The debut — and launch — of NASA’s massive SLS moon rocket is delayed again. But officials say it’s making significant progress.

But more recently, NASA has been making significant progress with the rocket. Last year it conducted a successful “hot fire” test, running all four RS-25 engines on the core stage of the rocket for over eight minutes, the same amount of time they would fire during flight. (A first attempt a couple months earlier was cut short after just about one minute.) After the successful test at the Stennis Space Center in Mississippi, which burned 733,000 gallons of propellant, the stage was shipped to the Kennedy Space Center, where the solid rocket motor side boosters were mounted, and then the Orion crew capsule was placed on top of the fully assembled system.