Premature shutdown of RS-68 engines on heavy booster was caused by problem in liquid-oxygen feed system
The premature shutdown of the first-stage engines on the first Boeing Delta IV Heavy booster, launched on 21 December, was caused by fluid cavitation in the liquid-oxygen feed system as the fuel changed to vapour within feedlines to the engines, investigators have confirmed.
The three Rocketdyne RS-68 engines shut down 8s early when sensors detected a "dry" fuel reading, which returned to normal after the shutdown sequence began. The premature shutdown resulted in the two burns of the Pratt & Whitney RL-10 upper stage being extended to make up the velocity loss. But the upper stage exhausted its propellant two-thirds through the second burn, resulting in a perigee 16,100km (10,000 miles) too low.
Cavitation originated at the entrance to the propellant feedline, where a filtration screen and elbow turn restricted the flow as it accelerated leaving the tank. As the tank drained, conditions at the inlet worsened, causing the cavitation to extend down the feedline until it reached and triggered the liquid depletion sensors, initiating the engine shutdown.
The feedline restriction was present in all previous Delta IV flights, but the unique conditions of the Heavy launch led to cavitation, say investigators. Boeing is "evaluating future missions across its Delta IV family of launch vehicles so that adequate margins for cavitation exist under the worst-case conditions". Adjustments can be made by changing the flight profile to throttle down the RS-68s earlier and pressurising the oxygen tanks to a higher ullage pressure later in flight.
Source: Flight International