Site map

Site update log

About this site

About the author



Searching for details:

The author of this page will appreciate comments, corrections and imagery related to the subject. Please contact Anatoly Zak.

Related pages:

Soyuz-2 rocket







Fregat upper stage

The high-performance Fregat space tug originated as a propulsion stage for the Soviet Mars probes. It was later adapted for many deep-space missions, thanks to its ability to fire its engine multiple times and function in space for prolonged periods.

Technical specifications of the Fregat upper stage (118; 207):

Fueled mass
6,415-6,535 kilograms (Fregat-MT: 7.5 tons)
Dry mass
980-1,100 kilograms
Height 1,500 millimeters
Diameter 3,350 millimeters
Fuel UDMH (Unsymmetrical dimethyl hydrazine)
Oxidizer NTO (Nitrogen tetroxide)
Maximum propellant load for main engine 5,350 kilograms
Main engine S5.92
Thrust 2,000 kilograms
Specific impulse 327-328.5 seconds
Number of engine firings up to 20
Propellant load (one component: hydrazine) for the attitude control system, RSU 85 kilograms (in 2012 reported as 63 kilograms)
Number of attitude control thrusters 12
Thrust of attitude control engines 5 kilograms
Specific impulse of the attitude control system 225 seconds

Formally, the development of the Fregat upper stage started in 1991-1992 in conjunction with the Rus project, envisioning a series of upgrades of the Soyuz rocket family. The Fregat was expected to serve as a fourth (upper) stage for the Soyuz-2 rocket, delivering payloads from low "parking" orbits into various high-altitude orbits or sending planetary probes into deep space. Potentially, Fregat could be adapted to serve as the third stage of the Soyuz rocket, instead of Block I stage, to form the Vostok-2/Fregat booster. In such configuration, the rocket could launch cargo into low- and mid-altitude orbits, as well as into sun-synchronous orbits.

Finally, Fregat could become the third stage of the Zenit rocket, or the fifth stage of the Proton M/Block D combination. When combined with Block D onboard the Proton, Fregat could deliver 3,500 kilograms of payload into geostationary orbit, compared to 2,600 kilograms without it.

NPO Lavochkin claimed that Fregat's performance characteristics would exceed those of any contemporary vehicle. The stage inherited many of its components from previous Soviet hardware, reducing the development cost and increasing the overall reliability of the system.

Design features

The Fregat's exotic architecture traces its roots in the Soviet lunar probes developed at NPO Lavochkin in 1960s. The stage is made up of six overlapping spherical sections, only four of which serve as propellant tanks. Two remaining spheres are actually instrument sections, one containing unpressurized avionics, which can function in vacuum of space and another, pressurized, section holding a flight control computer, which requires a temperature-controlled environment.

At least four spherical gas tanks with a diameter of 375 millimeters and a capacity of 23 liters are carried onboard Fregat.

The main propulsion unit of the Fregat upper stage consists of a single S5.92 engine. Capable of multiple firings and dual-thrust mode, it uses a turbo-pump and operates in a gas-generator cycle. The stage has also a single-component orientation system powered by 63 kilograms of hydrazine.

The "flat" architecture of Fregat allowes to minimize its mass and the inertia to be overcome during engine firings. As a result, developers can place the main engine of the ADU on special rail guides instead of a gimbal mechanism in order to steer the thrust of the propulsion system.

Technical specifications of the S5.92 engine used on Fregat upper stage (340):

Thrust in vacuum
1,400 kilograms and 2,000 kilograms
Specific Impulse
323 seconds and 329 seconds
Propellant component ratio
2.05 and 1.95
Thrust development time (to 90 percent from nominal)
2.5 - 3.0 seconds
Duration of firing
1,350 seconds
Restart capability
25 times
Engine gimbal capability (yaw and pitch)
±300 millimeters
Engine dry mass
75 kilograms
Engine dimensions
677 x 838 x 1,028 millimeters
Life span in orbit
Three years

Fregat modifications

Fregat SB

NPO Lavochkin also proposed the Fregat-2 version of the upper stage, equipped with a jettisonable external tank. If lifted by the Zenit rocket, the Fregat-2 could deliver 2,300 kilograms of payload into geostationary orbit, while in combination with the Proton M rocket, 4,000 kilograms could be inserted into the same orbit. (118) The vehicle was later renamed Fregat-SB, where SB stands for "sbrasyvaemye baki" or "jettisonable tanks," and its first mission was scheduled to be the launch of the Spektr-R scientific satellite onboard a Zenit-3M rocket. Routine preflight processing of the vehicle was planned at the assembly building at Site 31 in Baikonur.


