A commission studying the 24 August loss of a Soyuz-U carrying supplies to the International Space Station (ISS) has isolated the faulty component, a gas generator on the third stage RD-0110 engine.
"Members of the emergency commission have determined the cause of the failure of the Soyuz carrier rocket's third stage engine. It is a malfunction in the engine's gas generator," a Roscosmos spokesperson was quoted as saying by Itar-Tass, a state-owned news agency.
The Soyuz-U and its payload, a Progress M-12M, launched from Baikonur spaceport in Kazakhstan. After 325sec flight time, shortly after the second stage separation and third stage firing, an abrupt drop in fuel pressure led the onboard computer to automatically cut the engine for safety reasons. The nature and root cause of the gas generator malfunction remain unexplained.
Roscosmos was unavailable for immediate comment.
The rocket and capsule came down over the Altai region of southern Siberia, where residents reported hearing a loud boom. No wreckage has been found in the heavily wooded area, though weather prevented search parties from operation for several days.
All Soyuz variants are grounded until the issue is isolated and solved; it is unclear when they will again be cleared to fly and rendezvous with the ISS.
There have been a confirmed total of nine failed Soyuz launches due to third stage failures, according to Flightglobal's Ascend SpaceTrak data, though none have been traced to the gas generator.
The Proton launch system, another mainstay of Russian spaceflight, has been cleared for launch after the 18 August failure that destroyed a Proton-M and its communications satellite payload. Clearance to fly was resumed after the cause of the loss was traced to flight control problems with the Briz-M upper stage.