Cutaway diagram of Soyuz-Fregat launch vehicle (image courtesy of Starsem)

Soyuz was first launched in November 1963 and 1683 have been flown as of 13 October 2003. A manned version carries crews to the International Space Station, while unmanned versions are used to launch satellites, interplanetary spacecraft and Progress cargo vehicles. It is one of the most reliable launchers in the world, with a 98% success rate.

The Soyuz launch vehicle comprises a lower composite and upper composite. The lower composite is made up of four boosters (first stage), the central core (second stage) and the upper, third stage. The upper composite is made up of a fourth stage (Fregat) along with a payload adapter, a fairing, and the Venus Express spacecraft. The Fregat, payload adapter, and Venus Express spacecraft are all contained within the fairing.

The four side boosters and the core (second) stage ignite at the same time, shortly before liftoff. The boosters burn for slightly less than two minutes, then shut down and separate. After booster separation, the second stage continues to burn for about a further three minutes. Two seconds before second stage shutdown, the third stage ignites and then separates from the second stage. After the third stage burns out it then separates from the upper composite. All lower composite stages use liquid oxygen and kerosene as fuel.

The main engine on the fourth stage, the Fregat, is ignited twice. The first ignition moves the Fregat-Venus Express composite from the suborbital trajectory into which it is delivered by the Soyuz third stage into an almost circular parking orbit. The second injects Venus Express into its interplanetary flight trajectory. The Fregat uses unsymmetrical dimethyl hydrazine (UDMH) and nitrogen tetroxide as fuel.