Phobos Anomaly

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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.

In Their Own Story

On 22 August 2014, an Arianespace-operated flight of a Fregat MT ended in failure after depositing two EU/ESA Galileo navigation satellites into the wrong orbit. The Independent Inquiry Board concluded the failure occurred about 35 minutes after liftoff, at the beginning of the ballistic phase preceding the second ignition. The orbital error resulted from an error in thrust orientation of the main engine during its second powered phase, caused by loss of inertial reference. This loss occurred when the stage's inertial system operated outside its authorized envelope due to failure of two attitude control thrusters during the preceding ballistic phase. The failure was caused by freezing of hydrazine, resulting from proximity of hydrazine and cold helium feed lines connected by the same support structure acting as a thermal bridge. Ambiguities in design documents allowed this installation, and similar bridges were seen on other Fregat stages under production. The root cause was a shortcoming in system thermal analysis during stage design, not operator error. In July 2017, a Russian-operated rideshare flight ended with 9 of 72 small satellites dead-on-orbit. In November 2017, a Russian-operated flight failed when the guidance computer on the Soyuz rocket's Fregat upper stage was mis-programmed, causing an unnecessary turn that left it in wrong orientation for a critical engine burn, depositing the stage, a Meteor MS-1 weather satellite, and 18 secondary cubesats back into Earth's atmosphere.

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.

Did You Know?

• The Fregat upper stage entered operational service in February 2000 and completed four missions that same year.
• Fregat has a success rate of 97.3%, including two failures and one partial failure.
• The Fregat-SB version used to launch Spektr-R in 2011 broke into multiple pieces on May 8, 2020, creating debris.
• Adding a Fregat upper stage to a Soyuz-2 launch cost about US$13.5 million as of 2018.
• The Fregat's design was largely based on the spacecraft bus from the Soviet Phobos program of the late 1980s.