Every time you see the successful landing of the spacecraft return capsule and the safe return of astronauts to Earth, are you super excited?

But have you ever wondered why the return capsule in various countries is designed with a flat bottom?

The return capsule of Shenzhou 15

Schematic diagram of the Apollo return capsule

A "shield" for a safe return

In the process of exploring how to ensure the safe return of the capsule, solving the heat problem generated by the high-speed friction between the capsule and the atmosphere has become one of the key points. In the mid-20th century, Julian Allen, director of the Ames Space Research Center in California, devoted himself to related research. During his tests, he found that when a spacecraft re-entering the atmosphere (a ballistic missile warhead, a returner for a re-entry spacecraft, etc., re-enters the Earth's dense atmosphere from outer space from outer space) shuttles through the air fluid at extremely high speeds, the density of the surrounding air changes significantly. When light passes through these layers of air with different densities, it will produce different imaging effects due to refraction, which is called "shadow image".

Shadow image

Allen was keenly aware of the value of this discovery, and after in-depth thinking and research, he came up with an innovative idea: reentry vehicles should have a more gentle, blunt windward side. When the capsule re-enters the atmosphere, a strong shock wave will be generated in front of the bottom, which is like putting on a "super shield" for the capsule, which can block most of the heat and prevent the rest of the capsule from being burned too hot. Installing a heat-proof shield on the blunt bottom, that is, a heat-proof outsole, can firmly block most of the heat, and there is no need to make the entire return capsule into a high-strength heat-proof structure.

A breakthrough in the transformation of shape

We already know that the "flat-bottomed" design is common in the reentry capsule, but in addition to this well-known feature, what other factors played a role in the design of the reentry capsule shape?

When the re-entry capsule re-enters the atmosphere, it will encounter two "powerful characters", one is drag and the other is gravity. There are molecules such as nitrogen, oxygen, and carbon dioxide in the air, and when the return capsule falls, it hits them, and the speed of the fall will be slowed down, which is the effect of drag. However, the speed at which the return capsule falls is mainly affected by gravity. There is also a force that is very helpful for the return capsule - lift; Lift helps control the flight path of the capsule, so that the gravitational force acting on the capsule is smaller. As the capsule stays in the rarefied air at high altitudes, the less heat is absorbed and conducted, and the corresponding amount of heat enters the capsule.

The re-entry capsule is affected by various factors such as air resistance, gravity, heat, etc. These factors are critical to the safe return of the capsule. So was the return capsule a big bottom and a small head from the beginning? In the course of aerospace development, what changes have the shape of the return capsule undergone?

The earliest shape of the capsule was a spherical capsule. The design of the spherical return capsule conforms to certain aerodynamic principles and has a certain stability. But because it has almost no lift, a very small lift-to-drag ratio, and a small cross-section, it cannot be slowed down effectively with the help of air resistance. This means that during the return process, it is difficult to control the flight trajectory and is subjected to a large impact force, which can pose a threat to the safety of astronauts and the integrity of the equipment.

In order to solve these problems, scientists continue to explore, and then the bell-shaped and conical return capsules have emerged. The re-entry capsule of this shape is not afraid of airflow disturbance when entering the atmosphere, has good lift resistance, and has good stability during landing, which can effectively ensure the smooth progress of the re-entry mission.

Many people have the impression that the re-entry capsule always seems to be extremely stiff, able to withstand high temperatures, and used once before being used again. But in fact, aerospace engineers are working on a very innovative flexible inflatable return capsule. It can be called a new kind of landing protection device for the return capsule, and it has a very broad application prospect in the future. One of its major advantages is its small size, which allows the spacecraft to carry several of these capsules in future missions. This means that it is capable of multiple return missions, greatly improving the efficiency of transporting supplies from space downstream, while also effectively reducing costs.

At present, the technology related to flexible inflatable return capsule is still in the process of exploration in the world. In fact, as early as the 21s of the 0th century, the United States took the lead in starting a research journey of inflatable reentry technology. However, due to the limitations of the properties of exothermic materials at that time, this technology was never developed and put into practical use. At the beginning of the 0th century, the European Space Agency and the Russian Space Agency also joined the ranks of exploration, launching projects on inflatable reentry and deceleration technologies. Unfortunately, these attempts were ultimately unsuccessful.

If this flexible inflatable return capsule technology is successfully applied, the cost of cargo transportation between the space station and the Earth, and even between the Moon and the Earth will be greatly reduced in the future. This means that human beings can carry out space cargo at a very low cost, greatly reduce the burden of space transportation, make the development of space resources more economical, and help mankind take further steps in the universe.

Part of the information comes from: CNKI, Global Network, etc

(Scientific review: Li Liang, member of the Science Popularization and Education Committee of the Chinese Space Society)