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Galileo FOC Anomaly Traced to Design Ambiguity, Says Inquiry Board

October 8, 2014  - By
Image: GPS World

The root cause of the anomaly that sent two Galileo satellites into the wrong orbit is a shortcoming in the system thermal analysis performed during stage design, and not an operator error during stage assembly, according to findings by an independent inquiry board.

The Independent Inquiry Board was formed to analyze the causes of the anomaly occurring during the orbital injection of satellites in the Galileo constellation by a Soyuz rocket launched from the Guiana Space Center on August 22. The board announced its definitive conclusions on Tuesday following a meeting at Arianespace headquarters in Evry, near Paris.

The board was created on August 25 by Arianespace, in conjunction with the European Space Agency and the European Commission. It is chaired by Peter Dubock, former inspector-general of ESA. Its conclusions draw on data supplied by Russian partners in the program, and are consistent with the final conclusions of the inquiry board appointed by the Russian space agency Roscosmos.

The anomaly occurred during the flight of the launcher’s fourth stage, Fregat, designed and produced by NPO Lavochkin. It occurred about 35 minutes after liftoff, at the beginning of the ballistic phase preceding the second ignition of this stage.

The board’s conclusions confirm that the first part of the mission proceeded nominally, which means that the three-stage Soyuz launcher was not at fault. The Inquiry Board also eliminated the hypothesis that the anomaly could have been caused by the abnormal behavior of the Galileo satellites.

The scenario that led to an anomaly in the orbital injection of the satellites was precisely reconstructed, as follows:

  • The orbital error resulted from an error in the thrust orientation of the main engine on the Fregat stage during its second powered phase.
  • This orientation error was the result of the loss of inertial reference for the stage.
  • This loss occurred when the stage’s inertial system operated outside its authorized operating envelope, an excursion that was caused by the failure of two of Fregat’s attitude control thrusters during the preceding ballistic phase.
  • This failure was due to a temporary interruption of the joint hydrazine propellant supply to these thrusters. The interruption in the flow was caused by freezing of the hydrazine.
  • The freezing resulted from the proximity of hydrazine and cold helium feed lines, these lines being connected by the same support structure, which acted as a thermal bridge.
  • Ambiguities in the design documents allowed the installation of this type of thermal “bridge” between the two lines. In fact, such bridges have also been seen on other Fregat stages now under production at NPO Lavochkin.
  • The design ambiguity is the result of not taking into account the relevant thermal transfers during the thermal analyses of the stage system design.

The system thermal analyses have been reexamined in depth to identify all areas concerned by this issue. Given this identified and perfectly understood design fault, the board has chosen the following corrective actions for the return to flight.

  • Revamp of the system thermal analysis.
  • Associated corrections in the design documents.
  • Modification of the documents for the manufacture, assembly, integration and inspection procedures of the supply lines.

Arianespace said these measures can easily and immediately be applied by NPO Lavochkin to the stages already produced, meaning that the Soyuz launcher could be available for its next mission from the Guiana Space Center as from December 2014.

Beyond theses corrective actions, sufficient for return to flight, NPO Lavotchkin will provide Arianespace with all useful information regarding Fregat’s design robustness, which is proven by 45 successful consecutive missions before this anomaly.

Following the announcement of the Independent Inquiry Board’s conclusions, Stéphane Israël, chairman and CEO of Arianespace, said: “I would first like to thank Peter Dubock,  who chaired the board. Their work, with the support of Russian partners, enabled the rapid identification of the root cause of the anomaly and the corrective measures to be applied. Since the corrective measures are easy to deploy by NPO Lavochkin, we are looking at the resumption of Soyuz launches from the Guiana Space Center, as early as December 2014. The resolution of this anomaly will enable a consolidation of the reliability of Fregat, which had experienced 45 consecutive successes until this mission.”

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