The most powerful Atlas V rocket is scheduled to fly for the first time from Vandenberg Air Force Base on Thursday evening, carrying clandestine cargo into orbit, some 55 years after a very different version of the venerable booster made its debut on the West Coast.

Liftoff of the United Launch Alliance Atlas V booster from Space Launch Complex-3 East on South Base is expected to occur after the launch window opens at 7:17 p.m. Thursday. The length of the launch window is top secret, but reportedly doesn't extend past 8 p.m.

The actual launch window for the 200-foot-tall rocket remains classified due to the hush-hush spacecraft on board for the National Reconnaissance Office mission dubbed NROL-35.

While Atlas V rockets have flown 50 times since 2002, including nine from Vandenberg, this mission marks the first West Coast launch for this configuration — four boosters attached to the Common Booster Core to provide more thrust upon departure.

The first West Coast flight for this version of Atlas V rocket adds to the mission’s significance, according to Air Force Lt. Col. James Bodnar, commander of the 4th Space Launch Squadron.

“When you break it down, this is the most powerful Atlas vehicle ever launched from Vandenberg Air Force Base,” Bodnar said. “And that’s a history of decades of launching Atlas from here that goes back to the ’60s.”

Vandenberg’s first Atlas, actually an intercontinental ballistic missile, blasted off from the base on Sept. 9, 1959. Since then, VAFB has seen 293 various versions of the Atlas launch vehicle as both an ICBM, retired missiles converted into satellite-carrying space boosters and new rockets continuing the name and the workhorse reputation.

The most recent Atlas V rocket that launched from Vandenberg in August involved the 401 configuration that employs no boosters.

The Atlas V rocket’s 541 configuration puts out about 2 million pounds of thrust at liftoff, making it comparable to the Delta IV-Heavy rocket, said Capt. Jason Krahmer, launch mission manager and lead integrator.

The smaller Atlas V 401 version uses about 900,000 pounds of thrust at liftoff, he added.

“So it’s quite a jump for us, and we’re pretty excited about it,” Krahmer said.

To help explain the difference between an Atlas without solid rocket motors and one with four of them, Bodnar said he refers to the Minuteman III missile test earlier this year from Vandenberg.

“Each one of the solid rocket boosters on the Atlas is about the same size and produces approximately the same thrust as a Minuteman III ICBM, give or take a couple thousand pounds of thrust,” he said. “It’s a significant jump in performance to go from 900,000 pounds of thrust on a regular Atlas 501 or 401 configuration.

“So any of the folks who get to watch this launch, which will be an evening launch, will really get to see a show, both in sound and in light produced during the launch as long as the skies are clear,” Bodnar added.

Late last week, mission managers and launch weather officers were keeping a close eye on the forecast because a big storm is headed toward the Central Coast.

“We’re already a little bit nervous about what we see coming across toward us for Thursday,” Bodnar said, adding it was too early to say if the storm would hit “before, during or after our attempt.”

As of Sunday, the storm is forecast to arrive sometime Thursday evening.

Crews began stacking the rocket at SLC-3 in September after it arrived aboard a Russian-built Antonov airplane that landed on the Vandenberg runway.

“It’s quite an exciting time for us here and it’s been going very well,” Krahmer said. “At this point, we’re about three months into launch base processing so we have just a handful of other tasks that we have to complete before launch.”

To ready for the rocket, the team had to ensure the launch pad could accommodate this configuration.

“We actually went out and performed quite extensive analysis to determine where we needed to make modifications to our mobile service tower to ensure that the four solid-rocket motors were able to fit inside of the launch pad and successfully integrate those,” Krahmer said.

Some modifications were made to a couple of the levels inside the mobile service tower as a result of the review.

The 541 configuration fulfilled another mission for the National Reconnaissance Office earlier this year from the East Coast and carried NASA’s Curiosity rover to Mars in 2011, also from Florida.

Atlas V is part of the Air Force’s Evolved Expendable Launch Vehicle Program, which was developed more than a decade ago to create families of rockets to carry government payloads into orbit.

“This has been the busiest two years for EELV in Vandenberg’s 10-year history with the Atlas V and Delta IV,” Bodnar said, noting the base has seen three missions in the past couple of years.

Any delays of a Vandenberg mission could affect crews on the East Coast and vice versa. That’s one of many reasons the Vandenberg team breathed a sigh of relief at Friday’s successful liftoff of a Delta IV rocket from Florida with NASA’s Orion craft, according to Bodnar.

— Noozhawk North County editor Janene Scully can be reached at .(JavaScript must be enabled to view this email address) . Follow Noozhawk on Twitter: @noozhawk, @NoozhawkNews and @NoozhawkBiz. Connect with Noozhawk on Facebook.