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||MSFC/Pratt & Whitney|
|Manufacturer||Pratt & Whitney Rocketdyne|
|Application||Upper stage engine|
|Propellant||Liquid oxygen / Liquid hydrogen|
|Mixture ratio||5.5 or 5.85:1|
|Nozzle ratio||84:1 or 280:1|
|Thrust (vac.)||110 kN (25,000 lbf)|
|Isp (vac.)||450 to 465.5 seconds|
|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 (610 lb)|
|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 of America by Pratt & Whitney Rocketdyne, the RL10 burns cryogenic liquid hydrogen & liquid oxygen propellants, with each engine producing 64.7–110 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 373-470 seconds in a vacuum and has a mass ranging from 131–317 kg (290–700 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 the Florida Research and Development Center in West Palm Beach, Florida (SSFL) of Pratt and Whitney. It was first flown in 1962 in an unsuccessful suborbital test; 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 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 s (equivalent to an exhaust velocity of 4.55 km/s).
|Version||Status||First flight||Dry mass||Thrust||Isp (vac)||Length||Diameter||T:W||O:F||Expansion ratio||Burn time||Associated stage||Notes|
|RL10A-1||Retired||1962||131 kg (290 lb)||66.7 kN (15,000 lbf)||425 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 (290 lb)||65.6 kN (14,700 lbf)||444 s||2.49 m (8 ft 2 in)||1.53 m (5 ft 0 in)||51:1||5:1||57:1||470 s||Centaur B/C/D/E
|RL10A-4||Retired||1992||168 kg (370 lb)||92.5 kN (20,800 lbf)||449 s||2.29 m (7 ft 6 in)||1.17 m (3 ft 10 in)||36:1||5.5:1||84:1||392 s||Centaur IIA|||
|RL10A-4-1||Retired||2000||167 kg (370 lb)||99.1 kN (22,300 lbf)||451 s||1.53 m (5 ft 0 in)||60:1||84:1||740 s||Centaur IIIA|||
|RL10A-4-2||In production||2002||167 kg (370 lb)||99.1 kN (22,300 lbf)||451 s||1.53 m (5 ft 0 in)||61:1||84:1||740 s||Centaur IIIB
|RL10A-5||Retired||1993||143 kg (320 lb)||64.7 kN (14,500 lbf)||373 s||1.07 m (3 ft 6 in)||1.02 m (3 ft 4 in)||47:1||6:1||4:1||127 s||DC-X|||
|RL10B-2||In production||1998||277 kg (610 lb)||110 kN (25,000 lbf)||462 s||4.14 m (13.6 ft)||2.13 m (7 ft 0 in)||5.85:1||250:1||700 s||Delta Cryogenic Second Stage|||
|RL10B-X||Cancelled||317 kg (700 lb)||93.4 kN (21,000 lbf)||470 s||1.53 m (5 ft 0 in)||30:1||250:1||408 s||Centaur B-X|||
|CECE||In development||350 lb (160 kg)||66.7 kN (15,000 lbf)||>445 s||Base demonstrator
Original RL10 
- Thrust (altitude): 15,000 lbf (66.7 kN)
- Burn Time: 470 s
- Design: Expander cycle
- Specific impulse: 433 s (4.25 kN·s/kg)
- 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: LOX & LH2
- 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 
- RL10B-2 Specifications
- Thrust (altitude): 24,750 lbf (110.1 kN)
- Design: Expander cycle
- Specific impulse: 464 s (4.55 kN·s/kg)
- Engine weight - dry: 610 lb (277 kg)
- Height: 163 in (4.14 m)
- Diameter: 87 in (2.21 m)
- Expansion ratio: 250 to 1
- Mixture ratio: 5.85 to 1 
- Propellants: Liquid oxygen & liquid hydrogen
- Propellant flow: Oxidizer 41.42 lb/s (20.6 kg/s), fuel 7.72 lb/s (3.5 kg/s)
- Contractor: Pratt & Whitney
- Vehicle application: Delta III, Delta IV second stage (1 engine)
Other rockets using RL10 
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 (7) RL10 engines would be used in the proposed Jupiter Upper Stage, serving an equivalent role to the Ares V Earth Departure Stage.
Future use of the RL10 
Several potential uses of enhanced versions of the RL10 engine have been proposed:
Manned lunar missions 
In 2005 NASA announced the decision to use an Apollo-like spacecraft configuration for the proposed Orion spacecraft. At that time NASA decided that the descent stage of the new Lunar Surface Access Module (LSAM) would be powered by liquid hydrogen and liquid oxygen. The original plan called for the ascent stage to use liquid methane and liquid oxygen, but that has changed[when?] and the ascent stage will now also use LH2/LOX.
Because of the choice of propellents, along with the need to land the spacecraft in the polar regions of the Moon from an equatorial orbit, NASA decided to use the RL10 as the main powerplant for the descent stage engine. Current specifications call for four RL10 engines to be used on the descent stage and a single RL10 for the ascent stage. Currently, the RL10B-2 engines used on the Delta III and Delta IV can thrust at 20% of maximum thrust. Because of the need for the LSAM to hover above the lunar surface, along with providing a smooth landing, the new RL10 engines must be able to thrust as low as 10%. The use of the RL10 will allow NASA to keep costs on the lunar program down by using existing hardware, albeit modified to enhance performance or allow for manned spaceflight.
Common Extensible Cryogenic Engine 
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. In 2007 its operability (with some "chugging") was demonstrated at 11-to-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.
Advanced Common Evolved Stage 
As of 2009[update], 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. 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.
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 Pratt & Whitney Rocketdyne RL-10.
"We know the list price on an RL-10. 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 RL-10 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 RL-10 replacement?"
From the study, NASA hopes to find a less expensive RL-10-class engine for a third stage of the Space Launch System (SLS), which is on track to become the most powerful rocket ever built. Atop the Space Shuttle Main Engines (SSME)-powered first stage and Pratt & Whitney Rocketdyne J-2X second stage, a third stage is required to push massive payloads beyond Earth orbit.
USAF hopes to replace the Rocketdyne RL-10 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.
In 2003, Pratt & Whitney Rocketdyne had 90% of the work completed for the new RL-60 replacement of the RL-10. The RL60 cryogenic upper stage engine is designed to produce 60,000 pounds of thrust with a specific impulse of 465 seconds to meet the evolving needs of expendable launch requirements or human-rated missions. The RL60 was to be built and tested domestically with key components to be provided by four international industry strategic suppliers; Volvo Aero of Sweden (Regen cooled nozzle), Ishikawajima-Harima Heavy Industries (IHI) of Japan (Fuel Turbopump), Techspace Aero of Belgium (Fuel Inlet and LOX control valve) and Chemical Automatics Design Bureau (CADB) of Russia (LOX Turbopump).
Engines on display 
- There is a RL10A-1 on display at the New England Air Museum, Bradley International Airport, Windsor Locks, CT. 
See also 
- Spacecraft propulsion
- XCOR/ULA aluminum alloy nozzle engine, under development in 2011 ff.
- Pratt & Whitney Rocketdyne
|Wikimedia Commons has media related to: RL10|
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