Eighteen RS-25 engines are left over from the space shuttle program. Sixteen have flight experience, while two are used exclusively for testing. Today's test was made with development engine 0525.

While the space shuttle used three RS-25 engines to reach orbit, the Space Launch System is designed to operate with four. According to NASA, each RS-25 generated about 490,000 pounds of thrust during the shuttle era. That number is expected to increase to 512,000 for the Space Launch System's first four flights. A new engine controller, which is responsible for relaying commands between the rocket's avionics systems and engines, was also put through its first live test.

NASA intends to begin using expendable versions of the RS-25 after clearing its inventory of leftover shuttle engines. The agency plans to work with Sacromento, Calif.-based Aerojet Rocketdyne, which built the original shuttle engines, to make the new version more affordable.