See the rocket engine that has been boosting satellites out of Earth’s orbit since 1951.
First developed in 1959, the Pratt & Whitney RL10 was the first hydrogen/oxygen engine, yet it is still the most fuel efficient engine ever made, and the highest ISP engine flying today. It cranks out 400,000 horsepower of thrust. It has carried spacecraft to the Moon and the sun and every planet in our solar system and beyond (Mariner, Pioneer, Voyager, Cassini, Mars Viking, Helios solar probe, Lunar Surveyor, Venus 2, LCROSS). It powers the second stage of the Saturn, Atlas V, Titan and Delta III and IV.
— Steve Jurvetson
More photos and information about the rocket engine here.