Phobos Anomaly
A versatile upper stage for Soyuz and Zenit rockets, operational since 2000.
The Fregat upper stage, developed by NPO Lavochkin, entered operational service in February 2000. It is designed for universal compatibility with medium- and heavy-lift launch vehicles, primarily used with Soyuz and occasionally with Zenit rockets. Fregat uses a liquid-propellant engine burning unsymmetrical dimethylhydrazine (UDMH) fuel and dinitrogen tetroxide (N2O4) oxidizer, hypergolic propellants that ignite on contact. With a success rate of 97.3%, including two failures and one partial failure, it has deployed more than 300 payloads into various orbits. Fregat can restart up to seven times and operate for a total burn duration of up to 1,350 seconds, enabling it to place three or more spacecraft into distinct orbits during a single mission.
Lore & Background
The Fregat upper stage was developed by NPO Lavochkin in the 1990s, based largely on the spacecraft bus used in the Soviet Phobos program of the late 1980s, itself based on the architecture for Soviet lunar probes from the 1960s. It integrated several flight-proven subsystems and components from previous spacecraft and rockets, ensuring high reliability and accelerating development. Fregat was flight-qualified in February 2000 and successfully completed four missions that same year. It features six spherical tanks—four for propellants and two for avionics—arranged in a circle, with a centrally positioned main engine. Structural support is provided by eight struts passing through the tanks, which transfer thrust loads to the launcher. Fregat operates independently from lower stages, with its own guidance, navigation, attitude control, tracking, and telemetry systems. As of 2018, adding a Fregat upper stage to a Soyuz-2 launch costs about US$13.5 million.
Reader's Guide
The Fregat upper stage is currently used as the fourth stage on Soyuz launch vehicles. Its S5.92 engine is capable of up to 25 ignitions, with seven demonstrated during flight, allowing complex mission profiles. 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 (N2O4, also called NTO or amyl) as oxidizer. Versions include Fregat-M/Fregat-MT, which have ball-shaped additions on tank tops increasing propellant load from 5,350 kg to 6,640 kg without changing physical dimensions. Fregat-SB, used with Zenit-2SB, includes a block of drop-off tanks (SBB) weighing 360 kg containing up to 3,050 kg of propellant, with total dry weight 1,410 kg and maximum propellant capacity 10,150 kg. Fregat-SB launched first on 20 January 2011, lifting the Elektro-L weather satellite into geosynchronous orbit. Several Fregats have remained in orbit as space debris, as they did not have enough impulse capability to de-orbit themselves. The Fregat-SB used to launch Spektr-R in 2011 broke into multiple pieces on May 8, 2020, creating even more debris.
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