Fregat

Fregat (Russian: Фрегат, frigate) is an upper stage developed by NPO Lavochkin in the 1990s, which is used in some Soyuz and Zenit rockets. Its liquid propellant engine uses UDMH and N₂O₄.

Capabilities

The stage can be restarted up to 25 times, which has been used to place payloads such as the GIOVE-B satellite into their planned orbits.

Uses

Fregat stages are currently used as the fourth stage of some Soyuz launch vehicles.

NPO Lavochkin has built many interplanetary probes, and the Fregat stage follows their design traditions. The main part of the stage is six intersecting spheres placed on a single plane, four of which contain propellants. The remaining two contain the control equipment. The main engine is placed between the spheres, so Fregat is a tightly-packed stage with a diameter much larger than its height.

Fregat is a versatile spacecraft: for example, in addition to orbital insertion, it can be used as an escape stage to send modern space probes into interplanetary trajectories (e.g. Venus Express and Mars Express).