How people return from space. Everything you need to know about life on board the ISS

At first glance, it seems that the descent of a spacecraft to Earth should be much simpler than its ascent. Everyone knows well: it’s hard to go uphill, but it’s easier downhill. Unfortunately, this simple and obvious truth turns out to be not entirely true when dealing with the descent from the “space mountain”. We considered the design of a manned spacecraft suitable for long-term flights in outer space. It consists of two main parts: the orbital compartment and the so-called descent module (also called the reentry vehicle). In addition, the ship has a braking engine, a solar battery and a number of other systems. All these components of the ship are delivered into outer space from Earth. But not the entire ship returns to Earth, but only a small part of it, the one called the descent module.

Before starting the descent to Earth, all members of the spacecraft crew move into the descent module. It also houses the equipment necessary to support the life of the crew, as well as observation materials carried out by the crew in accordance with the flight plan. The remaining parts of the ship undock from the descent vehicle at the appropriate moment and after some time fall to Earth. The expression “fall to Earth” is not entirely accurate. The parts of the spacecraft that "fall to Earth" do not reach the Earth's surface. Passing through dense layers of air, they heat up and burn, just as iron and stone meteorites that enter the Earth's atmosphere burn up.

Man has visited not only near-Earth space, at a distance of 200 - 300 km from the Earth's surface, but also in the so-called deep space. The conditions for the descent to Earth of spacecraft returning from deep and near space are not the same. Being in outer space near the Earth, the ship moves at a speed = 8 km/sec, i.e. it has the first escape velocity. At such a speed of movement around the globe, at altitudes where there is no or almost no atmosphere, the ship can remain for a very long time without moving away from the Earth or falling on it. What needs to be done for the ship to begin to descend to Earth, that is, to fall? To do this, you should reduce the speed of its movement.

Although usually everyone returning from a long and distant trip wants to return home as quickly as possible, one should not return from space hastily because it is not easy, or, better said, not cheap to slow down a spacecraft. We have already said that every extra kilogram of cargo in a ship is an extremely undesirable thing. A spacecraft moving in orbit around the Earth can be slowed down by turning on an engine that develops thrust directed against the movement of the ship.

Let's assume that the spaceship and everything on it (without fuel) has a mass of 3 tons. How much fuel do you need to take on the ship to reduce its speed from 8 to 4 km/sec?

In order to reduce the speed of the ship by 4 km/sec, it is necessary to turn on the engine, which would create thrust directed in the direction opposite to its movement. Let us assume that the speed of the exhaust of fuel combustion products from the braking engine nozzle will be equal to 3000 m/sec (a value achievable for modern liquid-propellant rocket engines). The formula established by Tsiolkovsky allows us to determine that the initial mass of the spacecraft, i.e. its mass together with fuel, before turning on the braking engine should be 11.4 tons, therefore, the fuel in the ship should be = 8400 kg. Thus, the mass of fuel that needs to be burned in the braking engine exceeds the mass of the ship’s structure and the cargo located in it by almost 3 times. This method of braking spacecraft is very uneconomical and practically difficult to implement, since delivering such a large mass of fuel into outer space is neither easy nor cheap. But it turned out that it is not necessary to slow down a spacecraft making an orbital flight so much in order for it to begin its descent to Earth.

To begin moving along the descent trajectory, the ship must lose only a small part of its speed. It is quite enough to reduce the speed of the spacecraft by 200 - 250 m/sec. For the case considered by us, i.e., for a spacecraft weighing 3 tons, a loss of speed of 200 m/sec can be achieved by short-term operation of the braking engine when burning fuel in it, the mass of which is less than one tenth of the mass of the ship. But the spacecraft must land at almost zero speed, otherwise a catastrophe will occur - the ship and the crew in it will crash at the moment of landing. How can one take away from a ship all or almost all the kinetic energy it possesses? A practically feasible way to decelerate a spacecraft, without wasting fuel, was indicated by K. E. Tsiolkovsky. The air shell of the Earth, according to Tsiolkovsky, can play the role of a brake for spacecraft returning from an interplanetary journey to Earth. Air braking? Such a proposal may not seem entirely realistic. But remember how the wind blows in your face when you quickly ski down a steep mountain. Try sticking your hand out of a car window as it speeds down the highway. The air goes from almost weightless and imperceptible to elastic. You will have difficulty keeping the palm of your hand perpendicular to the direction the car is moving.

