The four engines that will help vault NASA's massive Space Launch System on its inaugural flight in 2019 are ready to be attached to the rocket's core stage, the agency said this week.

The four Aerojet Rocketdyne RS-25 engines, which flew on prior space shuttle missions, will join two solid rocket booster segments in lifting the rocket and an unmanned Orion spacecraft on Exploration Mission-1, or EM-1, to orbit the moon for several days and eventually splash down in the Pacific Ocean.

"The RS-25 engines that are being tested and prepared for SLS were proven during the years they were responsible for propelling 135 shuttle missions, and have been upgraded for the first SLS flight," a release from NASA reads.

The engines slated to fly on EM-1 are mostly the same with the exception of updated controllers, or "brains." The agency still has 16 of the engines left over from the space shuttle era, but has ordered six more from Aerojet Rocketdyne for future missions.

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The four liquid hydrogen and liquid oxygen-fueled RS-25s contributed to 21 successful shuttle missions:

E2045: The most veteran engine with 12 flights, including John Glenn's final flight in 1998.

E2056: Four total flights, including NASA's return to flight in 2005 after the Columbia disaster two years earlier.

E2058: Six total flights.

E2060: Three total flights, including the final space shuttle mission in 2011.

Despite their rich history with the shuttle program, however, the engines will not be recovered and reused after launching the rocket.

Progress on the core stage, meanwhile, continues as teams have completed work on all five of its structures, including the RS-25 engine mounts. When fully assembled, the stage will stand taller than a 20-story building and hold more than 700,000 gallons of fuel.

Contact Emre Kelly at aekelly@floridatoday.com or 321-242-3715. Follow him on Twitter and Facebook at @EmreKelly.