The final piece of Boeing's new generation Delta 4 rocket fleet makes its long-awaited debut this week when a booster blasts off from the U.S. West Coast for the first time, punctuating the company's work to develop a line of launchers and build pads in Florida and California.

The mission emblem is depicted here. Credit: Boeing

Conceived in the Air Force's Evolved Expendable Launch Vehicle program to field new American rockets to carry government payloads for the next two decades, the Delta 4 has flown five times in its medium- and heavy-lift configurations from Cape Canaveral, Florida. Now, the maiden mission from California's Vandenberg Air Force Base is poised for launch Tuesday evening.

"We committed back in 1998 to bring really everything that the government needed -- full family (of rockets) from the Medium through the Heavy and both coasts," said Dan Collins, vice president of Boeing Expendable Launch Systems. "There is a lot of pride in getting to this point -- finishing up and demonstrating the whole breadth of the Delta 4 family."

The Vandenberg launch site allows rockets to fly southward for delivery of spacecraft into orbit around Earth's poles, allowing coverage over most of the planet's surface. The earlier Delta 4 launches from the Cape have flown eastward to reach equatorial orbits.

The payload for Tuesday's mission is a classified spacecraft for the U.S. National Reconnaissance Office, the secretive government agency responsible for operating the nation's network of spy satellites. The Delta 4 will haul the craft into a highly inclined, highly elliptical orbit.

This launch has been in the works for years. The rocket itself was erected atop the launch pad on October 30, 2003, but delays with the payload have kept the booster earthbound.

"There have been a significant number of reviews and special tests because of the length this vehicle has been on the pad," Boeing flight director Rich Murphy said in an interview Friday.

The mission was within a day of blastoff in October only to be grounded due to concerns about fuel sloshing in the second stage. Analytical models differed about the potential for sloshing, which could have jeopardized the mission.

Liftoff will happen sometime between 7 and 9 p.m. PDT (10 p.m. and midnight EDT; 0200-0400 GMT). The actual target launch time has not been revealed.

Credit: Boeing

"Everything is looking great," Lt. Col. David Goldstein, 4th Space Launch Squadron commander and Air Force launch director, said in an interview Friday. "Of course, you always have certain ankle biter technical issues that pop up here and there especially with a first launch from a pad."

The weather outlook predicts an 80 percent chance of meeting the launch rules. But low clouds and fog could ruin the view of liftoff. See the full forecast here.

The California home of Delta 4 is commonly referred to as "Slick Six" -- the infamous Space Launch Complex-6 that has a star-crossed legacy. It was constructed in the 1960s for the Air Force's Manned Orbiting Laboratory space station project and then rebuilt in the 1980s for military space shuttle launches. However, both projects were cancelled before any liftoffs occurred, leaving the massive pad in mothball status.

In the 1990s, Lockheed Martin's tiny Athena booster flew a couple of launches from the pad. The first failed, the second mission's payload malfunctioned soon after launch, the third failed, but the final achieved complete success.

Boeing moved into the complex in early 2000, beginning renovations to transform the existing shuttle facilities to support the Delta 4, including the installation of a large erector arm to hoist the assembled rockets upright, modifications to the service tower, stripping the umbilical tower to add swing arms and building the Horizontal Integration Facility nearby.

Delta leaders picked Slick Six because the pad was designed from the start to launch large rockets, enabling much of its infrastructure to be reused.

"I would say the main driving reason was that we were going to do a modification rather than a build from scratch. That has a lot of advantages to it. I am very, very pleased with how the modifications, the construction phase and activation phase have gone at Vandenberg," Collins said.

"We specifically set up a program to take the lessons learned from our activation of SLC-37 on the East Coast, evaluate what had gone well on the East Coast and what we felt we could improve on. That program played a huge role in letting us have a very smooth activation here on the West Coast."

SLC 6 was overhauled from its shuttle days to support Delta 4. Credit: Boeing

Although Slick Six does not have all of the hardware in place to launch a Delta 4-Heavy -- the triple-body rocket that debuted from Florida in December 2004 -- Boeing officials say the pad can be upgraded quickly to support.

"We have designed Vandenberg to launch the Heavy. We have done all of the scarring and we've bought all of the hardware. We chose not to install the outboard tail service masts, which are the umbilical assemblies at the bottom of the rocket just because we wanted to keep them in a more protected situation. Without a current demand for the Heavy, we didn't want to install them and have them subjected to the elements just day-to-day as well as the different launch campaigns," Collins explained.

"When a need for a Heavy on the West Coast comes, we've got a plan in place that will install those tail service masts and we'll be fully ready within a matter of months to launch the Heavy."

The Air Force has ordered a couple of Delta 4 launches from Slick Six. The Delta 4-Medium rocket that will fly the second mission this November -- to loft a polar-orbiting military weather satellite -- is in storage at the base.

Watch our Mission Status Center page for live updates during Tuesday's countdown and launch.