The speed of the spacecraft when it enters the Earth's air envelope (after it has been slowed down by 100 - 200 m/sec) exceeds the speed of the fastest aircraft by approximately 28 times. At such enormous speeds, the air exhibits great resistance to movement. Any resistance is associated with the appearance of friction. Friction also occurs when bodies move in the air. Take two pieces of wood and quickly rub them together. - What do you notice? - Pieces of wood heat up - this is the result of the fact that the friction work you perform has turned into heat. Friction with air is also accompanied by the release of heat.

When spacecraft move in the Earth's atmosphere, not only air friction occurs. As the ship passes through the air envelope, it creates a wave of compressed air ahead of it. The air does not compress gradually, but over a very short period of time. How big is this compression? Calculations show that the pressure in compressed air during spacecraft motion can reach 50 atm. From your physics course, you know that rapid compression or expansion of a gas occurs practically without influx and without heat removal, since due to the short time, heat does not have time to either escape into the environment (during compression) or be transferred from the external environment (during expansion). Such processes are called adiabatic.

Due to adiabatic compression, the layer of air located in front of the flying spacecraft is heated to a high temperature. The temperature of the layer of air compressed by a flying spacecraft can reach 8000° K. This is a very high temperature. There are no substances on Earth that could remain in a solid state at this temperature. The most refractory substances begin to turn into gas or liquid at a temperature of 4000 - 4500 ° C. Will the spacecraft be able to withstand such high temperatures? In addition, you need to remember that there are people inside the ship, behind its hull.

Braking a spacecraft with an air brake requires compliance with certain precautions, otherwise the ship may not only slow down, but also burn out before reaching the Earth. The descent of a ship from near-Earth orbit begins with its deceleration in outer space, where there is no air. To do this, the braking engines are turned on for a while, which develop thrust directed in the direction opposite to the movement of the ship. After the braking engines fire, the spacecraft changes its trajectory and begins to descend, approaching the Earth.

A spacecraft usually flies in orbit around the Earth at some distance from the boundary of the air shell, so after braking the ship descends for some time in a space where there is practically no air. The time the ship descends in airless space must be no less than a certain value. During this time, preparatory work is carried out on the ship to enter the air envelope. Therefore, the height from which it is possible to change the trajectory of the spacecraft, i.e., begin the descent to Earth, is limited by the time required to complete the preparatory work.

What needs to be done on a spacecraft before it enters the Earth's air atmosphere? After the ship is braked by the engine, everything is thrown away from it, without which it can descend. The service compartment, braking motor and some systems are discarded. This is done in order to reduce the mass of the spacecraft, and therefore reduce the amount of energy that needs to be taken from the ship during its descent to Earth.

Rice. 14. The lander has the shape of a lentil.

The thermal protection on the bottom of the descent vehicle and its side surfaces, neither in terms of mechanical properties nor in terms of thermal characteristics, is designed to withstand the loads that the frontal part must withstand. Consequently, in order to prevent the descent vehicle from being destroyed or heated to an unacceptably high temperature during descent, it must enter the Earth's atmosphere with its frontal part directed forward. To do this, before entering the atmosphere, it must be appropriately oriented and, in such an oriented position, enter the Earth's air envelope.

Orientation also serves another purpose, namely to ensure that the descent vehicle enters the atmosphere at a certain angle. What is it for? The entry angle affects a number of parameters of the descent process. For manned spacecraft, the angle of entry into the atmosphere is determined by the amount of acceleration that a person can withstand. We have already said that when a spacecraft is lifted into outer space, overloads arise that exceed a person’s own weight several times.

Unlike ascent, during descent the spacecraft moves with negative acceleration. What forces will act on a person in the descent vehicle during its descent? Firstly, the force of gravity F = mg (m is the mass of the astronaut, g is the acceleration of gravity), directed towards the center of the globe. In addition, it will be subject to an elastic force directed in the opposite direction. These two forces impart acceleration a, directed in the opposite direction ~.

Consequently, when descending from orbit to Earth, the astronaut experiences a force directed from the Earth. This force presses the astronaut to the seat of the cabin or to the ceiling. In magnitude, this force exceeds the normal weight of the astronaut (his weight at rest) by one. A person can withstand overload, i.e., an increase in their own weight by 10 - 12 times. (Of course, in this case it becomes practically inoperable.) A large increase in weight, or, as they say, a large overload, is dangerous for human life.

The overload experienced by astronauts during the descent of the descent vehicle from orbit to the Earth's surface depends on the angle at which the descent vehicle moves in the Earth's atmosphere to the horizon.

