WASHINGTON — Tests of the main-stage engine for NASA’s planned Space Launch System (SLS) heavy-lift rocket continued at the Stennis Space Center in Mississippi June 11 as the agency fired up an RS-25 engine for the third time this year.

In an online post, NASA said it test-fired an RS-25 for 500 seconds on the A-1 test stand at Stennis. The latest hot-fire follows a 450-second test on May 28 that was the first for the RS-25 since Jan. 9. After the January test, NASA shut down the test stand to fix defects in its fuel lines.

The RS-25, made by Aerojet Rocketdyne of Sacramento, California, is the former space shuttle main engine. The motor powered 135 shuttle missions before being repurposed for SLS. Each SLS core stage will use four of the engines, along with a pair solid-fuel boosters provided by Orbital ATK of Dulles, Virginia.

NASA has scheduled two SLS flights: one in 2017 and one in 2021. Both will go to a distant lunar retrograde orbit, and only the second will be crewed.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.