Boeing appears to be a step closer to determining why the second stage of a Delta III rocket failed to fire, sending a communications satellite into the wrong orbit last month.

The mostly likely scenario was "an explosive-type event" caused by a breach in the combustion chamber of the rocket's Pratt & Whitney engine, Boeing said in a statement yesterday. That scenario was determined by reconstructing events with the aid of computer data transmitted back to Earth.

The failure on May 4, after an otherwise good launch from Cape Canaveral, Fla., was the second in as many tries for the competitively crucial Delta III.

The first attempt to launch a Delta III, last August, ended with the rocket self-destructing after a guidance malfunction.

It's possible something else caused the second-stage malfunction after May's launch, and the company's team of investigators is exploring several other possibilities. But the most likely cause was the explosive failure, said Russell Reck, director of engineering for Boeing's Huntington Beach, Calif.-based expendable launch systems program.

The next step is to determine what could have caused the combustion chamber to fail.

Boeing has firm customers for 18 more Delta III launches, the next one scheduled for this fall.

Delta III has twice the lifting power of the popular and reliable Delta II. It is a transitional vehicle to a family of even bigger Delta IV rockets, which is being developed for military and

civilian service.

Chuck Taylor's phone-message number is 206-464-2465. His e-mail address is: chucktaylor@seattletimes.com.