Fregat

Did You Know?

• The Fregat-SB upper stage used to launch the Spektr-R satellite in 2011 broke into multiple pieces on May 8, 2020, creating additional debris.
• As of 2018, adding a Fregat upper stage to a Soyuz-2 launch cost about a certain amount, though the exact figure is not specified.

Description

The Fregat is an upper stage that operates on its own, designed to carry heavy payloads to low, medium, and geosynchronous orbits, and can also push interplanetary probes like Venus Express and Mars Express out of Earth’s gravity. It was built by NPO Lavochkin in the 1990s. The stage has six round tanks arranged in a circle—four hold propellant, two hold avionics—with the main engine sitting in the middle. Eight struts run through the tanks to support the structure and transfer thrust to the rocket. Fregat handles its own guidance, navigation, attitude control, tracking, and telemetry, independent of the lower stages. Its design comes from the spacecraft bus used in the Soviet Phobos program of the late 1980s, which itself traces back to the lunar probes NPO Lavochkin developed in the 1960s. The stage burns hypergolic propellants—unsymmetrical dimethylhydrazine fuel and dinitrogen tetroxide oxidizer—that ignite on contact. Its S5.92 engine can fire up to 25 times, with seven ignitions proven in flight, enabling complex mission plans. Fregat offers both three-axis and spin stabilization for its payloads. It is the only upper stage that can put payloads into three or more separate orbits during a single launch.

Failures

On 22 August 2014, an Arianespace-operated flight of a Fregat MT ended in failure after the vehicle deposited two EU/ESA Galileo navigation satellites into the wrong orbit. Liftoff at 12:27:11 UTC from the Sinnamary launch site near Kourou, French Guiana, appeared normal, but the error became apparent after the second firing of the Fregat MT upper stage. An Independent Inquiry Board announced its definitive conclusions on 7 October 2014. 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 a loss of inertial reference for the stage. This loss occurred when the stage's inertial system operated outside its authorized envelope due to the failure of two attitude control thrusters during the preceding ballistic phase. The thruster failure was caused by a temporary interruption of hydrazine propellant supply due to freezing, which resulted from the proximity of hydrazine and cold helium feed lines connected by the same support structure acting as a thermal bridge. Ambiguities in design documents allowed such thermal bridges, which were also seen on other Fregat stages under production. The root cause was a shortcoming in the system thermal analysis performed during stage design, not an operator error during assembly.

Versions

The Fregat-M and Fregat-MT versions have ball-shaped additions on the tops of their tanks, increasing propellant load capability from 5350 kg to 6640 kg without changing physical dimensions. The Fregat-SB version, used with the Zenit-2SB launch vehicle, adds a torus-shaped drop-off tank block (SBB) weighing 360 kg and containing up to 3050 kg of propellant, raising maximum propellant capacity to 10150 kg. Fregat-SB was first launched on 20 January 2011, carrying the Elektro-L weather satellite into geosynchronous orbit.