The Soyuz-Fregat. Photo: Starsem

The Soyuz rocket was first flown in November 1963 and has since flown more than 1,500 times. A manned version carries crews to space stations, while an unmanned version is used to launch satellites and Progress cargo craft.

The Soyuz usually comprises three stages. The first stage has four conical boosters grouped around the central core stage; the second stage is a cylindrical core and the third stage also has the payload adapter and fairing.

The four side boosters and the core (first) stage ignite at the same time before lift off but the lateral boosters shut down and separate first. After the first stage has used all its fuel and separates, the second stage continues to burn. All three stages use liquid oxygen and kerosene fuel.

For the Cluster 2 launches, a fourth stage, known as Fregat, will be added. Its main engine will be ignited twice so that the stage changes position and the satellites end up in the correct orbit around the Earth. The Fregat uses UDMH and nitrogen tetroxide as fuel.

A number of modifications must be made to the Soyuz for the Cluster 2 launch. The most notable change is the use of a larger than normal (3.7m diameter) Soyuz fairing to protect the spacecraft. The third stage will also carry a flight control system which is slightly modified to match the larger Fregat fairing.