On November 1st, 2018 a press-conference was held at the Mission Control Center of TsNIImash in Korolyov, northeast of Moscow, to illustrate the findings of the State Committee investigating the cause of the contingency occurred during Soyuz-FG LV/ Soyuz MS-10 launch in October.
The press-conference participants were: the Chair of the Investigation Committee/ TsNIImash Deputy Director General Mr. Oleg Skorobogatov, TsNIImash Acting Director General Mr. Nikolai Sevastyanov, Roscosmos Deputy Director General for Rocket Building, Space-Related Ground Infrastructure and Quality Assurance Mr. Alexander Lopatin, Energia Director General Mr. Sergey Romanov, Progress Director General Mr. Dmitry Baranov, TsNIImash Designer General of Launch Vehicles and Space-Related Ground Infrastructure/ Deputy Director General Mr. Alexander Medvedev, Deputy Head of the Russian Federal Bio-Medical Agency Mr. Vyacheslav Rogozhnikov, Director of the Institute of Bio-Medical Problems at RAS Academician Oleg Orlov, and Head of the Gagarin Cosmonaut Training Center Mr. Pavel Vlasov.
Oleg Skorobogatov reported that “The launcher failure was caused by the abnormal separation of one of the four lateral boosters that hit with its nose (fig. 1) the rocket central core causing the rupture of the fuel tank and the consequent loss of rocket attitude control.”
The Soyuz lateral boosters are supposed to jettison simultaneously at the end of their use and fall back to Earth The sequence is in three steps: first, the control system sends simultaneous commands to shut off the boosters engines and pyrotechnically cut the lower connections to the core stage (fig.2). The residual thrust and a disalignment between the booster axis and the thrust vector causes a small rotation and disengagement of the ball joint at the booster’s tip (fig.3). At that moment the lid of the nozzle on the liquid oxygen tank in the upper section of the booster opens causing venting of pressurizing gas, which pushes the booster away from the central core stage (fig. 4).
The abnormal separation was caused by the non-opening of the lid of the nozzle due to a deformed pin of the separation sensor (bended by 6˚45‘). The pin was deformed during the assembling of the boosters on the core stage in Baikonur due to a procedural mistake (fig.5). Roscosmos has developed corrective actions to avoid such mistake in future.
Source: http://kik-sssr.ru/IP_4_Turatam_old_Razdel_1.htm & Roskosmos PR