Third stage of the Soyuz rocket

The third stage of the Soyuz rocket (also known as Block I) has a job of delivering the payload into an initial Earth orbit or on a ballistic path just short of orbital velocity. Block I replaced less powerful Block E stage, which performed the same function as part of the Vostok launcher. The upgraded third stage lifted off from the ground for the first time on Oct. 10, 1960, as part of the Molniya rocket. (120)

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Design variations in the third stage for Soyuz-U/FG and Soyuz-2-1a and 2-1b rockets. Credit: Starsem

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Beginning from the top, the stage consists of a transfer ring, fuel (kerosene) tank, oxidizer (liquid oxygen) tank, aft section (KxO) and the RD-0110 engine. Vapors of liquid oxygen coming out from a special gas generator in the engine are used to pressurize the oxidizer tank. Generator gas channeled through a heat exchanger then used to pressurize the fuel tank.

Thanks to an open lattice structure connecting the second and the third stage, the RD-0110 engine can start firing before the separation of two rocket elements 282.2 seconds after the liftoff. Pyrotechnic devices then cut ties between the second and third stage. In a typical mission, the third stage is firing until 814.2 seconds in flight, when RD-0110 shuts down and the rocket's payload is released into low Earth orbit. At T+815.2 seconds in flight, braking solid motors on the third stage fire and a special pressurization valve on the forward fuel tank opens, as the stage trails behind its payload. (512)

Stage III

Stage III of the Soyuz rocket attached to its payload.

Along with the rest of the Soyuz rocket, the third stage is manufactured at the Progress plant in the city of Samara.

As of 1996, a total of three versions of Block I had been developed for various missions. They differ primarily by their transfer rings, which provide interface with the payload and by some pneumatic and hydraulic hardware. The cylindrical forward transfer ring consists of two end stringers, two intermediate stringers, several ribs and external skin made of D-16T alloy. A forward (top) ring carries the payload fairing and pushers for the spacecraft separation system.

The top kerosene tank made of two hemispheres welded together and two cylindrical skirts on both sides. Inside, these skirts are strengthened with stringers and rings. The tank also contains a fuel level sensor for the synchronized consumption of the propellant, a sensor of full loading, a fueling pipe and a pressurization pipe. An oxidizer tank below has a similar design, however it has additional cylindrical insert between two hemispherical bulkheads. Also, due to cryogenic nature of the liquid oxygen, bulkheads are covered with fiberglass and capron thermal insulation blankets. The cylindrical transfer section between two tanks is used to attach flight control avionics of the stage. Six hatches on this section enable access to the avionics and one hatch is used for the installation of the pressurization valve on the oxidizer tank. A conical skirt designed to hold the engine is welded to the lower ring of the tank.

Five spherical tanks with a diameter of 500 millimeters and a capacity of 56 liters are installed inside the stage.

The aft section of the Block I connects the stage to the truss of the second stage (Block A). The bottom of the section hold thermal protection of the propulsion system. After the third stage separates from the second stage, the aft section also separates from the third stage and splits into three segments. To achieve that, the aft section is equipped with six pyrotechnic locks attaching it to the third stage and three spring pushers used for the separation from the third stage and three pairs of spring pushers, which are used to separate three individual segments of the aft section from each other. (264)


Specifications of the third stage (Block I) of the Soyuz rocket family:

Soyuz FG: 6.745 meters; other variants: 7.94 - 9.4 meters (512)
Maximum body diameter
2.66 meters (512)
Liftoff mass
Soyuz FG: 25.3 tons; Soyuz-ST-B: 27.8 tons**
Total propellant mass
22.5 tons (512); 22.8 tons (Soyuz-U)
Oxidizer (liquid oxygen) mass (Soyuz-U)
15.7 tons
Fuel (kerosene) mass (Soyuz-U)
7.1 tons
Dry mass
2.490 (Soyuz-2 for Vostochny) - 2.710 kilograms (512)
Total thrust
29.8 tons*
Burn time
approximately 240 seconds
First launch
1960 Oct. 10


Page author: Anatoly Zak

Last update: June 9, 2022

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A third stage of the manned version of the Soyuz rocket seconds before launch. A white frost is clearly visible around the area of the cryogenic liquid oxygen tank in the center of the photo. Copyright © 2001 Anatoly Zak


A third stage of the Soyuz rocket sans its aft section during the assembly. Copyright © 2001 Anatoly Zak


A fully assembled third stage of the Soyuz rocket. Copyright © 2001 Anatoly Zak


The aft section of the third stage of the Soyuz rocket. Copyright © 2000 Anatoly Zak

stage 3

A front side of the third stage of the Soyuz rocket with a propellant tank clearly visible. Copyright © 2001 Anatoly Zak


In April 2014, during the launch of the Sentinel-1A satellite from French Guiana, the Soyuz-2 rocket carried two pairs of cameras, which provided first ever close-up view of the launch and stage separation for this historic rocket. Credit: Arianespace


An external KL-152M camera transmitted first dramatic images during the Soyuz spacecraft launch of the third stage separation on June 6, 2018. Credit: NASA




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