Commercial launch services company International Launch Services (ILS) has blamed a production fault in the Proton M rocket's upper stage for the 6 August failure of a flight that stranded two satellites - Telkom 3 and Express MD 2 - in an incorrect orbit.

ILS's investigation agreed with the Russian State Commission's conclusion that the accident was caused by a fault in the Breeze M upper-stage fuel system which, in turn, was the result of a production fault.

The Proton M will be returned to flight status with the next launch set for 14 October. It will carry the Intelsat 23 commercial communications satellite.

After two successful firings, the third firing of the Breeze M upper stage shut down after 7s instead of the intended 18min 5s. The investigation found that pressure in the Breeze M's upper stage had fallen sharply following the vehicle's second engine burn, causing the rocket to spin out of control.

It further found that a small metallic hole inside a pressurisation line that was not manufactured according to specifications had caused the failure.

The faulty component was incorrectly built by the subcontractor, Polyot, of Omsk, Russia.

The manufacture of this component had been moved from the Proton M's main manufacturer, ILS parent company Khrunichev, to Polyot in 2011. The defect only occurred under certain pressure thresholds and was thus not found during component testing. All Breeze M upper stages for further planned launches are being checked for the fault.

Source: Flight International