Fregat (Russian: Фрегат, lit. 'frigate') 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 (N₂O₄) 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.

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.^[2]

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.^[3] 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.^[4]

Currently used as the fourth stage on Soyuz launch vehicles, the Fregat’s S5.92 engine is capable of up to 25 ignitions,^[3] with seven demonstrated during flight. This allows it to execute complex mission profiles that would be impossible for the launch vehicle alone.^[5] The stage provides both three-axis and spin stabilization for spacecraft payloads.^[6] Fregat uses storable, hypergolic propellants—unsymmetrical dimethylhydrazine (UDMH) as fuel and dinitrogen tetroxide (N₂O₄, 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.^[7]

As of 2018^[update], adding a Fregat upper stage to a Soyuz-2 launch costs about US$13.5 million.^[8][9]

VS missions are by Arianespace from the Guiana Space Centre.

The Arianespace-operated flight of a Fregat MT ended in failure on 22 August 2014 after the vehicle deposited two EU/ESA Galileo navigation satellites into the wrong orbit. The lift off at 12:27:11 UTC from the Sinnamary launch site near Kourou, French Guiana, appeared to go well. However, a failure was only apparent later when, after the second firing of the Fregat MT upper stage had taken place, the satellites were detected as being in the wrong orbit.^[10]

The Independent Inquiry Board formed to analyze the causes of the "anomaly" announced its definitive conclusions on 7 October 2014 following a meeting at Arianespace headquarters in Évry, near Paris.^[11] The failure occurred during the flight of the Fregat fourth stage. It occurred about 35 minutes after liftoff, at the beginning of the ballistic phase preceding the second ignition of this stage. The scenario that led to an error in the orbital injection of the satellites was precisely reconstructed, as follows:

• The orbital error resulted from an error in the thrust orientation of the main engine on the Fregat stage during its second powered phase.
• This orientation error was the result of the loss of inertial reference for the stage.
• This loss occurred when the stage's inertial system operated outside its authorized operating envelope, an excursion that was caused by the failure of two of Fregat's attitude control thrusters during the preceding ballistic phase.
• This failure was due to a temporary interruption of the joint hydrazine propellant supply to these thrusters.
• The interruption in the flow was caused by freezing of the hydrazine.
• The freezing resulted from the proximity of hydrazine and cold helium feed lines, these lines being connected by the same support structure, which acted as a thermal bridge.
• Ambiguities in the design documents allowed the installation of this type of thermal "bridge" between the two lines. In fact, such bridges have also been seen on other Fregat stages now under production at NPO Lavochkin.
• The design ambiguity is the result of not taking into account the relevant thermal transfers during the thermal analyses of the stage system design.

The root cause of the failure of flight VS09 is therefore a shortcoming in the system thermal analysis performed during stage design, and not an operator error during stage assembly.^[12]

Since 22 August 2014, Soyuz ST-B launch vehicles with Fregat-MT upper stages have performed three successful launches, six Galileo navigation satellites have been inserted into their target orbits in frame of Soyuz at the Guiana Space Centre ongoing ESA programme.^[13][14][15]

In July 2017, a Russian-operated rideshare flight of a Fregat upper stage ended with 9 of 72 small satellites dead-on-orbit.^[16]

The Russian-operated flight of a Fregat upper stage ended in failure after the vehicle deposited the upper stage, a Meteor MS-1 weather satellite, and 18 secondary cubesats back into Earth's atmosphere due to the first Fregat burn being ignited with the stage in the wrong orientation.^[17] The guidance computer on the Soyuz rocket's Fregat upper stage was mis-programmed, causing it to begin an unnecessary turn that left it in the wrong orientation for a critical engine burn required to enter orbit.^[18]

The Fregats did not have enough impulse capability to de-orbit themselves after placing their payload into orbit and so several have remained in orbit as space debris.

The Fregat-SB upper stage rocket used to launch the Russian Spektr-R satellite into orbit in 2011, broke into multiple pieces on May 8, 2020 creating even more debris than normal.^[19]

Fregat-M/Fregat-MT tanks have ball-shaped additions on the tops of the tanks. These additions increase the load capability of the propellant from 5,350 kilograms (11,790 lb) to 6,640 kilograms (14,640 lb), without causing any other changes to the physical dimensions of the vehicle.^[20]

A version called Fregat-SB can be used with Zenit-2SB launch vehicle. This version is a variation of Fregat-M with a block of drop-off tanks ("SBB" or Сбрасываемый Блок Баков in Russian) which makes increased payload capability possible. The torus-shaped SBB weighs 360 kg (790 lb) and contains up to 3,050 kg (6,720 lb) of propellant. The total dry weight of the Fregat-SB (including SBB) is 1,410 kg (3,110 lb) and the maximum propellant carrying capacity is 10,150 kg (22,380 lb).^[21]

Fregat-SB was launched for the first time on 20 January 2011, when it lifted the Elektro-L weather satellite into geosynchronous orbit.^[22]

• 👁 Image
  Spaceflight portal

• Fregat page on the manufacturer site
• Vestnik NPO Im. S.A.Lavochkina, issue 1, 2014 - special issue dedicated to Fregat