United Launch Alliance and the Air Force are readying the next Delta 4-Heavy rocket, the largest booster in the U.S. arsenal that is responsible for launching the nation's elite surveillance satellites.

File photo of the first Delta 4-Heavy rollout at Vandenberg. Credit: ULA video See archived photo gallery

The massive rocket was placed atop its West Coast pad this week, rolling out of the hangar at Vandenberg Air Force Base in California on Monday and going vertical Tuesday for a high-profile mission to deliver a National Reconnaissance Office spacecraft into polar orbit.

Liftoff is targeted for Aug. 28.

Located on the southern stretches of the base and hidden in a bowl of hills, the Space Launch Complex-6 pad avoids the prying eyes of the public for performing hush-hush military spaceflights.

The secluded site was first conceived in the 1960s for launching the Manned Orbiting Laboratory project using Titan 3 boosters and modified Gemini B capsules, but it was cancelled in the midst of construction in favor of unmanned satellites.

The site was reborn in the 1980s for classified Air Force missions of the space shuttle, but that effort was cancelled in the final phases of construction after Challenger in favor of unmanned rockets.

SLC-6 supported a handful of small Athena rockets in the late 1990s before becoming the West Coast home to the Delta 4, part of the Pentagon's Evolved Expendable Launch Vehicle fleet.

After launching an NRO bird and an Air Force weather satellite in separate flights in 2006 using single-core Delta 4-Medium-type rockets, the site underwent an upgrade to accommodate a Heavy, the triple-barrel power necessary to haul much larger cargos into space.

The first Heavy from Vandenberg flew in 2011 to successfully deploy a clandestine NRO satellite, becoming the largest rocket ever launched from the U.S. West Coast, and after another Medium went last year, rocket-maker ULA is getting ready to send up its next Heavy carrying a sizable payload for U.S. national security and the intelligence community.

"It is a very impressive rocket. It is the a largest, most powerful rocket in the U.S. inventory right now, produces approximately two million pounds of thrust at liftoff and is able to carry over 50,000 pounds to orbit. We only use this rocket, because of how large and complex it is, for our largest spacecraft, which tend to be the most strategically important national security payloads that we have. This is immensely critical," said Lt. Col. Dan Gillen, commander of the 4th Space Launch Squadron at Vandenberg and the Air Force launch director for the Heavy mission.

The first Vandenberg Heavy launch in 2011 as seen from an off-shore oil platform. Credit: Air Force photo provided courtesy of the 30th Communications Squadron Optics section

The Delta 4-Heavy is created by taking three Common Booster Cores -- the liquid hydrogen-fueled motor that forms a Delta 4-Medium's first stage -- and strapping them together to form a triple-wide rocket, then adding a cryogenic upper stage.

The rocket was constructed at ULA's factory in Decatur, Alabama, then shipped by sea to Vandenberg, completing the month-long, 4,000-mile voyage in December by docking at the base's harbor once envisioned for receiving the space shuttle fuel tank barge.

The stages were unloaded and moved up the road to Horizontal Integration Facility at SLC-6, an unassuming garage-like hangar were the rocket core was fitted with the port and starboard boosters and the upper stage was attached.

Then on Monday, riding horizontally on a motorized hauler, the 184,000-pound rocket was driven to the pad where hydraulic pistons on Tuesday pushed the 190-foot-tall vehicle upright to stand atop the pad.

"Over the last four-and-a-half months, we've been working multiple shifts a day in order to prepare for this milestone this week," Gillen said.

"All of that work has been in the horizontal where we've mated the three Common Booster Cores and also integrated the second stage with the first stage. All of that went extremely well and we were able to keep the milestone this week of rolling out to the pad and going to the vertical. On Monday we moved out from the Horizontal Integration Facility out to the pad and then Tuesday morning was when we went to the vertical, and everything went nominally," Gillen said.

File photo of the first Delta 4-Heavy going vertical at Vandenberg. Credit: ULA video See archived photo gallery

With the vehicle now standing on the pad, activities in May will focus on system testing and preparations for a realistic launch day simulation. That test, known as the Wet Dress Rehearsal, will occur in June and run through a full countdown and fueling of the rocket's eight liquid hydrogen and liquid oxygen tanks.

The secretive payload will be mounted aboard the Heavy in July. The fully assembled rocket, with the shrouded cargo in its nose cone, will measure over 230 feet tall for launch.

"This is extremely exciting," Gillen said of getting the rocket rolled out. "This really marks, in my mind, about the halfway point of the campaign. We've been going for four-and-a-half months and we've have another four months to get to launch. This is a huge step, it's great we were able to make this milestone. The whole team is just really excited."

The Delta 4-Heavy replaced the national capability to launch large satellites when the Titan 4 rocket was retired in 2005. It has flown five times from Cape Canaveral since 2004 and once from Vandenberg.