A liquid hydrogen and oxygen burning rocket engine, used in the S-II and S-IVB stages of the Saturn launch vehicle family. The S-II used five J-2s as the second stage of the Saturn V, while the S-IVB used one J-2 as the second stage of the Saturn IB and the third stage of the Saturn V.

Even as the liquid hydrogen fueled RL-10 rocket engine was being chosen for the Saturn Is S-IV stage, NASA engineers were looking toward more powerful engines. In June of 1960, the Rocketdyne Division of North American Aviation was awarded the contract to develop the RL-10 successor, the J-2.

Previous engines had been adapted from missiles or unmanned launchers, but as the J-2 was being developed exclusively for the manned Apollo project, reliability and rigorous testing was a watchword from day one.

As with the RL-10, extensive mathematical modelling was used prior to actual construction. Work proceeded quickly, and the first test firing of the engine happened only 18 months after the contract was awarded.

The J-2 engine had a difficult challenge to meet; Apollo lunar missions needed the ability to shut the engine down in flight and then later restart it as part of the S-IVB stage. To meet this challenge, some complicated plumbing was required and as a result the J-2 was more closely integrated into the rest of the stage than most rocket engines are.

Problems faced during the development of the J-2 included making a reliable propellant injector and the elimination of leaky joints.

The J-2 was 2 meters across and 3.4 meters high. It produced over 1 million newtons of thrust.