The instrument module of the Soyuz spacecraft, known by its Russian acronym as PAO, or Priborno-Agregatniy Otsek, in its turn is subdivided into three main sections:
Intermediate compartment, PkhO, or Perekhodnoi Otsek
Intermediate compartment, PKhO, is a truss-like structure, which is directly attached to the reentry capsule, SA, at 10 points. Five of the attachments are equipped with pyrotechnic locks and five with spring pushers, which ensure the separation of the PAO and SA before the reentry into the Earth atmosphere. The PKhO section also carries oxygen storage tanks and attitude control thrusters.
Instrumentation compartment, PO, or Priborniy Otsek
Instrumentation compartment, PO, is a drum-shaped pressure vessel, containing avionics, communications and control equipment. Among its avionics is primary guidance, navigation and flight control computers for the entire Soyuz spacecraft. A nitrogen gas is circulating through the section to remove heat generated by electronics.
Propulsion compartment, AO, Agregatniy Otsek
Propulsion compartment, AO, located at the very tail of the Soyuz, contains main engines, batteries, solar arrays and the main radiator with a total area of 8 square meters. The main engines inside AO are responsible for major orbit corrections and for the critical braking maneuver to reenter the atmosphere on the trip home. The engines, whose various versions were known as S5.35, S5.36, S5.37 and S5.62, burn nitrogen tetroxide and unsymmetrical dimetylhydrazin, also known as UDMH. The main engine and the backup S5.35.2000-0 engine, share their propellant tanks with smaller thrusters of the reaction control system, which provide precise orientation of the spacecraft in orbit, known as attitude control.
The Soyuz-T modification introduce the use of 11D726 engine and 11D428 thrusters.
Two wing-like, electricity-generating solar panels are attached to the opposite sides of the instrument module. Linked to rechargeable batteries, solar panels serve as the main source of power for the Soyuz during the autonomous flight. Made of four sections each, panels unfold only once, immediately after the separation of the spacecraft from the upper stage of the launch vehicle, upon reaching the orbit.
Instrument module also carries various antennas and flight control sensors on its exterior. The whole section separates as a single element from the crew capsule shortly after the braking maneuver and burns up in the atmosphere on the reentry.
Known specifications of the instrument module (Soyuz TM version):
The Soyuz TM-32 spacecraft photographed from the side of the instrument module by the crew of the International Space Station in October 2001. Credit: NASA
The Soyuz TMA-2 spacecraft docked to the International Space Station. Credit: NASA
The instrument module of the original Soyuz spacecraft. Copyright © 2000 Anatoly Zak