|First flight||1962 (RL10A-1)|
|Designer||Pratt & Whitney/MSFC|
|Manufacturer||Pratt & Whitney Space Propulsion |
Pratt & Whitney Rocketdyne
|Application||Upper stage engine|
|Propellant||Liquid oxygen / liquid hydrogen|
|Nozzle ratio||84:1 or 280:1|
|Thrust (vac.)||110.1 kN (24,800 lbf)|
|Isp (vac.)||465.5 seconds (4.565 km/s)|
|Burn time||700 seconds|
|Length||4.15 m (13.6 ft) w/ nozzle extended|
|Diameter||2.15 m (7 ft 1 in)|
|Dry weight||301 kg (664 lb)|
|Notes||Performance values and dimensions are for RL10B-2.|
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.
The RL10 was first tested on the ground in 1959, at Pratt & Whitney's Florida Research and Development Center in West Palm Beach, Florida.  the first successful flight took place on November 27, 1963. 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. The RL10 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.
The RL10 has been upgraded over the years. One current model, the RL10B-2, powers the Delta IV 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).
Aerojet Rocketdyne is working toward incorporating additive manufacturing into the RL10 construction process. The company conducted full-scale, hot-fire tests on an engine with a printed core main injector in March 2016, and on an engine with a printed thrust chamber assembly in April 2017.
Applications for the RL10Edit
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. Up to seven RL10 engines would be used in the proposed Jupiter Upper Stage, serving an equivalent role to the Ares V Earth Departure Stage.
Common Extensible Cryogenic EngineEdit
The Common Extensible Cryogenic Engine (CECE) is a testbed to develop RL10 engines that throttle well. NASA contracted with Pratt & Whitney Rocketdyne to develop the CECE demonstrator engine. In 2007 its operability (with some "chugging") was demonstrated at 11:1 throttle ratios. 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. In 2010, the throttling range was expanded further to a 17.6:1 ratio, throttling from 104% to 5.9% power.
Advanced Cryogenic Evolved StageEdit
As of 2009[update], an enhanced version of the RL10 rocket engine was proposed to power the upper-stage versions of the Advanced Cryogenic Evolved Stage (ACES), a long-duration, low-boiloff extension of existing ULA Centaur and Delta Cryogenic Second Stage (DCSS) technology for the Vulcan launch vehicle. 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.
SLS Exploration Upper StageEdit
In April 2016 it was reported NASA has chosen to use a design based on four RL10 engines for the Exploration Upper Stage to be used beginning with the crewed EM-2 mission of the Space Launch System. In October 2016 NASA confirmed these reports when it announced that the Exploration Upper Stage would utilize a new variant of the engine identified as the RL10C-3.
OmegA Upper StageEdit
In April 2018, Orbital ATK announced it would use two RL10C-5-1 engines for their OmegA to power the upper stage. Blue Origin's BE-3U and Airbus Safran's Vinci were also considered before Aerojet Rocketdyne's engine was selected.
Vulcan Centaur Upper StageEdit
On May 11, 2018 United Launch Alliance (ULA) announced that Aerojet Rocketdyne would be strategic partner with their RL10C-X upper stage engine for ULA’s next-generation Vulcan Centaur rocket following a competitive procurement process.
|Version||Status||First flight||Dry mass||Thrust||Isp (ve), vac.||Length||Diameter||T:W||O:F||Expansion ratio||Chamber pressure||Burn time||Associated stage||Notes|
|RL10A-1||Retired||1962||131 kg (289 lb)||67 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|||
|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|||
|RL10A-4-2||In production||2002||168 kg (370 lb)||99.1 kN (22,300 lbf)||451 s (4.42 km/s)||1.17 m (3 ft 10 in)||61:1||84:1||740 s||Centaur IIIB
|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|||
|RL10B-2||In production||1998||277 kg (611 lb)||110.1 kN (24,800 lbf)||465.5 s (4.565 km/s)||4.15 m (13.6 ft)||2.15 m (7 ft 1 in)||40:1||5.88:1||280:1||44.12 bar (4,412 kPa)||5-m: 1,125 s
4-m: 700 s
|Delta Cryogenic Second Stage|||
|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|||
|CECE||Demonstrator project||160 kg (350 lb)||67 kN (15,000 lbf), throttle to 5–10%||>445 s (4.36 km/s)||1.53 m (5 ft 0 in)|||
|RL10C-1||In production||2014||190 kg (420 lb)||101.8 kN (22,890 lbf)||449.7 s (4.410 km/s)||2.12 m (6 ft 11 in)||1.45 m (4 ft 9 in)||57:1||5.88:1||130:1||Centaur SEC|
|RL10C-1-1||In development||188 kg
|453.8 s||2.46 m
(8 ft 0.7 in)
(4 ft 9 in)
|RL10C-3||In development||230 kg
|460.1 s||3.15 m
(10 ft 4.3 in)
(6 ft 1 in)
|5.7:1||Exploration Upper Stage|||
|RL10C-1-5||In development||188 kg
|453.8 s||2.46 m
(8 ft 0.7 in)
(4 ft 9 in)
- Thrust (altitude): 15,000 lbf (66.7 kN)
- 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
- Thrust (altitude): 24,750 lbf (110.1 kN)
- Design: Expander cycle
- Specific impulse: 465.5 seconds (4.565 km/s)
- Engine weight, dry: 664 lb (301.2 kg)
- Height: 163.5 in (4.14 m)
- Diameter: 84.5 in (2.21 m)
- Expansion ratio: 280 to 1
- Mixture ratio: 5.88 to 1 
- Propellants: liquid oxygen, liquid hydrogen
- Propellant flow: Fuel, 41.42 lb/s (20.6 kg/s); Oxidizer 7.72 lb/s (3.5 kg/s)
- Contractor: Pratt & Whitney
- Vehicle application: Delta III, Delta IV second stage (1 engine)
In 2012 NASA joined 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 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. ... That's what this study will figure out, is it worthwhile to build an RL10 replacement?"— Dale Thomas, Associated Director Technical, Marshall Space Flight Center
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 methods of putting US government satellites into space. This relates to the requirements study of the Affordable Upper Stage Engine Program (AUSEP) liquid rocket engine for use on upper stages of medium- and heavy-class launch vehicles, including the Evolved Expendable Launch Vehicle (EELV) family of launch vehicles.
Engines on displayEdit
- An RL10 is on display at the New England Air Museum, Windsor Locks, Connecticut
- An RL10 is on display at the Museum of Science and Industry, Chicago, Illinois
- An RL10 is on display at the U.S. Space & Rocket Center, Huntsville, Alabama
- An RL10 is on display at Southern University, Baton Rouge, Louisiana
- Two RL10 engines are on display at US Space Walk of Fame, Titusville, Florida
- An RL10 is on display in the Aerospace Engineering Department, Davis Hall at Auburn University.
- An RL10A-4 is on display at the Science Museum in London, UK.
- An RL10 is on display at the Museum of Life and Science in Durham, NC
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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. ...
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