NASA's new Space Launch System isn't going anywhere without some monstrous engines. The second stage engine will be the J-2X, a descendant of the Apollo-era J-2, and NASA just lit one off for 500 seconds in a very loud, steam-filled test.
The J-2 rocket engine was developed back in 1960 by Boeing, and a cluster of five of them powered the second stage of the Saturn V moon rocket. Figuring that it's a good idea to just go with what works, NASA has decided to re-enlist the J-2 in the form of the J-2X to power the second stage of the SLS. The J-2X is an upgrade from the original J-2, offering simpler and more efficient operation while also being cheaper and easier to build. It's also more powerful, producing nearly 300,000 pounds of thrust, which is a whole heck of a lot, as you can see from the vid below.
Of course, the J-2X is only powering the second stage of the SLS. The first stage will be driven by five (!) RS-25D/Es, which are the same engines that powered the space shuttle. Oh, and with a couple solid rocket boosters thrown in for good measure. The SLS isn't scheduled to launch for at least five years, but when it goes up, it's going to be one of the most epic rocket launches the world has ever seen.