Rice. 15. Descent of the spacecraft to Earth.

However, the angle of descent cannot be made too small, since in this case another threat to the safety of the crew arises, associated with overheating.

Let's consider how the shape of the flight path of the descent vehicle affects its heating. We have already said that most of the kinetic and potential energy that a spacecraft possesses while in orbital flight in outer space is converted into internal energy when descending to Earth. How will the descent vehicle heat up when descending to Earth along a steep trajectory, compared to moving along a certain curve located at a small angle to the horizon? During a steep descent, the reentry vehicle decelerates faster and, as a result, loses energy faster. When descending along a gentle curve, the device spends longer in rarefied layers of air and therefore reduces speed not as sharply as in the first case. Obviously, the flatter the trajectory, the slower the vehicle will lose speed. Consequently, the amount of heat generated per unit time when the vehicle descends along a steep trajectory will be significantly greater than when descending along a trajectory making a small angle with the horizon.

From the above, the conclusion suggests itself that the steeper the descent trajectory, the less the danger of overheating of the descent vehicle, and therefore the less danger for the crew. But this conclusion is incorrect. From the point of view of maintaining acceptable temperature conditions for the crew inside the descent vehicle cabin, a too smooth descent is undesirable. What explains this? You know that when putting out fires, rescue teams often have to enter a burning house, fighting their way through the flames. A person is doused with water, and he, in wet clothes, passes through the wall of fire without any harm to himself. He could have done this in a dry suit, if the latter had been made of non-flammable fabric. The flame temperature of objects burning in air is usually 450 - 500°C. This is a fairly high temperature, but due to the fact that the firefighter in his non-flammable suit is in the flame for a very short time, the suit does not have time to warm up, and therefore such a high temperature is not dangerous for a person.

How would a person feel in the same suit made of non-flammable fabric if the environment around him had a temperature even two to three times lower than the temperature of the flame, but the time spent in it was calculated in several minutes? Apparently, this would be unsafe not only for health, but also for human life. A suit made of non-flammable fabric would not have helped him - in such a long time the human body would have warmed up to the ambient temperature, that is, it would have overheated. A similar picture occurs when the descent vehicle moves in the atmosphere. If the apparatus descends along a steep trajectory, a greater amount of heat is supplied to it per unit time than when moving along a flat trajectory. But in order for the heat to reach the cabin of the device, where the crew is located, it takes time. This time depends on the nature and thickness of the heat-protective layer applied to the outer surface of the descent vehicle, and the characteristics of the thermal insulation, which is located under the heat-protection layer.

If the descent of the vehicle occurs quickly, then the time for warming up may not be enough and then, despite the large amount of heat supplied to the descent vehicle per unit time from the outside, from the hot gases of the air atmosphere, the air inside the cabin will not have time to heat up much. During a long descent (along a flat trajectory), although less heat will be supplied per unit time from less hot air, some of it will still have time to pass inside the cabin of the descent vehicle through the heat-protective coating and thermal insulation of the vehicle’s skin, which will lead to heating of the air and all items located inside the cabin.

Thus, two indicators on which the safety of the spacecraft crew’s descent to Earth depends, such as overload and heating, change differently depending on the type of descent trajectory of the descent vehicle in the dense layers of the atmosphere. Reducing overload requires a smooth trajectory and a long descent time. The inadmissibility of overheating of the cabin of the descent vehicle, on the contrary, requires descent along a steeper trajectory with a short time for the vehicle to remain in dense layers of air. The descent trajectory is chosen in such a way that the overload would not exceed the value permissible for the human body, and at the same time, the temperature inside the cabin of the vehicle, where the crew is located, would not be higher than 40 - 50 ° C. A person can easily tolerate this temperature. The already extensive practice of lowering manned spacecraft from orbit to Earth shows that the permissible values ​​of overloads and air temperatures inside the cabin are ensured when the descent time in the dense layers of the atmosphere is 20 - 25 minutes.

We examined the conditions for the descent of a reentry vehicle from near or near-Earth space. Being near the Earth and moving around it, the space object has a speed of ~ 8 km/sec (first escape velocity). In order for a spacecraft to go into deep space and visit any celestial body in our solar system, it must reach a speed of 11.2 km/sec (i.e., the second escape velocity). And he will also have to return from deep space at the second cosmic speed. How does this affect descent conditions?

Before considering the descent of a spacecraft to Earth after returning from an interplanetary flight, let us find out how the approach of space objects to such a celestial body as the Moon occurs.

