Main Engine Cut Off

In the Soyuz/Fregat launch vehicle, the first three booster stages of the rocket and the Fregat upper stage have their two separate guidance systems controlled by their own gyroscopic platforms. The guidance reference axis used by the gyroscopes on the Soyuz and on the Fregat had a 10-degree difference. The geographical azimuth of previous Soyuz/Fregat launcher from Baikonur, Plesetsk and Kourou normally laid within a range from positive 140 to negative 140 degrees. To bring the gyroscopic guidance system into operational readiness, its main platform has to be rotated into a zero-degree position via a shortest possible route. The azimuth of the ill-fated Vostochny launch was 174 degrees, and with an additional 10 degrees for the Fregat's reference axis, it meant that its gyro platform had to turn 184 degrees in order to reach the required "zero" position.

In the Soyuz rocket, the gyro platform normally rotated from 174 degrees back to a zero position, providing the correct guidance. However on the Fregat, the shortest path for its platform to a zero-degree position was to increase its angle from 184 to 360 degrees. Essentially, the platform came to the same position, but this is not how the software in the main flight control computer on the Fregat interpreted the situation. Instead, the computer decided that the spacecraft had been 360 degrees off target and dutifully commanded its thrusters to fire to turn it around to the required zero-degree position. After a roughly 60-degree turn, the gyroscope system on the Fregat stalled, essentially leaving the vehicle without any ability to orient itself in space…