The J-2, commonly known as Rocketdyne J-2, was a liquid-fuel cryogenic rocket engine used on NASA's Saturn IB and Saturn V launch vehicles. Built in the United States by Rocketdyne, the J-2 burned cryogenic liquid hydrogen (LH<sub>2</sub>) and liquid oxygen (LOX) propellants, with each engine producing of thrust in vacuum. The engine's preliminary design dates back to recommendations of the 1959 Silverstein Committee. Rocketdyne won approval to develop the J-2 in June 1960 and the first flight, AS-201, occurred on 26 February 1966. The J-2 underwent several minor upgrades over its operational history to improve the engine's performance, with two major upgrade programs, the de Laval nozzle-type J-2S and aerospike-type J-2T, which were cancelled after the conclusion of the Apollo program.
The engine produced a specific impulse (I<sub>sp</sub>) of in a vacuum (or at sea level) and had a mass of approximately . Five J-2 engines were used on the Saturn V's S-II second stage, and one J-2 was used on the S-IVB upper stage used on both the Saturn IB and Saturn V. Proposals also existed to use various numbers of J-2 engines in the upper stages of an even larger rocket, the planned Nova. The J-2 was America's largest production LH2-fuelled rocket engine before the RS-25. A modernized version of the engine, the J-2X, was considered for use on the Earth Departure Stage of NASA's Space Shuttle replacement, the Space Launch System.
Unlike most liquid-fueled rocket engines in service at the time, the J-2 was designed to be restarted once after shutdown when flown on the Saturn V S-IVB third stage. The first burn, lasting about two minutes, placed the Apollo spacecraft into a low Earth parking orbit. After the crew verified that the spacecraft was operating nominally, the J-2 was re-ignited for translunar injection, a 6.5 minute burn which accelerated the vehicle to a course for the Moon.
Components
thumb|400px|A diagram showing the flow of propellant through a J-2 engine
Thrust chamber and gimbal system
The J-2's thrust chamber assembly served as a mount for all engine components, and was composed of the thrust chamber body, injector and dome assembly, gimbal bearing assembly, and augmented spark igniter.
Thrust was transmitted through the gimbal (mounted to the injector and oxidizer dome assembly and the vehicle's thrust structure), which consisted of a compact, highly loaded () universal joint consisting of a spherical, socket-type bearing. This was covered with a Teflon/fiberglass coating that provided a dry, low-friction bearing surface. The gimbal included a lateral adjustment device for aligning the combustion chamber with the vehicle, so that, in addition to transmitting the thrust from the injector assembly to the vehicle thrust structure, the gimbal also provided a pivot bearing for deflection of the thrust vector, thus providing flight attitude control of the vehicle.
Rocketdyne launched the development of the J-2 with an analytical computer model that simulated engine operations and aided in establishing design configurations. The model was supported by a full-sized mockup which was used throughout development to judge the positioning of the engine's components. The first experimental component, the engine's injector, was produced within two months of the contract being awarded, and testing of the engine's components began at Rocketdyne's Santa Susana Field Laboratory in November 1960. Other test facilities, including a vacuum chamber and full-size engine test stand, were used during the development, with the engine's turbopumps entering testing in November 1961, the ignition system in early 1962, and the first prototype engine running a complete 250-second test run in October 1962. In addition to flight hardware, five engine simulators were also used during the development process, assisting in the design of the engine's electrical and mechanical systems. Contracts were signed between NASA and Rocketdyne in the summer of 1962, requiring 55 J-2 engines to be produced to support the final designs for the Saturn rockets, which required five engines for each S-II second stage of the Saturn V and one engine for each S-IVB Saturn IB and Saturn V third stage.
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, including powering the Space Shuttle in a number of early designs as well as on the Comet HLLV.
J-2T
While work on the J-2S continued, NASA also funded a design effort to use the J-2S turbomachinery and plumbing to a toroidal combustion chamber with a new aerospike nozzle. This would improve performance even further. Two versions were built, the J-2T-200k that provided thrust, allowing it to be "dropped in" to the existing S-II and S-IVB stages, and the J-2T-250k of .
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
What became a different engine with a similar name, called the J-2X, was chosen in 2007 for the Project Constellation crewed lunar landing program. A single J-2X engine, generating 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 predecessor, and cost less than the Space Shuttle Main Engine (SSME). Design differences include the removal of beryllium, modern electronics, a centrifugal turbo pump versus the axial turbo pump of the J-2, a different chamber and nozzle expansion ratios, a channel-walled combustion chamber versus the tube-welded chamber of the J-2, a redesign of all the electronics, supersonic injection and the use of 21st-century joining techniques. On September 8, 2008, Pratt & Whitney 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 21 September 2010.
Project Constellation was cancelled by President Barack Obama on 11 October 2010, but development of the J-2X has continued for its potential as the second stage engine for the new, heavy-lift Space Launch System. The first hot-fire test of the J-2X was scheduled for late June 2011.
On 9 November 2011 NASA conducted a successful firing of the J-2X engine of 499.97 seconds in duration.
On 27 February 2013 NASA continued testing of the J-2X engine of 550 seconds in duration at NASA's Stennis Space Center.
In later times, however, the planned upper stage engine for what would become the Exploration Upper Stage for the SLS rocket has since been chosen instead as a variant of the RL-10, the RL-10C-3. As a result of this, development of the J-2X has ceased, with the program officially being "idle" since the end of the prototype's testing regime in 2014.
<gallery class="center">
Image:J-2X concept image June 2006.png|Concept image of the J-2X engine
Image:Test of the J-2X Engine.JPG|Test of the J-2X engine 'workhorse' gas generator
Image:NASA's J-2X Engine.jpg|Cold Flow nozzle testing for the J2X program
</gallery>
Specifications
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