Fregat () is an upper stage developed by NPO Lavochkin for universal compatibility with a wide range of medium- and heavy-lift launch vehicles. Fregat has been used primarily with Soyuz and on a few occasions with Zenit rockets, and entered operational service in February 2000.
Fregat uses a liquid-propellant engine burning unsymmetrical dimethylhydrazine (UDMH) fuel and dinitrogen tetroxide () oxidizer, a pair of hypergolic propellants that ignite on contact. With a success rate of 97.3%, including two failures and one partial failure, Fregat is among the most reliable upper stages in operation. It has deployed more than 300 payloads into a variety of orbits and is capable of placing three or more spacecraft into distinct orbits during a single mission, owing to its ability to restart up to seven times and operate for a total burn duration of up to 1,350 seconds.
Description The Fregat upper stage is a versatile and autonomous vehicle designed to inject large payloads into a range of orbits, including low, medium, and geosynchronous. Additionally, it serves as an escape stage for sending space probes on interplanetary missions, such as the Venus Express and Mars Express.
Developed by NPO Lavochkin in the 1990s, the Fregat features six spherical tanks—four for propellants and two for avionics—arranged in a circle. Its main engine is centrally positioned, allowing for a compact design with a diameter larger than its height. Structural support is provided by eight struts passing through the tanks, which also transfer thrust loads to the launcher. Fregat operates independently from the lower stages of its launch vehicle, with its own guidance, navigation, attitude control, tracking, and telemetry systems.
The Fregat’s design was largely based on the spacecraft bus used in the Soviet Phobos program of the late 1980s, itself based on the architecture used for the Soviet lunar probes developed at NPO Lavochkin in the 1960s. Fregat also integrated several flight-proven subsystems and components from previous spacecraft and rockets. This approach ensured high reliability and accelerated development. Fregat was flight-qualified in February 2000 and successfully completed four missions that same year.
Currently used as the fourth stage on Soyuz launch vehicles, the Fregat’s S5.92 engine is capable of up to 25 ignitions, with seven demonstrated during flight. This allows it to execute complex mission profiles that would be impossible for the launch vehicle alone. The stage provides both three-axis and spin stabilization for spacecraft payloads. Fregat uses storable, hypergolic propellants—unsymmetrical dimethylhydrazine (UDMH) as fuel and dinitrogen tetroxide (, also called NTO or amyl) as the oxidizer, which ignite spontaneously upon contact. To date, Fregat has successfully deployed over 300 payloads into various orbits and remains the only upper stage capable of placing payloads into three or more distinct orbits in a single launch.
, adding a Fregat upper stage to a Soyuz-2 launch costs about .
Fregat upper stage launches VS missions are by Arianespace from the Guiana Space Centre.