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The Russian investigation into the December 1 launch failure of a Progress freighter has come up with two possible causes, both centered on the third stage of the Soyuz rocket.
According to one theory some unforeseen dynamic loads on the tank’s structure were exacerbated by low-quality welding of the tank and led to its rupture.
Another hypothesis presumed that an anomalous operation of the RD-011o engine, (as a result of higher-than-normal vibrations in its bearings or in its turbopump), could apply unforeseen loads onto the aft bulkhead of the oxidizer tank, which is located right above the engine.
Both scenarios were based on theoretical assumptions and could not be proven without doubt due to lack of telemetry.
It appears that they might be delaying any further manned launches while they evaluate the third stages of rockets intended for future flights.