Yet another unique modification of the Fregat upper stage was developed specifically for the Phobos-Grunt mission. It was dubbed Flagman and it also featured a jettisonable external tank, as well as additional ball-shaped inserts on the upper hemispheres of the propellant tanks. Similar tank extensions were added to the particular versions of Fregat meant to fly on the Soyuz-ST rocket from Kourou, such as those intended to carry Europe's Galileo navigation satellites. This version of the stage was identified as Fregat-MT. According to unofficial sources, propellant tank inserts enabled to increase the propellant load from 5,350 kilograms to 6,640 kilograms, without any changes in the critical physical dimensions of the vehicle. In the meantime, Arianespace officials confirmed that some preliminary consideration had been given to equipping Europe's flagship Ariane-5 rocket with Fregat in order to fly trajectories, which would require multiple firings of an upper stage in space.


During an expanded session of the Scientific and Technical Council, NTS, on May 29, 2018, Sergei Ishin, Deputy Designer General, presented plans for the Fregat-SBU variant, which would be adapted for the Soyuz-5 and Angara-3 launch vehicles. According to NPO Lavochkin, the council approved the development of the Fregat-SBU variant.

Fregat-M/Fregat-MT specifications (410):

Total (dry) mass
920 kilograms
1,030 kilograms
Maximum propellant load
5,250 kilograms
6,550 kilograms
1.55 meters
Maximum diameter
3.35 meters
3.38 meters
Engine thrust (high-thrust mode)
2,000 kilograms
Engine thrust (low-thrust mode)
1,400 kilograms
Specific impulse
333.2 seconds
Number of possible engine firings

In-flight problems

In 2012, posters in the Novosti Kosmonavtiki web forum reported that during the mission to deliver the COROT space observatory, the Fregat upper stage had experienced problems. As it transpired, a valve tasked to reduce pressurization of propellant tanks from 320 bars to 38 bars had leaked. As a result, the pressurization system worked below specifications during the mission, failing to provide needed pressure into the propellant tanks during the Fregat maneuvers. Fortunately, it was still enough to deliver propellant for all firings of the main engine and keep the vehicle on the right trajectory. A similar problem took place onboard Fregat during the launch of the Globalstar satellites on May 30, 2007.

The investigation later concluded that specific design of the valve and the loss of flexibility in its membrane had been likely culprits. One source claimed that KBKhM design bureau, which built the Fregat's propulsion system possible used a new materials, later reverted back to previous design, thus solving the problem.

Production rates

In 2011, NPO Lavochkin built 12 Fregat stages, 11 of which were expended during missions. The production of 10 more stages was ordered during 2012, while nine stages were scheduled to fly, including three on the Soyuz rockets from French Guiana. NPO Lavochkin reconfirmed the annual production rate of 12 Fregat stages again in May 2018.

Writting, photography and illustrations by Anatoly Zak; Last update: June 18, 2018

Page editor: Alain Chabot; Last edit: March 7, 2011

All rights reserved

Fregat upper stages at NPO Lavochkin's testing and checkout station, KIS. The development and test version is on the foreground, a demo version is on the background. Click to enlarge Copyright © 2001 Anatoly Zak

An original concept of the Fregat-2 upper stage with a jettisonable external tank, which was evaluated around 2001. One proposed version of Fregat would reportedly reach 17 tons. Credit: NPO Lavochkin

Upgrading Soyuz with the off-the-shelf Fregat upper stage would enable it not simply "loop" around the Moon, but enter orbit around the Earth's natural satellite. The Soyuz/Fregat combination could be launched by an upgraded version of the Soyuz rocket. Copyright © 2007 Anatoly Zak

A view of an open avionics section on the Fregat upper stage. Click to enlarge. Copyright © 2001 Anatoly Zak

S5.92 engine

The S5.92 engine serves as the main propulsion unit of the Fregat upper stage. Click to enlarge. Copyright © 2008 Anatoly Zak

Flagman upper stage

The Flagman cruise stage of the Phobos-Grunt spacecraft would closely resemble the Fregat orbital tug, minus its flight control system. Apparent changes would include ball-shaped tank extensions, enabling larger propellant loads. Click to enlarge. Copyright © 2009 Anatoly Zak