SpaceX is launching its massive Falcon Heavy rocket for the second time this year on Monday night in an experimental mission for the U.S. Air Force.

The rocket is delivering 24 spacecraft into three separate orbits, in what CEO Elon Musk called SpaceX's "most difficult launch ever." Lifting off from Kennedy Space Center in Florida, the mission will take about 3 1/2 hours to complete.

Falcon Heavy is the most powerful rocket in the world, with 27 engines to power the colossus. In essence, Falcon Heavy is three of SpaceX's Falcon 9 rockets strapped together and strengthened.