The most critical and dramatic phase of a manned space flight is the return to Earth. For a Soyuz mission, it starts with the orientation of the spacecraft for the braking manoeuvre and ends approximately 90 minutes later with the landing of the descent module on the Kazakh steppe and the evacuation of the crew. This phase involves a sequence of twelve specific actions, the successful completion of which is vital for the safety of the crew. Indeed, to date, the worst accident in the history of the Soviet manned space programme - the death of Vladimir Komarov - occurred during the return from orbit.
The OKB-1 designers based the return operation on the presumption of excellent visibility in orbit for the orientation and braking manoeuvres, as well as on Earth for the landing. Traditionally, the in-space activities were done on the daylight part of the orbit so that the crew could confirm the orientation of their spacecraft relative to the illuminated horizon, and landing was timed to occur at dawn. Setting up for reentry is crucial, as even a small misalignment of the braking engine in relation to the direction of travel could result in the descent module missing the landing site by hundreds of kilometres. In addition, if the entry angle were too shallow, the descent module might only 'graze' the atmosphere and remain in an extremely low orbit which, although it would soon decay, would likely not do so before the crew ran out of air. The orientation and control system (SOUD) developed in Department No. 27 of the OKB-1 under the leadership of Boris Raushenbakh was used to orientate the spacecraft with its main braking engine facing in the direction of travel. Normally, the braking manoeuvre to initiate the descent trajectory occurs 25 minutes after the completion of the orientation manoeuvre, while travelling northeast at an altitude of about 250 km over the Gulf of Guinea towards the coast of Africa. The KTDU-35 had a single combustion chamber, and was designed by Isayev's OKB-2 bureau. It delivered a thrust of 417 kg, and could be fired up to 25 times for periods between one and several hundred seconds, accumulating a total time of at least 500 seconds. It was this engine that performed the manoeuvres of the rendezvous with Salyut. An almost identical engine with a thrust of 411 kg served as a backup for the braking manoeuvre. The propulsion module contained four tanks (two for fuel and two for oxidiser) containing approximately 900 kg of propellant.
At the onset of the braking manoeuvre the cosmonauts feel a gentle jolt, followed by uniform deceleration. Depending on the ballistics of the descent, the engine fires for between 145 and 194 seconds to reduce the speed from the 8 km/s required for orbit by 100-120 m/s to initiate the descent. In passing over the Mediterranean at an altitude in the range 110-150 (usually 130) km, the spacecraft adopts an orientation in which its longitudinal axis is more or less perpendicular to the direction of travel, with the orbital module 'on top' and the propulsion module 'beneath' so that when the three modules are separated, aerodynamic drag cannot cause a collision with the descent module. At the time of separation, less than ten minutes after the braking manoeuvre, explosives simultaneously jettison the orbital and propulsion modules and discard from the descent module all unnecessary elements such as its antennas and periscope. Only the descent module is equipped with the shielding required to survive the thermal stress of entry into the atmosphere; all the discarded items burn up. Owing to mass limitations and the relatively short time of its autonomous flight -about 30 minutes - the descent module is not equipped to issue telemetry. Instead, at all stages of the descent following separation the commander loudly calls out the progress of the automated sequence of operations and on conditions in the descent module, and this commentary is encoded in the form of Morse code and transmitted by a small VHF antenna on the outer part of the hatch at the top of the capsule - the
one which had provided access to the orbital module, and had thermal protection on its exterior. In addition, telemetry from various systems on board is recorded by the 'Mir-3' device, which has a duration of 76 minutes.1
In contrast to the spherical Vostok and Voskhod capsules, the descent module of the Soyuz is capable of controlling its path through the atmosphere. This phase of the descent starts over eastern Turkey, 16 minutes after the braking manoeuvre and about 6 minutes after separation. The module has six 10-kg thrusters positioned on its sides which draw their propellant from tanks located in the base, directly behind the couches. The flight control system fires these thrusters as necessary to maintain the broad base facing the direction of travel. In addition, because the module has an offset centre of mass to generate aerodynamic lift, the thrusters can roll the capsule to steer left or right and upward or downward so as to aim for a given landing point. Furthermore, an aerodynamic flight subjects the crew to a lesser g-load than does a ballistic path. The entire module is coated with an ablative material for protection against the heat of re-entry, but the base, which is subjected to the most extreme thermal stress, is covered by a thick shield of azbetextolite material. The maximum thermal and deceleration forces occur while over the Caspian Sea. The Kazbek-U couches enable the cosmonauts to return with their backs facing the direction of travel and in the optimal body-position to endure the deceleration.2 At this time, the module is sheathed by a hot plasma which, being opaque to radio waves, inhibits communication. The module bounces and shakes in response to the aerodynamic forces of its passage. It is a very noisy time. After the time of greatest thermal stress, the incandescence of the surrounding plasma fades to show blue sky. As the module continues to slow down, the strong vibrations cease and there is a welcome silence.
The parachute deployment begins at an altitude of about 9.5 km. First a cover is jettisoned to allow a small pilot chute to pull out a drogue chute with a canopy area of 14 square metres. This is designed to stabilise the module, and it is released after 17 seconds to initiate the deployment of the main chute at an altitude of about 7 km. This chute is stowed in an egg-shape container behind the heads of the crew that has a volume of just 0.27 cubic metres. It deploys in several stages to produce a canopy of 1,000 square metres by an altitude of 5 km. Small VHF and short-wave antennas on the shrouds transmit signals to the recovery helicopters. By 50 seconds after the start of the deployment of this chute the rate of descent ought to have been reduced to 6-8 m/s. If the rate of descent exceeds the maximum permissible value, the main chute will be jettisoned and the reserve chute of 570 square metres deployed. This is stowed in a separate container adjacent to that for the main chute, with a volume of 0.17 cubic metres. If used, the reserve chute will deploy at an altitude of 4.6 km and achieve the minimal landing speed of 10 m/s.
1 When Vladimir Komarov's capsule struck the ground at high speed, the 'black box' was destroyed by a combination of the shock and the subsequent fire. The design had been strengthened in order to survive a recurrence of such an event.
2 When the direction of the force is from the feet toward the head, the body is exposed to the maximum load. The optimal position is when the force acts at an angle of 10-15 degrees to the chest-to-backbone direction (known as 'breast-spin') because this minimises the component from the head to the feet.
Immediately after the braking manoeuvre, the Soyuz spacecraft separates into three modules to enable the descent module (in the middle) to re-enter the atmosphere on its own. (From the book Soyuz - A Universal Spacecraft, courtesy Rex Hall)
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