The Delta 4 Heavy is massive, standing taller than the space shuttle and designed to carry payloads of up to about 50,000 pounds to low-Earth orbit where the International Space Station and some remote sensing spacecraft fly.

The rocket can loft about 29,000 pounds into orbits 22,300 miles above the planet, a position favored by communication satellite operators and other entities that need their spacecraft to attain geosynchronous orbit to remain in a relatively fixed position above Earth's surface.

The booster also can carry spacecraft weighing about 22,000 pounds to the moon or and about 17,500 pounds to trajectories leading to Mars.

The first flight, originally scheduled for Friday from Cape Canaveral Air Force Station, will carry a 6.5-ton dummy payload, nicknamed DemoSat. It is instrumented to relay flight and engineering information to ground-control teams, but it has no purpose beyond the launch itself.

Boeing had hoped to carry a commercial payload to demonstrate the Delta 4 Heavy's launch capabilities, but was unable to find a customer. So the Air Force, which is counting on the Delta 4 to launch its national security and communication satellites into orbit, stepped in to sponsor DemoSat.

First and foremost (the goal) is to place the payload into its proper orbit, said Dan Collins, vice president of Boeing Expendable Launch Systems.

The rocket is based on Boeing's successful Delta 4 Medium booster, a design that debuted two years ago as the launcher for a European communications satellite. Two more successful missions occurred in 2003.

The Delta 4 family is built around identical liquid hydrogen-burning boosters powered by a newly developed main engine manufactured by Rocketdyne. The heavy-lift version of the rocket features three of Boeing's so-called Common Booster Cores strapped side-by-side to form a triple body, and a powerful upper-stage motor. The rocket measures almost 235 feet in height - about 50 feet taller than a space shuttle.

Together, the trio of boosters generates almost 2 million pounds of thrust at liftoff, with each rocket consuming about a ton of propellants per second, or about five tanker trailers' worth of fuel per minute.

The Delta 4 Heavy is the latest offering from an Air Force-sired program called the Evolved Expendable Launch Vehicle. It is an effort to reduce launch costs, simplify operations and get spacecraft into orbit quicker than has been possible under traditional custom-order rocket manufacturing programs. The U.S. government seeded development efforts for EELV boosters from both Boeing and competitor Lockheed Martin, which builds the Atlas rockets.

In addition to the DemoSat mission, the Air Force has ordered two Delta 4 Heavy rockets for critical national security payloads: a Defense Support Program missile detection satellite, slated for launch in late 2005, and a classified spy satellite for the National Reconnaissance Office. Beyond those missions, however, the future of the Delta 4 Heavy is uncertain. Boeing initially had counted on a commercial satellite market to carry its heavy-lift line.

The original strategy for demonstrating the Heavy capability was to utilize the perceived burgeoning commercial market. In 1998, this vehicle would have been a big player in what was projected back in those times.

So the Air Force was in a great position. They were going to be able to benefit from the commercial launches, Collins said, in an interview with the online trade industry Web site, SpaceflightNow.com.

When that commercial launch market started to go away and signs that it wasn't going to allow the demonstration to happen, the Air Force stepped in and said 'Hey, we've got some important payloads to go. We want to get data before we put those on top of the rocket,' Collins said. So they came in and purchased an amendment to the development of the contract for this mission.

As hopes for commercial customers dwindled, a new prospect appeared. After the shuttle Columbia accident on Feb. 1, 2003, NASA was advised to retire the space shuttle by 2010 and develop a new vehicle to carry astronauts to the space station and other destinations, including the moon and eventually Mars.

Managers decided to return to using capsules launched on expendable boosters, such as what had been used in the early days of the space program. NASA now is considering heavy-lift Delta and Atlas rockets, as well as shuttle-derived booster concepts for its proposed Crew Exploration Vehicle.

Closer on the horizon is an expected bid from the Air Force for heavy-lift launch services. Both Boeing and Lockheed plan to compete, though the number of launches is not yet known. Liftoff is scheduled to occur between 2:31 and 5:27 p.m. ET Saturday, though meteorologists were predicting winds, clouds and rain, which might force another postponement.

Boeing first needs to get its Delta 4 Heavy off the ground and successfully into space. The rocket has been on the launch pad for a year, delayed by technical problems and a trio of hurricanes that blew through Central Florida over the summer and into September.

In addition to DemoSat, the rocket will carry a tiny pair of university research satellites, nicknamed Ralphie and Sparky. The nanosats had been slated to fly on the space shuttle, but delays after the Columbia accident prompted program managers to find another ride to orbit.

The twin, 35-pound spacecraft are designed to test imaging systems, miniature propulsion systems and satellite-to-satellite communications. The satellites, which will be released just beyond Earth's atmosphere, were designed and built by researchers and students at Arizona State University, New Mexico State University and the University of Colorado.

The rest of the mission, slated to last about six hours, will be devoted to engine burns, coasts and re-ignitions to test the rocket's performance in orbit.

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