Errant readings from sensors inside the inaugural Boeing Delta 4-Heavy rocket triggered the premature shutdown of its three main engines during ascent last month, causing a massive underspeed that the vehicle's upper stage could not overcome and resulting in a final orbit lower than planned, the U.S. Air Force said Friday. A team investigating results of the test launch are confident the problem can be resolved.

The three hydrogen-fueled Common Booster Cores were ignited during the final seconds of the December 21 countdown, generating 1.9-million pounds of thrust to propel the 23-story rocket away from pad 37B at Cape Canaveral Air Force Station, Florida.

About 50 seconds into flight, the center booster's main engine throttled back to 58 percent thrust as a fuel conservation effort. The starboard and port boosters continued to operate at their maximum power setting of 102 percent thrust, each guzzling a ton of propellant per second.

The strap-on boosters were scheduled to fire until T+plus 4 minutes, 5 seconds when the Rocketdyne-made RS-68 engine on each stage would cut off. About three seconds later, the 15-story starboard and port boosters, which provided the vast majority of thrust during the first four minutes of flight, would peel away from the center stage and tumble into the Atlantic Ocean below.

But the reconstruction of data received from the rocket shows the engines shut down 8 seconds early after sensors temporarily indicated "dry" fuel conditions despite the stages having plenty of propellant remaining to accomplish the scheduled firing time. The sensors returned to "wet" readings after the shutdown sequence was already activated.

Once the outer boosters were shed, the center stage's RS-68 engine revved back to full throttle. Although the booster was identical to the outer strap-on stages, carrying the same propellant supply and engine package, it employed a more conservative fuel consumption strategy by the lower-throttle setting for the past three minutes and saved enough propellant to operate almost 90 seconds longer.

But the same sensor "phenomenon" repeated on the center booster, causing its engine to shut down 9 seconds prematurely, the Air Force said.

After the center booster had been jettisoned, the Delta 4-Heavy rocket's upper stage found itself with a speed deficit of 1,500 feet/second due to the early shutdowns of the main engines. The upper stage ignited for the first of three firings planned over the 6-hour mission to geosynchronous orbit.

That first burn of the Pratt & Whitney RL10 upper stage engine was supposed to last seven minutes to reach an initial parking orbit around Earth where a pair of university-built nanosatellites would be released into space. The rocket motor was designed to extend its firing time to compensate for any performance shortfalls experienced by the Common Booster Cores, and it did that. But even through the stage fired much longer than planned it still failed to reach a stable orbit, deploying the nanosats into a suborbital trajectory that took them into the atmosphere before completing a lap around the planet.

The upper stage then reignited for its second scheduled burn, shaping the rocket's track into a highly elliptical egg-shaped geosynchronous transfer orbit. It was in this orbit that the vehicle coasted for five hours to reach the high point about 19,600 nautical miles above the planet where the final engine blast would occur.

This firing should have lasted three minutes to circularize the orbit. However, the stage's precious fuel supply was greatly impacted by the extended maneuvers battling back from the Common Booster Core problem. The stage ran out of fuel about two-thirds of the way through the burn, leaving the instrumented satellite simulator payload -- the rocket's main cargo for this test flight -- with an orbit featuring a high point of 19,600 nautical miles (36,400 km), low point of 9,600 nautical miles (19,000 km) and inclination of 13.5 degrees. The orbit's low point was 10,000 miles off the target and inclination was 3.5 degrees higher than planned.

Rocket maker Boeing and its Air Force customer are conducting a planned two-month post-flight review of data obtained during the Delta 4-Heavy's demonstration mission. The military bought the test launch to put the rocket through a full rehearsal before critical national security satellites begin flying on the big vehicle.

Despite missing the targeted orbit, Boeing and the Air Force consider the launch a success since all events during test mission occurred and the rocket completed the entire 6-hour flight.

"The overall purpose of this mission was to demonstrate the capability of the Delta 4-Heavy ground and flight systems. The mission profiles were selected to stress the system and reduce the risk to flying an operational mission. This is as close as space systems come to a flight test," said Col. John Insprucker, Evolved Expendable Launch Vehicle program director at the Air Force's Space and Missile Systems Center.

To unravel the problem with the RS-68 engines shutting down too soon, Boeing and the Air Force, supported by The Aerospace Corporation, have established an Anomaly Investigation Team to determine the root cause of the anomaly.

"The goal is to understand and mitigate the risk of this premature shutdown from occurring on an operational mission," the Air Force said in a statement Friday.

"All data is being reviewed, a sequence of events has been constructed, and a Fault Tree analysis has begun to categorize the potential root causes to include propulsion, avionics, and flight environments, etc. This investigation has a deliberate process to ensure no potential causes are missed, and is getting the proper attention at all levels of the Air Force; however, it is difficult to say how long the investigation will take to complete.

"There are several findings, so far," the statement continued.

• "First, the initial flight data indicates the Common Booster Cores shut down prematurely and left the second stage approximately 1500 ft/sec short of the planned velocity.
• "Second, analysis indicates there was sufficient propellant left in the boosters to achieve the mission velocity if not for the premature shutdown. This indicates the vehicle has the necessary performance to achieve the mission as designed. The RS-68 engine performance appears to be healthy.
• "Third, the engine shutdown sensors appeared to operate properly; however, the sensors temporarily indicated 'dry' approximately eight seconds before expected for the strap-ons, and then returned to 'wet' after triggering the shutdown sequence. The same phenomenon was repeated on the center booster core nine seconds before expected. Preliminary data checks show the flight software and control system reacted to the engine shutdown sensors properly.

"The second stage was programmed to compensate for a first stage velocity deficit if required, and it did this successfully until it expended its propellant reserves. The stage flew flawlessly through all three RL-10 engine burns and a large number of maneuvers, achieving a record-setting 1,115 seconds of burn time. Other than the early Common Booster Core shutdown, the launch, six-hour mission and payload deployment appeared nominal.

"The Air Force chose to reduce risk to operational flights by conducting this 'all-up' demonstration. Given the repetitive nature of the anomaly on all three Common Booster Cores, the team is confident this anomaly will be resolved prior to an operational Heavy mission," the Air Force statement said.

Two operational Delta 4-Heavy rocket launches were planned for 2005. What, if any, impact the investigation will have on those launch dates is not clear. An August liftoff is scheduled to carry the final Defense Support Program craft that detects enemy missile launches and nuclear weapon detonations from space. A secret National Reconnaissance Office payload is slated to fly on the second operational Heavy mission in December.