Zarya - Soviet, Russian and International Spaceflight
carousel image
Soyuz 4 and Soyuz 5

Tyneside, UK
2019 Jan 19
Saturday, Day 19

Maintained by:

Spacecraft Heading For Landing

Landing is the most critical stage of a space flight. How do spacecraft return to Earth?

As the spacecraft orbits the Earth, centrifugal force balances gravity. In orbit, a spacecraft develops vast energy. To provide for safe landing this energy must be absorbed. In principle, rocket thrust may be employed for this, but the most expedient way of absorbing the energy is by atmospheric drag. This is achieved as the re-entry compartment moves through dense atmosphere and induces the action of aerodynamic forces as a result of air resistance.

The landing process comprises two stages. Stage one involves departure from orbit. The second stage consists of re-entry into the atmosphere and soft landing. Before putting the spacecraft on the re-entry trajectory it is oriented in space at the appropriate time. At a strictly assigned moment the retro-rocket unit is switched on to decrease speed of the spacecraft. As a result, the force of gravity exceeds the centrifugal force and the spacecraft starts descending. During the first, extra-atmospheric stage the re-entry compartment is separated from the orbital compartment and service module which re-enter the atmosphere and burn up. The re-entry compartment is oriented by attitude control jets in such a way that it enters the atmosphere in a specific position.

All this is necessary for a safe passage through the atmosphere and precise landing in the assigned area. Passage through the atmosphere has to be prolonged so that acceleration stresses do not exceed the limits of human endurance. Special attention is paid to the direction of acceleration effects on a cosmonaut. For example, a cosmonaut can endure relatively short-time acceleration stresses in the direction "front-back" of about 10 g-units, but an acceleration of 4-5 g-units in the direction head-feet is much harder to bear.

In the course of the controlled re-entry of the Soyuz spacecraft the peak acceleration stresses are considerably lower than in a ballistic re-entry. This is due to the fact that in the course of re-entry aerodynamic forces come into play prolonging the process of landing and decreasing deceleration stresses.

Heat exchange is the most important re-entry problem. Even one-third of the energy of the moving re-entry compartment is enough, when turned to heat, to evaporate the structure, if it is not supplied with heat protection. In the course of descent of the re-entry compartment through the atmosphere a heavy incandescent shock wave is formed in front of it.

The temperature of the wave front reaches several thousands degrees. But the temperature inside the compartment must not exceed twenty-five degrees. This temperature is maintained by a heat exchange system.

It is extremely important for the spacecraft to land in the assigned area. Ballistic re-entry and landing may involve a considerable deviation from the assigned point of landing as a result of atmospheric disturbances affecting the spacecraft. Controlled descent with reliance on aerodynamic forces helps considerably to achieve precision landing.

By turning the spacecraft about the lateral axis (changing the bank angle) it is possible to adjust the lift and thus control movement of the spacecraft. The re-entry compartment of Soyuz is of a definite shape, and centre-balanced in such a way that during re-entry the angle between the direction of movement and the surface of the compartment is constant. This generates lift. Directed upwards, lift makes the trajectory less steep, increasing the length of the atmosphere flight, and reducing acceleration stresses. By selecting the required value of lift, by rotating the re-entry compartment about its lateral axis, it is possible to achieve a controlled length of re-entry flight at permissible acceleration stresses.

Controlled re-entry makes possible precise landing of the re-entry compartment in an assigned area, as well as minimising peak acceleration stresses affecting the crew.
Copyright © Robert Christy, all rights reserved
Reproduction in whole or in part without permission is prohibited