ULA’s Atlas 5 prior to the OA-6 mission. Photo: ULA

[Via Satellite 06-17-2016] United Launch Alliance (ULA) has traced the cause of the Atlas 5 rocket’s first stage underperformance during the March 22 OA-6 Cygnus mission to an anomalous propellant mixture ratio. This caused the first stage to shut down prematurely, though the mission, for Orbital ATK, was ultimately successful thanks to an extended burn by the rocket’s Centaur upper stage.

After the mission, ULA formed a review team consisting of its own engineers, members from the engine supplier and several government customers. The review team assessed all flight and operational data to determine direct and root causes and implemented the appropriate corrective actions for future flights.

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.

ULA performed engine hot-fire testing, extensive component and assembly level testing and analyses to confirm the findings. The company is implementing detailed inspections and minor hardware replacement on all engines to support its current launch manifest. ULA intends to complete all missions manifested for 2016 within the year. Its next mission is an Atlas 5 launch of MUOS 5, currently scheduled for June 24 from Space Launch Complex-41 at Cape Canaveral Air Force Station, Florida.