Fregat upper stage
Entering service at the turn of the 21st century, the Fregat (frigate) became the main fourth stage for the Soyuz family of rockets flying from four launch sites around the world. The Fregat was also adapted as a third stage of the Zenit rocket for some of its most critical missions.
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.
Formally, the development of the Fregat upper stage started around 1991 or 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.
The Fregat's exotic architecture traces its roots in the Soviet lunar probes developed at NPO Lavochkin in the 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 attitude control system, SOiZ, which has its own propellant cache of between 63 and 85 kilograms of hydrazine. The small engines of the SOiZ system have a thrust of five kilograms and specific impulse of 225 seconds.
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.
According to the Technical Project developed in the 1990s, the main propulsion system aboard Fregat was supposed to have an protective cover to address thermal requirements during a passive flight lasting from 1.5 hours up to 10 days. The electrically driven cover would swing 150 degrees to expose the main nozzle ahead of each maneuver. However, because Fregat ended up operating at the lower end of the projected temperature range in most launches during missions lasting a few hours, the developer made a decision to remove the thermal cover in 2002, after three launches. An extra package of batteries, known as KhIT (from Chemical Current Source) was removed as as well to save mass. However, the thermal cover was retained in the design of the Fregat derivative, known as the Main Propulsion System, MDU, developed for the Phobos-Grunt project.
Technical specifications of the S5.92 engine used on Fregat upper stage (340):
The successful operation of the Fregat stage spurred a number of its upgrades for the the Soyuz and other launch vehicles.
NPO Lavochkin proposed the Fregat-2 version of the upper stage equipped with a jettisonable external tank. When launched on a Zenit rocket, Fregat-2 could deliver 2,300 kilograms of payload into the 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, but not much was heard about the project after that.
At the turn of the 21st century, NPO Lavochkin considered the Flagman two-stage space tug, which would combine the 11S824M upper stage (Block-D) and Fregat, serving as the second stage. The combo was apparently intended for placing satellites directly into the geostationary orbit, GSO. The space tug could deliver 3,100 kilograms of cargo on the Proton rocket or 3,500 kilograms either on Proton-M flying from Baikonur or an Angara-5 rocket based in Plesetsk.
Fregat-M/Fregat-MT specifications (410):
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.
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.
A complete list of Fregat missions:
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
Project of the Flagman upper stage, which combines Block D and Fregat stages.
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
The S5.92 engine serves as the main propulsion unit of the Fregat upper stage. Click to enlarge. Copyright © 2008 Anatoly Zak
Soyuz-FG rocket launches European Mars Express orbiter on June 2, 2003. Credit: ESA
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
The Fregat-SB variant. Click to enlarge. Credit: NPO Lavochkin
A Separable Tank Block (Sbrasyvaemy Blok Bakov) SBB used with the Fregat-SB variant. Click to enlarge. Credit: NPO Lavochkin
A Fregat-SBU variant as of 2018. Credit: NPO Lavochkin