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J-2 Rocket Engine

J-2 rocket engine
Country of origin	United States
Manufacturer	Rocketdyne
Propellant	LOX / LH2
Thrust (Vac)	232,250 lbf (1,033.100 kN)
Thrust (SL)	109,302 lbf (486.200 kN)
Thrust-to-weight ratio	73.18
Isp(Vac)	421 sec
Isp(SL)	200sec

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.

J-2S

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

An experimental program to improve the performance of the J-2 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 a change from the gas generator cycle to a tap-off cycle that supplied 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" that produced little thrust 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 no follow-on orders for Saturn boosters were coming, and the program 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.

J-2T

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. 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 (1,100 kN).

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

J-2X

Concept image of the J-2X engine.

A J2 engine

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

A new variant of this engine, called the J-2X, was chosen in 2007 for the Project Constellation manned lunar landing program. A single J-2X engine,generating 294,000 lbf (1,310 kN) of thrust, was to be used to power the Earth Departure Stage (EDS).

NASA began construction of a new test stand for altitude testing of J-2X engines at Stennis Space Center (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 is designed to be more efficient and simpler to build than its Apollo J-2 ancestor, and cost less than the Space Shuttle Main Engine (SSME).

On July 16, 2007 NASA officially announced the award to Pratt and Whitney Rocketdyne, Inc. of a $1.2 billion dollar contract "for design, development, testing and evaluation of the J-2X engine" intended to power the upper stages of the Ares I and Ares V launch vehicles. On Sept. 8, 2008 Rocketdyne announced successful testing of the initial J-2X gas generator design. The completion of a second round of successful gas generator tests was announced on September 21, 2010.

As of November 19, 2010, the first hot-fire test of a J-2X was scheduled to take place in early 2011 at SSC.

Project Constellation was cancelled by President Barack Obama on October 11, 2010. As of November 2010, no concrete plans for a J-2X application have been announced.

Specifications