Rocketdyne J-2 rocket motor, 1960s.
© Science Museum / Science & Society Picture Library
The J-2 engine is one of the most important engines in the history of manned space flight. It was the first manned booster engine to make use of liquid hydrogen as a propellant, as well as being the first large booster engine designed to be restarted multiple times in the course of a flight. The J-2's versatility meant that it was used on both the second and third stages of the Saturn V rocket used to launch NASA's succesful Apollo misions to the Moon. The second stage used five of the engines, which burned for six minutes, while the third used a single J-2, which was fired twice during each mision and then sent into orbit around the Sun.