A powerful United Launch Alliance Delta 4 Heavy, one of only four remaining on the company’s books, blasted off from California on Monday afternoon, lifting a classified National Reconnaissance Office spy satellite into space.

The Aerojet Rocketdyne RS-68A engines powering the rocket’s three side-by-side “common booster cores” thundered to life with a roaring rush of flame at 4:47 p.m. EDT, pushing the 233-foot-tall vehicle away from launch complex 6 at Vandenberg Air Force Base northwest of Los Angeles.

Generating 2.1 million pounds of thrust — the equivalent of 51 million horsepower — the hydrogen-fueled engines quickly propelled the rocket skyward, consuming 5,000 pounds of liquid hydrogen and oxygen propellants per second as it arced away on a southerly path over the Pacific Ocean.

A truncated look at the Delta 4 Heavy climbing away from Space Launch Complex 6 at Vandenberg Air Force Base, California.

ULA webcast

The two outboard common core boosters, after helping lift the rocket out of the dense lower atmosphere, were jettisoned about four minutes after liftoff. The central booster continued firing another minute and 40 seconds before it, too, was discarded.

The flight continued on the power of the rocket’s single RL10B-2 second stage engine, but as usual with classified NRO missions, United Launch Alliance ended its mission commentary a few moments later and the remainder of the flight was carried out in secrecy.

The timing of the launch and the rocket’s trajectory, tilted about 98 degrees to the equator based on safety notices posted before liftoff, closely matched the paths to orbit used by previous classified optical imaging satellites capable of collecting extremely detailed views of ground targets.

A camera on the side of the Delta 4’s central core captures a view an outboard booster falling away as the rocket climbed toward space.

ULA webcast

But the National Reconnaissance Office does not provide any information about its classified payloads or their orbits and does not comment on outside speculation.

United Launch Alliance is a partnership between Lockheed Martin, builder of the Atlas 5 family of rockets, and Boeing, which designed the Delta 4. Monday’s launch was the 13th of the most powerful Delta 4 Heavy variant.

ULA is phasing out the Delta 4 and, eventually, the Atlas 5 in favor of new Vulcan rockets that will be available in multiple configurations featuring reusable first stage engines.