A final executive review board was convened on March 25, 2005, to officially close the investigation.

The root cause of the anomaly has been identified as a fluid cavitation within the liquid oxygen feed system.

Analyses showed that the cavitation originated at the entrance of the propellant feedline, where a filtration screen, an elbow and an internal gimbal strut alter the propellant flow (changing the fluid velocity and decreasing the pressure) as it leaves the tank.

This feedline design has been present in all previous Delta 4 flights, but the unique combination of vehicle acceleration, liquid level in the tank, and propellant flow rate for the Heavy 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 further down the feedline. A pocket of gaseous oxygen continued to enlarge until it reached the Engine Cut-Off (ECO) sensors and caused the ECO sensors to momentarily indicate dry.

This ECO sensor dry indication 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. In reality, flight data showed that sufficient propellant remained in the tank to complete the planned first stage burn time.

The investigation team used computer models to simulate the flow in the liquid oxygen feedline between the bottom of the propellant tank and the ECO sensors, approximately five feet downstream.

The example simulation result depicts the cavitation vapor pocket (shown in red) after it has grown beyond the ECO sensor location.

Simulation results correlated well with measurements taken by the special instrumentation sensors on board the vehicle, giving the team high confidence in the root cause conclusions.

Col John Insprucker, Evolved Expendable Launch Vehicle (EELV) System Program Director, Space and Missile Systems Center (SMC), and the Air Force mission director for the demonstration launch said, "The combined Government and contractor team did an outstanding job of working together to understand the root cause behind the Delta 4 flight anomaly, giving us the confidence that cavitation will be eliminated for future flights."

Boeing is implementing corrective actions for the Delta 4 fleet to include both hardware and software fixes.

The primary remedy is to increase the pressure in the liquid oxygen tank to offset the pressure losses in the upper part of the liquid oxygen feedline.

The pressure will be increased a sufficient amount to eliminate the possibility that liquid oxygen could begin to vaporize in the feedline

In previous missions, the liquid oxygen tank pressure was allowed to decrease from its maximum value at lift-off as the vehicle climbed out of the earth's atmosphere.

Two modifications to the Delta 4 launch vehicles will instead increase tank pressure later in flight to avoid cavitation.

First, the liquid oxygen tank's existing pressure relief valve will be replaced with one having a higher relief pressure, and second, the flight software will be modified to provide in-flight booster commands necessary to increase the tank pressure later in flight.

The liquid oxygen tank is already qualified to operate at the required higher pressure.

Additional enhancements to the flight software will further reduce susceptibility to false ECO indications on future flights.

These changes involve adjusting the time during flight when the flight computer will accept an "ECO dry" signal from the liquid oxygen sensors. The executive review board concurred with these corrective actions.

"Implementation plans for the required pressure relief valve and flight software modifications have been defined and are being executed," stated Dan Collins, Boeing's Vice President of Expendable Launch Systems.

None of these corrective actions are required for the next Delta 4 flight, which is scheduled to launch a Geostationary Operational Environmental Satellite (GOES-N) from Cape Canaveral in May.

Based on an analysis of this mission's flight profile, there is no potential for cavitation to form within the liquid oxygen feed system.

However, all future Delta 4 missions will incorporate the new pressure relief valve and software modifications to eliminate cavitation and maintain fleet standardization.

The next EELV Delta 4 launch is NROL-22, currently scheduled for August; the next Delta 4 Heavy launch is DSP-23, scheduled for October. At this time, the corrective action plan does not change Delta 4 launch dates.

"All in all, the Heavy demo mission was successful in providing a wealth of vehicle design, environment and performance validation data. We are now taking all our knowledge and understanding from this Demo mission and applying it as we prepare for our operational missions," concluded Dan Collins.

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