from last week:
The complex failure scenario of the second Soyuz rocket launch from Vostochny continued emerging in the days following the accident. Although the culprit had quickly been pinned down by flight control specialists, even seasoned space engineers, who were not directly involved in the intricacies of guidance systems, struggled to fully comprehend the bizarre nature of the accident.
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 angle of a roll maneuver for rockets lifting off from Baikonur, Plesetsk and Kourou, which was required to guide them into a correct azimuth of ascent, normally laid within a range from positive 140 to negative 140 degrees. To bring the gyroscopic guidance system into a position matching the azimuth of the launch, its main platform has to be rotated into a zero-degree position via a shortest possible route. The ill-fated launch from Vostochny required a roll maneuver of around 174 degrees (which was apparently conducted from the 5th to 22nd second of the flight), and with an additional 10 degrees for the Fregat's reference axis, it meant that its gyro platform had to turn around 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 the nearly 60-degree turn at a rate of around one degree per second, the Fregat began a preprogrammed trajectory correction maneuver with its main engine. Unfortunately, the spacecraft was in a wrong attitude and, as a result, the engine was fired in a wrong direction.
Soyuz fails to deliver 19 satellites from Vostochny