CAPECANAVERAL, Fla. - The inaugural Boeing Delta 4-Heavy rocket suffered prematureengine shutdowns during its December test launch because of bubbles in theliquid oxygen plumbing, investigators have concluded, and now correctivemeasures are being devised to prevent a repeat problem during the next launchin October.
The three engines weresnuffed out several seconds early after internal sensors were fooled intobelieving the liquid oxygen fuel supply had been expended. That left the rocketwith a massive underspeed in which the vehicle's upper stage could not overcomeand resulted in a final orbit lower than planned.
"The root cause of theanomaly has been identified as a fluid cavitation within the liquid oxygen feedsystem," the Air Force said Wednesday in announcing the investigation'sfindings.
The cavitation, orbubbling, is a localized condition where the super-cold oxidizer changed fromliquid to vapor within the feed lines running from the rocket's tanks toengines.
The Delta 4-Heavy is thelargest member in Boeing's next-generation rocket family. It takes three CommonBooster Cores, each featuring a cryogenic main engine, and straps them togetherto form a vehicle capable of launching hefty cargos into space.
Trouble on trip to space
The three Common BoosterCores 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 frompad 37B at Cape Canaveral Air Force Station, Florida. It was meant to be a fulldress rehearsal flight -- with only a dummy payload aboard -- to test therocket before critical national security satellites begin using the vehiclelater this year.
About 50 seconds intoflight, the center booster's main engine throttled back to 58 percent thrust asa fuel conservation effort. The starboard and port boosters continued tooperate at their maximum power setting of 102 percent thrust, each guzzling aton of propellant per second.
The strap-on boosters werescheduled to fire until T+plus 4 minutes, 5 seconds when the Rocketdyne-madeRS-68 engine on each stage would cut off. About three seconds later, the15-story starboard and port boosters, which provided the vast majority ofthrust during the first four minutes of flight, would peel away from the centerstage and tumble into the Atlantic Ocean below.
But the engines shut down 8seconds early after sensors temporarily indicated "dry" fuelconditions despite the stages having plenty of propellant remaining toaccomplish the scheduled firing time. The sensors returned to "wet"readings after the shutdown sequence was already activated.
Once the outer boosterswere 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 thesame propellant supply and engine package, it employed a more conservative fuelconsumption strategy by the lower-throttle setting for the past three minutesand saved enough propellant to operate almost 90 seconds longer.
Butthe same sensor "phenomenon" repeated on the center booster, causingits engine to shut down 9 seconds prematurely, according to investigators.
After the center boosterhad been jettisoned, the Delta 4-Heavy rocket's upper stage found itself with aspeed deficit of 1,500 feet/second due to the early shutdowns of the mainengines. The upper stage ignited for the first of three firings planned overthe 6-hour mission to geosynchronous orbit.
That first burn of thePratt & Whitney RL10 upper stage engine was supposed to last seven minutesto reach an initial parking orbit around Earth where a pair of university-builtnanosatellites would be released into space. The rocket motor was designed toextend its firing time to compensate for any performance shortfalls experiencedby the Common Booster Cores, and it did that. But even through the stage firedmuch longer than planned it still failed to reach a stable orbit, deploying thenanosats into a suborbital trajectory that took them into the atmosphere beforecompleting a lap around the planet.
The upper stage thenreignited for its second scheduled burn, shaping the rocket's track into ahighly elliptical egg-shaped geosynchronous transfer orbit. It was in thisorbit that the vehicle coasted for five hours to reach the high point about19,600 nautical miles above the planet where the final engine blast wouldoccur.
This firing should havelasted three minutes to circularize the orbit. However, the stage's preciousfuel supply was greatly impacted by the extended maneuvers battling back fromthe Common Booster Core problem. The stage ran out of fuel about two-thirds ofthe way through the burn, leaving the instrumented satellite simulator payload-- the rocket's main cargo for this test flight -- with an orbit featuring ahigh point of 19,600 nautical miles (36,400 km), low point of 9,600 nauticalmiles (19,000 km) and inclination of 13.5 degrees. The orbit's low point was10,000 miles off the target and inclination was 3.5 degrees higher thanplanned.
