Soyuz’ Fregat upper stage is an autonomous and flexible upper stage designed to operate as an orbital vehicle. Flight qualified in 2000, it extends the Soyuz launcher’s capability to provide access to a full range of orbits (medium-Earth orbit, Sun-synchronous orbit, geostationary transfer orbit, and Earth escape trajectories).
To ensure high reliability for the Fregat stage right from the outset, various flight-proven subsystems and components from previous spacecraft and rockets are used. The upper stage consists of six spherical tanks (four for propellants, two for avionics) arrayed in a circle and welded together. A set of eight struts through the tanks provide an attachment point for the payload, and also transfer thrust loads to the launcher. The upper stage is independent from the lower three stages, since Fregat has its own guidance, navigation, attitude control, tracking, and telemetry systems.
The Fregat uses storable propellants (UDMH/NTO) and can be restarted up to 20 times in flight – enabling it to carry out complex mission profiles. It can provide three-axis or spin stabilization of the spacecraft payload.