The Delta IV Heavy rises from the launch pad Dec. 21, 2004, at Cape Canaveral Air Force Station in Florida. Photo: Carleton Bailie, The Boeing Co.

Team of Experts Identify Root Cause of

Delta IV Heavy Mission Anomaly

EL SEGUNDO, Calif. (3/23/05) -- A team of experts from The Aerospace Corporation, the Air Force, and The Boeing Co. has identified the root cause for the premature main engine cut-off on the Delta IV Heavy-Lift Vehicle demonstration mission that was launched Dec. 21, 2004.

The root cause of the anomaly was a fluid cavitation within the liquid oxygen feed system, according to a Space and Missile Systems Center news release dated March 16.

“Over the last several months Aerospace Delta IV program office and Engineering and Technology Group personnel have worked extremely hard on this anomaly investigation,” said Ray Johnson, vice president, Space Launch Operations at The Aerospace Corporation.. “Our technical staff has been instrumental in identifying and validating the root cause through fluids analysis and component testing. We utilized all of our relevant experts, including casual employees like Dr. John Murdock, a recognized expert in fluid dynamics.”

According to the Air Force news release, analyses showed that the cavitation originated at the entrance of the propellant feedline, where a filtration screen and turning elbow restrict the propellant flow as it accelerates leaving the tank. This feedline restriction has been present in all previous Delta IV flights, but the unique combination of vehicle acceleration, the liquid level in the tank, and the propellant flow rate for this mission reduced the fluid pressure enough to enable the creation of gaseous oxygen at this location as the tanks emptied.

Further draining of the liquid oxygen tank worsened the conditions at the feedline inlet, causing the cavitation effect to extend down the feedline until it reached the liquid depletion sensors and caused them to momentarily toggle dry, the release stated. This action was sensed by the flight computer, which initiated the sequence to throttle-down and shut off the main engines as it is programmed to do. Flight data shows that sufficient propellant remained in the tank to complete the planned first stage burn time.

“This investigation has followed a deliberate process to ensure no potential causes were missed,” said Maj. Rod Houser, investigation lead for the Air Force. “The fault-tree analysis, which we’ve used to categorize potential causes in propulsion, avionics, structures, and flight environments, has ruled out 49 of the 50 fault-tree branches. Our attention is now focused on the final open branch of the fault tree dealing with cavitation within the liquid oxygen feed system,” Houser said.

Engineering experts researched the other 49 “closed” fault-tree branches and other possible causes to determine they were not credible as causes of the anomaly. All closures were thoroughly documented, citing multiple sources of supporting evidence drawn from flight data, a range of focused technical analyses, and computer simulation results.

The Boeing Co. has started to take steps toward corrective actions for the entire Delta IV fleet. Additional computer simulations that fully analyze the liquid oxygen flow between the bottom of the propellant tank and the engine cut-off sensors will continue through March to help the engineers select and validate corrective actions. A final executive review board will then be held to officially close the anomaly investigation.

The Aerospace Corporation, based in El Segundo, Calif., is an independent, nonprofit company that provides objective technical analyses and assessments for national security space programs and selected civil and commercial space programs in the national interest.