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Rocketdyne's J-2 was America's largest production liquid hydrogen fueled rocket engine before the Space Shuttle main engines (SSME), and is being revived in support of NASA's return to the Moon.

The J-2 was a major component of the Saturn V rocket. Five J-2 engines were used in the S-II, the second stage of the Saturn. One J-2 engine was used in S-IVB, the third stage of the Saturn V, and the second stage of the Saturn IB. There were proposals to use various numbers of J-2 engines in the upper stages of the planned Nova rocket.

A unique characteristic of the J-2 engine (at the time) was its ability to re-start after shutdown. The J-2 engine on the S-IVB was intended to burn twice. The first burn, lasting for about two minutes, placed the Apollo spacecraft into earth orbit, and then shut down. After the crew members verified that the spacecraft was operating nominally, the J-2 was re-ignited for translunar injection. This 6.5 minute burn accelerated the Apollo spacecraft to escape velocity, and on a course for the moon.

Contents 1 J-2S 2 J-2T 3 J-2X 4 Specifications 4.1 J-2 4.2 J-2S 5 References 6 Notes 7 External links

An experimental program to improve the performance of the J-2 was started in 1964 as the J-2X (not to be confused with a later variant by the same name). The main change to the original J-2 design was to change from the gas generator cycle to a tap-off cycle which supplied the hot gas from a tap on the combustion chamber instead of a separate burner. In addition to removing parts from the engine, it also reduced the difficulty of starting up the engine and properly timing various combustors.

Additional changes included a throttling system for wider mission flexibility, which also required a variable mixture system to properly mix the fuel and oxygen for a variety of different operating pressures. It also included a new "Idle Mode" which produced little thrust and could be used for on-orbit maneuvering or to settle the fuel tanks on-orbit prior to a burn.

During the experimental program, Rocketdyne also produced a small run of six pre-production models for testing, the J-2S. These were test fired many times between 1965 and 1972, for a total of 30,858 seconds burn time. In 1972 it became clear that follow-on orders for further Saturn boosters would not be forthcoming, and the program was shut down. NASA did consider using the J-2S on a number of different missions, and for some time a set of five were to power the Space Shuttle, a configuration that can be seen on early diagrams.

While work on the J-2S continued, NASA also funded a design effort to use the J-2S turbomachinery and plumbing with a new aerospike nozzle. This would improve performance even further, notably for the lower S-II stage that operated primarily in the atmosphere. Two versions were built, the J-2T-200k that provided 200,000 lbf (890 kN) thrust, allowing it to be "dropped in" to the existing S-II and S-IVB stages, and the J-2T-250k of 250,000 lbf.

Like the J-2S, work on the J-2T had progressed to a lengthy series of ground-based test runs, but further development was ended in the post-Apollo draw-down.

A new variant of this engine, called the J-2X, is being designed to support the upcoming Project Constellation and its Apollo-based Orion Spacecraft, which will replace the Space Shuttle upon its retirement in 2010. Originally the plan called for two J-2X engines to be used as the powerplant only for the Earth Departure Stage, a stage originally based on the Shuttle's external tank, but with the modification and redesign of the Ares V core stage to accommodate five modified RS-68 rocket engines (also built by Rocketdyne and currently in use on the Delta IV rocket), NASA redesigned the EDS to only use one engine, thus making the current EDS base similar in design and function to the Apollo-Saturn S-IVB upper stage.

With the expense of converting the SSME from a ground-started engine to an air-startable engine, along with the expense of constructing and pre-firing new SSMEs for each mission, NASA decided to also adopt the J-2X engine for the second stage of the Ares I. This decision, made on February 18, 2006, would allow NASA to be able to launch the Ares I rocket within 3 years after the retirement of the Shuttle in 2010 and allow the launching of the Orion spacecraft by 2014. In addition, the use of the J-2X on both rockets will allow NASA to simplify Orion support construction. Testing of the J-2X is scheduled to begin at Stennis Space Center (SSC) in the spring of 2007.^{[citation needed]} NASA began construction of a new test stand for altitude testing of J-2X engines at SSC on 23 August 2007.^[1]

The new J-2X will be designed to be more efficient and simpler to build than its Apollo J-2 ancestor, and cost less than the SSME.

On July 16, 2007 NASA officially awarded Pratt and Whitney Rocketdyne, Inc. a $1.2 billion dollar contract "for design, development, testing and evaluation of the J-2X engine that will power the upper stages of the Ares I and Ares V launch vehicles."^[2]

Thrust (altitude): 200,000 lbf (890 kN)
Burn time: 500 s
Specific impulse: 418 s (4099 m/s)
Engine weight - dry: 3,480 lb (1,579 kg)
Engine weight - burnout: 3,609 lb (1,637 kg)
Exit to Throat Area ratio: 27.5 to 1
Propellants: LOX & LH2
Mixture ratio: 5.50
Contractor: NAA/Rocketdyne
Vehicle Application: Saturn IB & Saturn V / S-IVB upper stage - 1-engine
Vehicle Application: Saturn V / S-II 2nd stage - 5-engines

Vacuum thrust: 265,000 lbf (1179 kN)
Vacuum specific impulse: 436 s (4276 m/s)
Basic engine weight - dry: 3,235 lb (1,467 kg)
Engine weight with accessories - dry: 3,800 lb (1,724 kg)
Propellants: LOX & LH2
Mixture ratio: 5.50
Contractor: Rockwell International/Rocketdyne

• Bilstein, Robert E. (2003). Stages to Saturn: a technological history of the Apollo/Saturn launch vehicles. Gainesville: University of Florida Press. ISBN 0-8130-2691-1. 

• Encyclopedia Astronautica link about the J-2
• Encyclopedia Astronautica link about the J-2S

v • d • e Project Constellation
Sub-orbital	Ares I-X · Ares I-Y
Earth orbital	Orion 1 · 2 · 3 · 4 · 5 · 6 · 7 · 8 · 9 · 10 · 11 · Ares V-Y · Orion 12 · 14 · 16 · 18 · 20
Lunar	Altair 1 / Orion 13 · Altair 2 / Orion 15 · Altair 3 / Orion 17 · Altair 4 / Orion 19 · Altair 5 / Orion 21
Solar System	Orion Asteroid Mission · Orion Mars Mission
Components	Orion · Ares (I • IV • V) · Earth Departure Stage · Altair · J-2X · RS-68
Launch sites	Kennedy Space Center LC-39
Abort	Orion abort modes · In-flight aborts and rescue options · Orion Abort Test Booster · Launch Abort System (LAS) · Max Launch Abort System (MLAS)
Other	Vision for Space Exploration · Exploration Systems Architecture Study
List of missions

v • d • e  J-2 Rocket Engine
	Main Article: J-2
	Technologies: Bipropellant | Gas-generator cycle | LOX | LH2
	Historic Spacecraft: Saturn IB (S-IVB) | Saturn V (S-II, S-IVB)
	Future Spacecraft: Ares I | Ares IV | Ares V | Earth Departure Stage
	Other LOX & LH2 Engines: SSME | RS-68 | RL-10 | Vulcain