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Descent module

The descent module (Russian: Спуска́емый Аппара́т, romanized: spuskáyemy apparát), also known as a reentry capsule, is used for launch and the journey back to Earth. Half of the descent module is covered by a heat-resistant covering to protect it during reentry; this half faces forward during reentry. It is slowed initially by the atmosphere, then by a braking parachute, followed by the main parachute, which slows the craft for landing. At one meter above the ground, solid-fuel braking engines mounted behind the heat shield are fired to give a soft landing. One of the design requirements for the descent module was for it to have the highest possible volumetric efficiency (internal volume divided by hull area). The best shape for this is a sphere – as the pioneering Vostok spacecraft's descent module used – but such a shape can provide no lift, resulting in a purely ballistic reentry. Ballistic reentries are hard on the occupants due to high deceleration and cannot be steered beyond their initial deorbit burn. Thus it was decided to go with the "headlight" shape that the Soyuz uses – a hemispherical upper area joined by a barely angled (seven degrees) conical section to a classic spherical section heat shield. This shape allows a small amount of lift to be generated due to the unequal weight distribution. The nickname was thought up at a time when nearly every headlight was circular. The small dimensions of the descent module led to it having only two-man crews after the death of the Soyuz 11 crew. The later Soyuz-T spacecraft solved this issue. Internal volume of Soyuz SA is 4 m (140 cu ft); 2.5 m (88 cu ft) is usable for crew (living space).

The thermal protection system on the slightly conical side walls is stood off from the structure to also provide micrometeoroid protection in orbit. The slightly curved heat shield on the bottom consists of "21mm to 28mm thick ablator (glass-phenolic composite) whichis held by brackets approximately 15mm from the 3.5mm thick aluminum AMg-6 substrate. VIM low-density silica fibrous insulation (8mm thick) is contained in the gap between the heat shield ablator and aluminum substrate."