On March 10, 2016 NASA conducted a successful 500 second test firing of the first Space Launch System (SLS) flight dedicated RS-25 engine. This major milestone means that the first of the four RS-25 engines that will form the core 1st stage of SLS is ready to go. The next time it lights for that length of time will be during the maiden flight of SLS hopefully in 2018.
The RS-25 is the engine that propelled 135 Space Shuttle missions and is the most reliable rocket engine on the planet. 16 RS-25 engines were left over from the Space Shuttle program and are being used for SLS. The Rs-25 production line was recently reopened so that new engines could be produced.
Unlike the Space Shuttle, SLS RS-25 engines will be "one and done" for each mission.
Here is a closeup of an RS-25 engine from when I visited Stennis in 2013.
Sky Guy in VA