A new spy satellite launched into orbit on a secret mission for the U.S. military Friday, roaring spaceward atop the world's most powerful rocket in use today.

The NROL-15 reconnaissance satellite blasted off from a launch pad at Cape Canaveral Air Force Station in Florida at 9:15 a.m. EDT to begin its classified mission for the National Reconnaissance Office. It rode a towering Delta 4-Heavy booster equipped with new RS-68A first stage engines — the most powerful liquid hydrogen rocket engines ever built, according to their manufacturers.

Built by Pratt and Whitney Rocketdyne, the new RS-68A engines are each capable of generating 702,000 pounds of thrust and are more efficient than a previous design, the rocket engine builders said, adding that the engine generates 36,000 more pounds of thrust than its predecessor. Three of the new rocket engines made their space launch debut in Friday's Delta 4-Heavy flight by rocket provider United Launch Alliance.

ULA

The NROL-15 spy satellite takes off Friday from Cape Canaveral, Fla., powered by a Delta 4-Heavy booster with new RS-68A first stage engines — the most powerful liquid hydrogen rocket engines ever built.

"And the RS-68A engines roar to life!" a ULA launch commentator said during liftoff. "The NRO systems continue to ensure vigilance from above." [ Photos: Secret Spy Satellite NROL-15 Blasts Off ]

The Delta 4-Heavy rocket is currently the largest and most powerful rocket in service. The booster features three core rocket boosters and is topped with a second stage to place payloads into orbit. It is 235 feet tall (72 meters) and can carry payloads of up to 24 tons into low-Earth orbit and 11 tons to geosynchronous orbits.