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Saturn V second stage, with five J-2 engines.

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 on the S-II, the second stage of the Saturn V. One J-2 engine was used on the 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.


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.


Concept image of the J-2X engine.

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 (EDS). One J-2X engine will generate 294,000 lbf for the EDS.

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. NASA began construction of a new test stand for altitude testing of J-2X engines at Stennis Space Centermarker (SSC) on 23 August 2007. Between December 2007 and May 2008, nine tests of heritage J-2 engine components were conducted at SSC in preparation for the design of the J-2X engine.

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. It will use a gas generator power cycle.

On July 16, 2007 NASAmarker 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." On Sept. 8, 2008 Rocketdyne announced successful testing of a gas generator like those which will be used for J-2X engines. The first hot-fire test of a J-2X is scheduled to take place in 2010 at SSC.


A J2 engine
Test of the J-2X engine 'workhorse' gas generator.

J-2 J-2S J-2X
Vacuum thrust: 1,033.100 kN (232,250 lbf) 1,138.500 kN (255,945 lbf) 1,310.000 kN (294,490 lbf)
Specific impulse -Isp: 421 sec. 436 sec. 448 sec.
Burn time: 475 sec. 475 sec. 465 sec. (Ares I, upper stage)
Engine weight - dry: 1,438 kg (3,170 lb). 1,400 kg (3,000 lb). 2,472 kg (5,450 lb).
Propellants: LOX & LH2 LOX & LH2 LOX & LH2
Mixture ratio: 5.50 5.50 5.50
Diameter: 2.01 m (6.60 ft). 2.01 m (6.60 ft). 3.05 m (10.00 ft).
Length: 3.38 m (11.08 ft). 3.38 m (11.08 ft). 4.70 m (15.40 ft).
Thrust to Weight Ratio: 73.18. 85.32 55.04
Contractor: Rocketdyne Rocketdyne Rocketdyne
Vehicle application: Saturn V / S-II 2nd stage - 5-engines, Saturn IB & Saturn V / S-IVB upper stage - 1-engine Planned replacement for J-2 on Saturn V / S-II 2nd stage / S-IVB upper stage Proposed for Ares I upper stage - 1 engine & Ares V upper stage - 1 engine



  1. The Agena engine could also be restarted.

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