CAPE CANAVERAL, Fla. -- The most powerful model yet of the Atlas 5 rocket is set to blast off Thursday from Cape Canaveral Air Force Station.

Ready to fly is an upgraded version of the historic Lockheed Martin-built rocket that is equipped with a pair of brand new solid rocket boosters (SRB's) and a larger nose cone.

The Atlas 5 targeted for liftoff between 7:20 and 9 p.m. EDT (2320 and 0100 GMT) will have a fresh look and enough lifting power to leap off the launch pad at twice the speed of the previous Atlas 5. If successful, Lockheed Martin will have demonstrated in flight every major component it has at its disposal to mix and match in building all models of the Atlas 5 family.

"There's an anxiety because it's new," Adrian Laffitte, Lockheed Martin's director of Atlas launch operations at Cape Canaveral Air Force Station, said Tuesday. "But it doesn't matter -- you always get nervous for a launch, whether or not it's a new configuration."

The Atlas 5 expected to debut Thursday is the first of the 500-series -- more specifically a model 521, because it has the larger five-meter diameter fairing, two SRB's attached and a single engine Centaur. While there is significant overlap in their payload carrying capabilities, at the high end of the chart an Atlas 5 500-series rocket can carry about a half-ton more weight to geosynchronous transfer orbit than the most powerful 400-series version.

The family of launch vehicles was developed by Lockheed Martin as part of the U.S. Air Force's Evolved Expendable Launch Vehicle program to increase assured access to space while at the same time reduce costs. International Launch Services markets the Atlas family in the commercial marketplace.

"We keep improving over what we did before because we're trying to create more flexability for the market," Laffitte said.

All systems are go for launch, officials said. The weather forecast predicts a 70 percent chance of acceptable conditions during the 100-minute window. The 196-foot-tall (59.7-meter-tall) launcher is to carry into geosynchronous transfer orbit a direct broadcast satellite called Rainbow 1, which will be operated by Cablevision.

Growing up

The Atlas 5 booster can trace its heritage back to the missile hey days of the 1950s, a fact that is particularly on the minds of program officials with the introduction on this flight of the Aerojet-built SRB's. The reason: the Atlas rocket has grown so much since the first Atlas A intercontinental ballistic missile (ICBM) was launched in 1957 that the Atlas 5 SRB's are now about as big as the original ICBM.

"Essentially we're flying an Atlas 5 with two Atlas A rockets strapped on," said Laffitte. "Those are huge solids."

Assembled by Aerojet in California, the Atlas 5 SRB's are 67 feet (20 meters) long and weigh 102,000 pounds (46,270 kilograms) at launch. Compare that to the original Atlas A ICBM, which was 76 feet (23 meters) long and weighed 180,000 pounds (81,650 kilograms) at launch.

While the Atlas A missile was just a little bit larger than today's Atlas 5 SRB's, both generate more than 300,000 pounds of lifting thrust. And although the Aerojet design is based on more than 40 years of experience, mission managers will be paying close attention to this launch's first use of the new motors.

"We're very confident the boosters will work as expected," said Pete Cova, Aerojet's Atlas 5 SRB program manager.

Booster club

Cova said there are some technology enhancements in these Atlas 5 SRB's that should make them a better booster, including a new method of attaching the motors to the first stage and using an ogive-shaped nose cone instead of the cone-shaped design seen on competing SRB's.

Another innovation is the use of an enhanced solid propellant grain that burns slower, providing more energy for the same amount of volume compared to other motors.

The type of propellant grain used in the Atlas 5 SRB's is the same that was going to be used in NASA's Advanced Solid Rocket Motor for the space shuttle before that program was shut down in 1993.

Cova also noted that the Atlas 5 SRB is now the world's longest motor in which the propellant is poured in a single casting. Other motors are bigger in diameter and more powerful, but this is the longest, which adds to the complexity during manufacture.

If solid rocket boosters get too long they are usually divided into segments. For example, the space shuttle's 149-foot-tall SRB's are made up of eight segments, which are joined to create four segments in the factory. Those four are then shipped to the launch site and joined there.

The Atlas 5 SRB's were test fired five times on the ground, failing once during the initial test due to a burn-through of the nozzle on the aft end. Investigators determined what happened and why, and subsequent tests proved the design was sound.

"When we were all done we had not too many doubts we had the right root cause and the right solution," said Mike Przekwas, Lockheed Martin's manager in charge of the Atlas 5 SRB's.

For Thursday's mission the SRB's will be ignited at launch and burn for about 90 seconds. Another 30 seconds or so later they will be jettisoned to fall into the Atlantic Ocean. NASA's solid rocket booster recovery ships will be waiting to attempt to retrieve the spent casings to study -- not to re-use, but the chances are maybe less than one percent it can be done, Laffitte said.

"If there's an opportunity by some miracle that we are able to recover them, particularly the nozzles, we'll be able to get post flight data that will be very useful to our engineers," Laffitte said.

Lockheed Martin is paying NASA an undisclosed amount to be on station, while NASA and its contractors who operate the vessels are using the opportunity as a training exercise.

Cover up

The other visible change to the model 500-series is the inclusion of the 17.8-foot (5.4-meter) diameter fairing, which is designed to completely encapsulate the Centaur upper stage and the satellite cargo. Built by Contraves Space AG of Zurich, the two-piece nose cone splits right down the middle and remains in place until about three minutes and 45 seconds after launch, when it falls away.

A moment later a structure that was inside the fairing and connected it to the Centaur upper stage also is jettisoned, clearing the way for the Centaur to separate from the first stage less than a minute later.

Engineers designed the nose cone to cover up the entire upper stage and spacecraft because it was a simpler solution to some aerodynamic and structural challenges. Also helping was the fact that the Atlas 5's RD-180 Russian main engine is so powerful that it could easily handle the extra weight from the larger nose cone.

Contraves is offering three different lengths for the 500-series fairing. The short one is 68 feet (20.7 meters) high, the medium is 77 feet (23.4 meters) and the long fairing is 87 feet (26.4 meters) tall. This first Atlas 5 is carrying the short fairing.

Made of a composite material, the fairing has a layer of cork on the exterior to protect it and the spacecraft from excessive heat during the climb through the dense lower atmosphere. To help ward off any build-up or discharge of static electricity, an electrically conductive white paint is applied to the outside of the cork, while the inside of the fairing has a lining of material that helps keeps noise levels down around the satellite.

Air pressure inside the nose cone is controlled during launch with a series of vents, and each fairing can be manufactured with tailor-made access doors and special windows that allow only certain radio frequencies to pass through depending on the satellite.