Tracking down the glitch
Each Common Booster Corehas a large liquid hydrogen tank and a much smaller liquid oxygen tank for itsRS-68 engine. The liquid oxygen tank is located at the top of each rocketstage, with a long feedline running down the booster's side to reach theengine.
"Analyses show thatthe cavitation originated at the entrance of the propellant feedline, where afiltration screen and turning elbow restrict the propellant flow as itaccelerates leaving the tank. This feedline restriction has been present in allprevious Delta 4 flights, but the unique combination of vehicle acceleration,liquid level in the tank, and propellant flow rate for this mission, reducedthe fluid pressure enough to enable the creation of gaseous oxygen at thislocation as the tanks emptied," Wednesday's Air Force statement said.
"Further draining ofthe liquid oxygen tank worsened the conditions at the feedline inlet, causingthe cavitation effect to extend down the feedline until it reached the liquiddepletion sensors and caused them to momentarily toggle 'dry.' This action wassensed by the flight computer, which initiated the sequence to throttle-downand shut off the main engines as it is programmed to do. Flight data shows thatsufficient propellant remained in the tank to complete the planned first stageburn time."
A Fault Tree analysis wasused to examine potential causes of the problem, including propulsion,avionics, structures and flight environments. Forty-nine of 50 Fault Treebranches were "closed" after being ruled not credible.
"All closures werethoroughly documented, citing multiple sources of supporting evidence drawnfrom flight data, a range of focused technical analyses and computer simulationresults," the Air Force said.
Other propellant phenomenalike sloshing and "vapor pull-through" were analyzed and determinedto be highly unlikely.
"Thisinvestigation has followed a deliberate process to ensure no potential causeswere missed," said Maj. Rod Houser, investigation lead for the Air Force."Our attention is now focused on the final open branch of the Fault Treedealing with cavitation within the liquid oxygen feed system."
Engineers have spent thepast two months examining various scenarios to explain the cavitation occurringin the region near the engine cut-off sensors.
"Our team usedcomputer models to simulate the flow in the liquid oxygen feedline between thebottom of the propellant tank and the engine cut-off sensors, approximatelyfive feet downstream," said Mark Baldwin, Boeing's Delta propulsionanalysis manager.
"The team enhanced itssimulation models incrementally to include the more complex internal featuresof the liquid oxygen tank and feedline. Simulation runs have been completedwith the higher fidelity models, resulting in an increasingly accuratesimulation of the flow conditions experienced during the Heavy demonstrationflight. These conditions correlated well with measurements taken by the sensorsonboard the vehicle."
Boeing is examining optionsto fix the bubbling problem. Throughout this month, additional computersimulations are being performed to fully analyze the liquid oxygen flow betweenthe bottom of the tanks and the engine cut-off sensors to assist in picking andverifying the corrective actions, the Air Force said.
"Boeing is evaluatingfuture missions across its Delta 4 family of launch vehicles so that adequatemargin for cavitation exists under the worst case conditions," a companyspokesman said. "Cavitation margin adjustments, if required, can be madeby changing the flight profile to throttle the RS-68 earlier, and can also bemade by pressurizing the oxygen tank to a higher ullage pressure later in flight."
Boeing is scheduled tolaunch the GOES N civilian weather satellite from Cape Canaveral atop a Delta4-Medium rocket on May 4. A Medium vehicle uses just one Common Booster Core --a configuration that has flown three times without fault.
That will be followed bythe first Delta 4 launch from Vandenberg Air Force Base in California, alsoflying in the Medium version. It is targeted for late August to loft aclassified National Reconnaissance Office Payload.
The first operational Delta4-Heavy with a real satellite payload is planned for late October when the 23rdand final Defense Support Program missile-warning spacecraft is launcheddirectly into geostationary orbit.