Pratt & Whitney Rocketdyne, the rocket-engine manufacturer that helped power American astronauts to the moon during the Apollo era, has completed the last in a series of hot-fire tests on a J-2X engine with a stub-nozzle extension at simulated altitude conditions. This latest chapter in the development of America’s next rocket engine paves the way toward full-motion testing of the J-2X engine, which is designed to power humans to Mars. NASA has selected the J-2X as the upper-stage propulsion for the evolved 143-ton (130-metric-ton) Space Launch System (SLS), an advanced heavy-lift launch vehicle.