ULA pinpoints cause of Atlas 5 premature shutdown

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In the heat of competition: ULA has completed its investigation into the premature shutdown of the Atlas 5 first stage during its March launch of Orbital ATK’s Cygnus capsule and has corrected the problem.

An extensive review of all post-flight data observations determined that at approximately T+222 seconds, an unexpected shift in fuel pressure differential across the RD-180 Mixture Ratio Control Valve (MRCV) and a reduction in fuel flow to the combustion chamber caused an oxidizer-rich mixture of propellants and a reduction in first stage performance. The imbalanced propellant consumption rate resulted in depletion of the first stage oxidizer with significant fuel remaining at booster engine shutdown. The engine supplier has implemented a minor change to the MRCV assembly to ensure the anomaly does not occur on future flights.

Engine hot-fire testing, extensive component and assembly level testing and analyses have been performed to confirm the findings. Detailed inspections and minor hardware replacement on all engines will support the current launch manifest.

They have now scheduled the next Atlas 5 launch for June 24.

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