Being in near-Earth orbit, the spacecraft has a speed equal to the first cosmic speed. Possessing this speed, it cannot fall to the Earth, but it also cannot move away from the Earth or fly to other celestial bodies.

Rice. 16. Trajectories of an artificial Earth satellite at different speeds relative to the globe.

It must be said that almost all artificial Earth satellites located in low-Earth orbit move not in a circle, but in an ellipse. Why? Sometimes the elliptical trajectory of an artificial Earth satellite is necessary for it to perform its tasks in space. In these cases, the satellites are deliberately given a speed slightly higher than the first cosmic speed. For the most part, the trajectory of artificial satellites turns out to be elliptical because it is simply difficult to ensure that the speed of the satellite at the calculated altitude exactly corresponds to the first cosmic speed.

As the speed of a spacecraft increases, its trajectory changes from elliptical to parabolic. The speed at which the spacecraft acquires a parabolic trajectory is called the second cosmic speed, it is equal to ~ 11.2 km/sec. A parabolic trajectory, like a circular one, has only theoretical significance. Flights of spaceships and uninhabited spacecraft to the Moon and other planets of the solar system (Mars, Venus) take place not along parabolic trajectories, but along hyperbolic ones. A spaceship can move along a parabola only if its speed exactly corresponds to the second cosmic speed, and if it is slightly less, then it will move along a closed curve - an ellipse, i.e. it will not be near the Earth and will not be able to fly to other planets of the solar system . If the ship is given a speed slightly greater than the second cosmic speed, its trajectory no longer becomes a parabola, but a hyperbola. A hyperbola is an open curve, and a spacecraft, having switched to a hyperbolic trajectory, cannot approach the Earth when moving along it. He will move further and further away from her and will eventually lose contact with her, that is, he will cease to feel the action of gravity.

Thus, in order to fly to the Moon or any planet of the solar system, a spacecraft located in near-Earth orbit must be given a speed equal to or slightly greater than the second cosmic speed. If, after the spacecraft reaches a speed slightly greater than the second cosmic speed, the engine is turned off, the ship will continue to move along a hyperbolic trajectory.

Rice. 17. At point A, the force of attraction of a body by the Earth (F h) is equal to the force of attraction of this body by the Moon (F l)

Due to the fact that the mass of the Moon is 81 times less than the mass of the Earth, the acceleration of gravity for the Moon is less than the acceleration of gravity for the Earth, and the first lunar escape velocity is only 1.7 km/sec. What is necessary for a spacecraft to leave lunar orbit and fly to Earth? Obviously, just as in the case of leaving the Earth for the Moon, it needs to be given the so-called second lunar escape velocity. For near-Earth space, the second escape velocity is 11.2 km/sec; for near-lunar space it is significantly less. The spacecraft can leave the gravitational zone of the Moon and fly to other celestial bodies of the solar system if its speed slightly exceeds 2.4 km/sec. At this speed, the spacecraft will begin to move away from the Moon, rising upward relative to its surface.

Moving along a hyperbolic trajectory, the spacecraft will move away from the Moon, gradually decreasing its speed. Its kinetic energy will transform into potential energy. Having reached the neutral point, where the gravitational force of the Moon is balanced by the gravitational force of the Earth, the spacecraft will begin to fall towards the Earth. At the neutral point, the spacecraft will have maximum potential energy (relative to Earth).

As you approach the Earth, the potential energy will decrease, and the kinetic energy will increase. Approaching the Earth, the spacecraft will acquire a speed of approximately 11.2 km/sec, i.e., the second cosmic speed. It is unsafe to begin our descent to Earth at such a speed. Before starting the descent, it is necessary to reduce the speed of the ship. But how?

We have already determined the amount of fuel that needs to be burned in a rocket engine in order to reduce the speed of the spacecraft from 8 to 4 km/sec. It turned out that this requires too much fuel for such a path of braking of space objects to be of practical importance. It is even more difficult to brake a body moving at a speed of 11.2 km/sec. Calculations and practice of space flights in the Soviet Union and the USA show that the problem of braking spacecraft moving at the second escape velocity can be successfully solved if the braking effect of the air envelope of the globe is used. When a spacecraft returns to Earth from an orbital flight, when its speed is not much higher than the first cosmic speed, a safe descent using the braking effect of the atmosphere can be achieved if the appropriate angle of entry of the ship into the dense layers of the atmosphere is ensured. The ship, gradually entering more and more dense layers of air, will heat up and at the same time slow down until it reaches the surface of the Earth.