Soyuz' Habitation Module, BO
The spherical habitation module, BO, was originally designed to provide extra convenience for the crew during missions, which could last as long as a week.
Inside the early version of the habitation module onboard the Soyuz spacecraft. The docking port is at the top, the transfer hatch into the descent module is at the bottom of the photo. The main equipment section nick-named "servant" (cupboard) is on the left and the smaller section known as "divan" (coach) is on the right.
The habitation module allowed to minimize the size and mass of the crew module, SA, which had to be covered with heavy thermal protection layers to protect it during the fiery descent in the Earth atmosphere. The thin-walled habitation module essentially offered the crew a second room for both work and rest. In addition, by merely being a forward docking section of the spacecraft, the habitation module provided additional safe clearance for the crew during rendezvous with other spacecraft, such as lunar modules or space stations. When docked to another spacecraft, the habitation module would serve as a hallway for crew transfer. (52)
In most cases, but not always, the docking hardware and the rendezvous antennas would be located in the front of the habitation module. Beginning with the mission of Soyuz-10, the habitation module was equipped with the docking and internal transfer system known by the Russian abbreviation SSVP - Sistema Stykovki i Vnutrennego Perekhoda. (429) SSVP-type docking ports have a hatch, enabling transfer between spacecraft in a short-sleeve environment.
The opposite end of the habitation module is always connected to the reentry capsule with a transfer hatch. Finally, a third hatch on the side of the habitation module, enables this compartment to serve as an airlock, in case the crew needs to get outside the spacecraft in orbit for a spacewalk (Extra-Vehicular Activity, EVA). In such cases, spacewalking cosmonauts dressed in spacesuits can let the air out of the habitation module, while still keeping the reentry capsule of the Soyuz pressurized. Spacewalks from the habitation module are still possible, when the Soyuz is docked to another spacecraft. The only condition is closing the docking port hatch between the Soyuz and the vehicle it is docked to. The same EVA hatch also serves as entry point for the crew members, when they board the ship on the launch pad.
A majority of the internal hardware of the habitation module is grouped into two sections located on both sides of the module, thus leaving the middle of the compartment as a free space for the crew. These sections became known in Russian jargon as "divan" (coach) and "servant" (cupboard) after two most common furniture pieces in Soviet-era living rooms.
The habitation module also contains a toilet, docking avionics and communications gear. In several "solo" missions in the 1970s, (not involving rendezvous with other spacecraft), the docking hardware in the habitation module was replaced with custom-built payload sections.
Structurally, the habitation module is made of two semispherical magnesium sections with a 0.3-meter cylindrical segment between them. Originally, the section had two windows. During the evolution of the spacecraft, a special blister was introduced, providing a forward view in the course of rendezvous and docking.
Originally, the habitation module was designed to separate from the descent module, SA, prior to the reentry maneuver at the end of a mission. However in the wake of a nearly disastrous incident in 1988, the habitation module would only be jettisoned after the braking maneuver, despite the extra propellant required to push the whole spacecraft off the orbit. Otherwise, any major failure of the braking system would strand the crew inside the cramped descent module with no access to toilet and other critical life-support systems.
During the return trip of the Soyuz spacecraft from the station, bags of trash are routinely loaded into the habitation module to be disposed of along with it. The habitation module burns up during reentry into the atmosphere, as does the instrument module, PAO.
One critical sub-system providing thermal control inside the three pressurized compartments of the Soyuz crew vehicle is ventilation hardware. It is designed to operate not only in space, but also during pre-launch processing of the spacecraft, on the launch pad and after the completion of the flight on land or at sea.
Known specifications of the Habitation Module (Soyuz TM version):