After Kennedy announced his crazy Moon idea, NASA scrambled to get rockets powerful enough to achieve that mission. Rocketdyne created the J-2, the 232,250 pounds of force beast that powered the Saturn's second stage. This is the J-2X.
It's the next generation engine that was intended to power Ares I and Ares V, the two spacecraft that were supposed to take us to the Moon and Mars. Capable of generating 294,490 pounds of force, the Pratt and Whitney Rocketdyne J-2X is being now tested for the heavy-life Space Launch System in NASA's A-2 Test Stand at Stennis.
This image shows a 1.9-second ignition test, known as The Burp Test. [NASA]