# How to land a spacecraft lander

The spacecraft entered the assigned near-Earth orbit. The research program has been completed. The earth is waiting for its courageous messengers. Difficult and unsafe was the ascent into outer space. But perhaps no less difficult is the return to Earth. How to land the descent module of the spacecraft?

The desire for a soft landing has a strict technical connotation: the descent vehicle of the spacecraft must land at a speed of no more than 2 m/s. Only then the design of the apparatus, the instruments in it, and, most importantly, the crew members do not experience a sharp, hard blow. To do this, you need to slow down the apparatus - to select all the kinetic and potential anergy. Suppose its mass is 4 tons, the height from which the descent begins is 200 km, and the descent begins at the first cosmic velocity - 8 km/s. It is easy to calculate that the total energy of the apparatus before the descent will be equal to 37800 kWh. This energy is more than enough to first melt and then evaporate the descent vehicle of the spacecraft!

How to take away this colossal energy without harming the device itself!? It would seem that the most reliable and easiest way is to take the necessary supply of fuel with you into space and, before starting the descent, turn on the brake engine so that its thrust is directed in the direction opposite to the movement of the ship. K.E. Tsiolkovsky thought about this. He derived a formula for calculating the required amount of fuel for landing with a braking engine. Such a supply of fuel would have weighed the launching ship about twenty times!.. But the scientist was the first to consider another possibility - the deceleration of the descent vehicle of the spacecraft by the air shell of the Earth. All ships returning to Earth use this idea.

Let's follow the descent from orbit. Moving at a speed of approximately 8 km/s, the spacecraft does not fall to Earth. The first stage of descent is the switching on of the braking engine for a short time. As soon as the speed drops by only 0,2 km/s, the descent begins. Now the first step is to undock the orbital compartment and the braking propulsion system. The operation requires speed. Even before entering the dense layers of the atmosphere, it is necessary to turn the descent vehicle of the spacecraft so that it enters the air ocean at a strictly defined angle. If this angle turns out to be too large, the speed will drop sharply, and the astronauts will experience the strongest g-forces, so it is sent into the atmosphere at an angle at which the g-forces acting on the crew do not exceed 4g.

And yet, why should the descent trajectory be such that the crew members must experience a weight that exceeds their own weight four times! Is it possible to choose a more gentle trajectory?

Overloading is one danger that comes with deorbiting. An even greater danger is overheating during deceleration of the spacecraft descent vehicle by the atmosphere. A steep descent leads to greater heating of the shell, but it reduces the flight time: the device will reach the Earth before the sizzling heat penetrates inside it. Thus, overload limits the steepness of the trajectory, and heating limits its flatness.

A wave of compressed air is formed in front of the apparatus descending at high speed. A force of 50 tons presses on each square meter of the frontal surface of the apparatus. The material of the surface layer of the shell of the spacecraft descent vehicle must withstand such enormous mechanical loads.

The walls of the body of the apparatus are made of light aluminum alloy. The temperature in a compressed air wave is approximately 7800–8000 C. Even graphite, the most heat-resistant structural material, evaporates already at a temperature of 4000 C. More precisely, it immediately passes from a solid to a gaseous state. Therefore, a thin metal case is covered on the outside with a protective sheath. It is made from two layers. The first layer, heat-shielding, has sufficiently high mechanical properties and thermal conductivity. Under it, a layer of thermal insulation is mounted - with low mechanical strength and low thermal conductivity.

An attentive reader must have noticed a contradiction with what was said before - that not a single structural material can withstand a temperature of 8000 C. The engineering cunning of the creators of thermal protection can be understood by this example. If you put a pot of water even on the hottest fire, its walls will still never heat up above 100 C. All the heat from the gas stove will go to the evaporation of water. Thus, the temperature of the heated body, no matter how much heat is supplied to it, can always be controlled. This m is used to maintain a certain temperature of the surface of the heat-shielding coating.

The protection material is a polymer compound reinforced with fiberglass. Strong heating leads to slow evaporation of the material. This is where the heat of hot gases goes. The oncoming air flow, as it were, gradually blows away the layer of thermal protection. The particles carry away a lot of heat. The temperature on the surface of the spacecraft's descent vehicle does not exceed 3000 C. During the descent, the cosmonauts see through the porthole a raging sea of fire, reliably tamed by thermal protection.

As it enters into ever denser layers of the atmosphere, the speed of the descent vehicle of the spacecraft decreases, and finally there comes a moment when it flies towards the Earth as a freely falling body. The braking effect of the atmosphere is over. But the speed is still high - about 250 m/s. Now it's time for the parachute system to operate. On board the descent module of the spacecraft, as a rule, there are three parachutes: two main and one auxiliary. One of the main, brake, has a relatively small size and is thrown out with the help of a small explosion, a squib. Its purpose is to slightly reduce the speed of movement. The second main parachute is much larger than the first. It provides a smooth approach of the spacecraft descent module to the surface. It is dangerous to put this parachute into operation immediately at a speed of 250 m/s. Having a large dome area, it may not withstand the pressure of the air flow and break. A smooth landing parachute has not only a large canopy area, but also a significant mass. It is impossible to pull it out of the nest by the same means as the brake one - using a squib. This is the third one - the auxiliary parachute.

Calculations show that reducing the speed of the apparatus to 2 m/s requires a very large parachute. Therefore, for a soft landing, another means is used - the propulsion system of a soft landing. Its task is to develop such a counterthrust at the right moment so that the descent vehicle of the spacecraft hovered over the surface and landed at a speed of no more than 2 m/s from a strictly defined height. It should not exceed 0,15–0,2 m. It is impossible to make a mistake in determining the moment the engine is turned on even for a fraction of a second: the distance to the Earth is measured in centimeters. Solved this problem surprisingly simply. Before the apparatus approaches the Earth, a pin pops out of it. Its length strictly corresponds to such a distance of the device to the surface, at which it is necessary to turn on the soft landing engine. The moment the pin meets the Earth is the signal to turn on the engine. Landing operation completed!