Why is it that the rocket is fine when it flies out, but the spacecraft burns up when it comes back? The key is speed

The process of a rocket igniting and taking off from the earth is spectacular, but when the return capsule comes back it always burns into charcoal due to friction with the atmosphere. Why is this?

Obviously, this is a question related to speed. What speed does the rocket have to reach to send the spacecraft into Earth orbit? Of course, it is 7.9 kilometers per second, which is what we call the first cosmic speed. If it is lower than this speed, it will not be able to escape the shackles of the Earth's gravity. So how fast is the return capsule when it returns to Earth? It is also 7.9 kilometers per second, because no matter where the return capsule comes from, it must fly in Earth orbit for a while in order to adjust the flight trajectory and angle, and wait to start the engine to return to Earth when it reaches the return position. To orbit the Earth, it must have a speed of 7.9 kilometers per second, so the initial speed of the return capsule when it returns will not be lower than this number.

Since both were 7.9 kilometers per second, why was it fine when it flew out but burned into charcoal when it came back?

Because the rocket is accelerating when it goes out. The Earth's atmosphere can be divided into the troposphere, stratosphere, mesosphere and thermosphere from bottom to top. When the rocket is launched, the speed gradually increases. When it reaches the upper edge of the mesosphere, which is about 80 kilometers above the surface, the speed of the rocket is only 4 or 5 kilometers per second. This speed is not enough to cause the outer shell of the rocket to burn through friction with the atmosphere. The higher it goes, the faster the speed, and the thinner the atmosphere becomes, so the outer shell will not burn due to friction. When the return capsule returns to Earth, the situation is different.

When the return capsule returns to Earth, it is in free fall, with an initial speed of 7.9 kilometers per second.

Since the upper atmosphere is very thin and has little resistance, when the return capsule enters the middle layer about 80 kilometers from the surface, the speed does not drop significantly. Due to the high speed, the outer shell of the spacecraft rubs against the increasingly dense atmosphere and burns. The outer shell temperature can even reach about 2000℃. The process of astronauts returning to Earth is a dangerous process. In the history of human spaceflight, there has been an accident in which a spacecraft disintegrated during the return process. It was the Columbia Space Shuttle of the United States. Since it is so dangerous, why don't humans change the way the spacecraft returns? Why must we rely on free fall instead of slowing down in advance?

There are only two ways to slow down the return capsule in advance when returning to Earth. One is reverse propulsion, and the other is to open the parachute in advance.

We know that the return capsule of the spacecraft is equipped with a parachute, which is a necessary guarantee for the safe landing of the return capsule. At about 10 kilometers from the ground, the return capsule will open the parachute. The first one to open is a parachute for deceleration and position adjustment, which we call the "deceleration parachute". After the return capsule has successfully decelerated and adjusted its position, the main parachute will be opened, which is the big umbrella we finally see. Its total area is about 1,200 square meters. Compared with it, the return capsule is like a marble hanging under the umbrella, so after landing, the parachute will quickly detach from the return capsule to prevent the return capsule from being dragged around by the parachute under the action of the wind.

Whether it is the main parachute that opens last or the deceleration parachute that opens first, they both utilize the principle of air resistance.

The density of the upper atmosphere of the earth is very low, and the air is extremely sparse. Even if the parachute is opened in advance, it cannot slow down the vehicle. Moreover, in the friction with the atmosphere, the return capsule was burned into charcoal. If the parachute is opened in advance, it will be even more difficult to escape, so it is impossible to open the parachute in advance. So what if reverse propulsion is not possible? In theory, it is feasible, but this will greatly reduce the carrying capacity of the rocket. In order to achieve deceleration through reverse propulsion, the return capsule must carry a large amount of fuel, and this fuel must be carried with it from the beginning. The main weight of chemical-powered rockets is fuel, so the payload is so unsatisfactory. If you have to bring a lot of extra fuel for deceleration, there will be very little left to transport to the sky.

Taking various factors into consideration, it is most convenient and economical to let the return capsule return to the ground in a free fall manner. All we have to do is to insulate the return capsule to ensure the safety of the interior of the return capsule, especially the astronauts.

In fact, for astronauts, in addition to the intense burning of the outer shell of the return capsule during their return to Earth, there is another thing they need to face, which is the loss of communication for several minutes. In the middle layer of the Earth's atmosphere, the atmosphere is ionized. When the return capsule passes through this area, it will be wrapped in ions and lose its communication ability. This period of time happens to be the stage when the outer shell of the return capsule burns violently. Only the astronauts have to face this thrilling process alone.