Second stage of the Soyuz rocket
All rockets in the Soyuz family inherited their first and second stages from the original Soviet ICBM -- R-7. The four boosters of the first stage are clustered around a single central booster identified as the second stage, even though it begins firing on the ground, simultaneously with the first stage. Thanks to its larger propellant reserve, the core booster operates much longer than the first stage.
The central "core" stage (Stage II) of the R-7 ICBM. Copyright © 2014 Anatoly Zak
One of a kind architecture
Like the first stage, the second stage (a.k.a. Block A, central block or core stage) of the Soyuz rocket, was borrowed from the R-7 ballistic missile mostly unchanged. It is propelled by a single RD-108 engine with four combustion chambers. The steering of the launch vehicle in flight after separation of the four strap-on boosters is achieved with four vernier thrusters surrounding the nozzles of the main engine. Right above the engine, the tail section also houses a thorus-shaped tank with liquid nitrogen and further up a larger tank with hydrogen peroxide. The nitrogen is used to pressurize the rocket's tanks, while the peroxide is needed for spinning the engine's turbopump.
The cylindrical fuel tank is located right above the propulsion (tail) section, followed by the oxidizer tank made of two conical sections. The intersection of the two conical sections of the oxidizer tank forms the main structural backbone of the core stage. The two main tanks are transported separately and bolted together at the launch site, as the core stage is too long for even the largest railway cars.
An avionics bay is located at the top of the oxidizer tank. It houses the flight control equipment responsible for the mission during the firing of the first and second stages. All the equipment can be accessed during pre-launch processing through a series of hatches.
To accommodate the third stage, the core booster is crowned with a lattice structure, featuring 12 connectors and six locks, which are designed to be cut with pyrotechnics to separate the second and third stage in flight.
The lattice structure makes it possible to ignite the engine of the upper stage moments before the separation of the core stage below it. A special cone-shaped, titanium-covered deflector above the instrument section is designed to facilitate the flow of hot exhaust gas from the third-stage engine, when it begins firing.
The central "core" stage (Stage II) of the Soyuz rocket during manufacturing in Samara.
Two sections of the core stage are bolted together after their delivery to the launch site.
Specifications of the second stage of the Soyuz rocket family*:
Assembly of the first and core stage of the Soyuz rocket. Click to enlarge. Copyright © 2000 Anatoly Zak
A close up view of the core-stage engine on the Soyuz rocket. Copyright © 2000 Anatoly Zak
A propulsion (tail) section of the core stage. Click to enlarge. Copyright © 2011 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, including an image of the second stage (center) falling away from the third stage. Credit: Arianespace