Orbital Focus - International Spaceflight Facts and Figures
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Soyuz 4 and Soyuz 5

Tyneside, UK
2024 Jun 15
Saturday, Day 167

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Controlled Re-entry And Landing Of Soyuz

Two major re-entry trajectories for spacecraft returning from orbit are employed: ballistic (used by Vostok and Voskhod), and gliding.

Prior to retro-fire, a spacecraft is oriented in space so that the impulse of the retro-engine is directed at a calculated angle relative to the speed vector. This provides for re-entry at the required angle to the Earth's horizon. Once the spacecraft has been oriented, the on-board electronic programming device sends a command to switch on the retro-engine, which applies the retro-impulse of the assigned value and direction.

The retro-engine is cut off at a pre-determined point. At the point of departure from orbit, the speed of the spacecraft is altered by a pre-determined value, thus initiating the movement of the spacecraft along the elliptical re-entry path. Throughout the process the orientation of the spacecraft remains the same as during the firing of the retro engine.

Next, the re-entry compartment is separated from the spacecraft. At the same moment the controlled re-entry system is put into operation. The system stabilises the attitude of the re-entry compartment and ensures maintenance of its orientation. At this moment the re-entry compartment travels at zero bank angle, which provides for maximum lift-drag ratio.

After separation of the re-entry compartment comes its programmed manoeuvre by pitch prior to entry into dense atmosphere at the balanced angle of incidence. The program of manoeuvre is prepared on Earth in accordance with the nominal re-entry parameters. The manoeuvre is performed by means of on-board rocket engines. Then follows pre-programmed bank manoeuvre after which the re-entry compartment continues stabilised flight until re-entry proper.

If the retro-impulse is equal by value and direction to the pre-programmed one the re-entry compartment will follow the required descent trajectory. If the value or direction of the impulse is different from the pre-programmed one, or if the density of the atmosphere is not equal to the pre-calculated value, the re-entry compartment may descend along a trajectory above or below the selected re-entry path.

The re-entry control system comprises a special electronic computer. It is designed for controlling the bank angle and compensating for the difference between the actual and the pre-programmed re-entry values. This is achieved by measuring the acceleration and comparing it with a pre-programmed value.

If the acceleration effect does not begin at the expected time, the program is automatically corrected. For example, if deceleration begins two seconds after the pre-calculated time, it means that the spacecraft is re-entering along a trajectory passing above the expected one. In this case the computer determines the 'miss' value, makes corrections in the program, and sends the command for a bank manoeuvre of the re-entry compartment (increases the bank value). As a result, the lift of the re-entry compartment decreases and the compartment will proceed along a steeper trajectory, gradually approaching the pre-programmed re-entry path.

If, on the other hand, the expected deceleration starts early, it means that the re-entry compartment is descending along a lower trajectory. The command then is to turn it in the reverse direction so that the bank angle is reduced, increasing lift.

After landing the cosmonauts had high praise for the performance of all the on-board systems, which functioned properly throughout the flight. They were particularly satisfied with the performance of the re-entry control system, which ensured landing in an assigned area. The cosmonauts noted that re-entry stresses were moderate and were in fact easier to withstand than stresses generated during manoeuvres in aircraft.
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