WASHINGTON — Boeing Co. and the U.S. Air Force have a lot more riding on the first mission of the heavy-lift Delta 4 rocket than the demonstration payload it is supposed to carry into space, officials said.

The launch, which Boeing hopes to carry out no later than Dec. 22, is intended to show that the heavy-lift variant of the Delta 4 will be a reliable vehicle for lofting the nation’s largest military payloads into orbit, officials said.

A successful flight is crucial, because the Air Force is slated to launch its final two Titan 4 rockets, the service’s current heavy-lift vehicle, in 2005, said Phil McAlister, program manager for the space and telecommunications industry analysis unit at Bethesda, Md.-based Futron Corp.

For the U.S. Department of Defense, “this is a very important mission,” McAlister said. “For Boeing, it’s even more important.”

Boeing, headquartered in Chicago, developed the Delta 4 rocket family under the Air Force Evolved Expendable Launch Vehicle (EELV) program. The heavy lift variant, composed of three of the vehicle’s RS-68 core-stage engine joined together, is designed to loft up to 13,000 kilograms to geosynchronous-transfer orbit, twice the capacity of its single-core cousin.

The first Air Force launch of the heavy-lift Delta 4 is a Defense Support Program missile warning satellite scheduled for fall 2005. In the event that the demonstration launch does not go as expected, Boeing and the Air Force will have to weigh their options, officials said.

“We will continue to make this vehicle successful if we have an anomaly and hope to do so immediately if that scenario happens,” company spokesman Robert Villanueva said.

The next step would depend on the nature of any potential problem discovered during the demonstration launch, Alicia Garges, of the Air Force Space and Missile Systems Center at Los Angeles Air Force Base, said in a written response to questions.

One option for the Air Force could come from Lockheed Martin Corp., company spokesman Julie Andrews said. Lockheed Martin, headquartered in Bethesda, Md., developed its Atlas 5 rocket family under the EELV program, and the vehicles are marketed by International Launch Services of McLean, Va.

While there is no heavy-lift version of the Atlas 5, variants that already have flown have the capability to launch most of the military’s largest payloads, Andrews said. Only a single military launch scheduled for the next several years would require a heavy-lift variant of the Atlas 5, and the company could produce that vehicle in 30 months, she said.

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