The “common face” of the “super shield”: Why do the return capsules of spacecraft look similar?

Every time you see the spacecraft's return capsule successfully land and send astronauts back to Earth safely, aren't you super excited?

But have you ever wondered why the return capsules of various countries all use a flat-bottom design?

The return capsule of Shenzhou 15

Apollo return capsule diagram

Shield for safe return

In the process of exploring how to ensure the safe return of the return capsule, solving the problem of heat generated by high-speed friction between the return 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, USA, devoted himself to related research. In his tests, he found that when a spacecraft reenters the atmosphere (a ballistic missile warhead, a returning spacecraft's returner, and other reentry bodies reenter the Earth's dense atmosphere from outer space) travels through the air fluid at a very high speed, the surrounding air density will change significantly. When light passes through these air layers of different densities, it will produce different imaging effects of light and dark due to refraction, which is the "shadow image."

Shadow Image

Allen was keenly aware of the value of this discovery. After in-depth thinking and research, he put forward an innovative view: the reentry vehicle should adopt a more gentle blunt windward surface. When the reentry capsule reenters the atmosphere, a strong shock wave will be generated in front of the bottom. This shock wave is like a "super shield" for the reentry capsule, which can block most of the heat and prevent other parts of the reentry capsule from being burned too hot. Installing a heat shield, that is, a large heat-proof bottom, on the blunt bottom can firmly block most of the heat, without having to make the entire reentry capsule into a high-strength heat-proof structure.

A breakthrough in shape change

We already know that the return capsule generally adopts a "flat bottom" design. So, in addition to this well-known feature, what other factors play a role in the design process of the return capsule shape?

When the return capsule re-enters the atmosphere, it will encounter two "powerful characters", one is resistance and the other is gravity. There are molecules such as nitrogen, oxygen and carbon dioxide in the air. When the return capsule falls, it will hit them, and the speed of falling will be slowed down. This is the effect of resistance. However, the speed of the return capsule's fall is mainly affected by gravity. There is another force that is very helpful to the return capsule - lift; lift can help control the flight path of the return capsule, so that the gravity acting on the return capsule becomes smaller. Because the longer the return capsule stays in the thin air at high altitudes, the less heat it absorbs and conducts, and the corresponding heat entering the return capsule will also decrease.

When the return capsule reenters the atmosphere, it will be affected by various factors such as air resistance, gravity, and heat. These factors are crucial to the safe return of the return capsule. So has the return capsule been shaped like a large base and a small top from the beginning? In the course of the development of spaceflight, what changes has the shape of the return capsule undergone?

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

To solve these problems, scientists continued to explore, and bell-shaped and conical return capsules emerged. This shape of return capsule is not afraid of air disturbance when entering the atmosphere, has better lift and drag, and has good stability when landing, which can effectively ensure the smooth progress of the return mission.

In many people's impressions, the return capsule always seems to be extremely hard, able to withstand high temperatures, and will never be used again after being used once. But in fact, aerospace engineers are working on a very innovative flexible inflatable return capsule. It can be called a brand-new return capsule landing protection device, and it has extremely broad application prospects in the future. One of its major advantages is its small size. In future launch missions, spacecraft can carry multiple such return capsules. This means that it can perform multiple return flight missions, greatly improving the efficiency of downlink transportation of materials from space, while also effectively reducing costs.

At present, the technology related to flexible inflatable return capsules is still in the process of exploration around the world. In fact, as early as the 1960s, the United States took the lead in starting the research journey of inflatable reentry technology. However, due to the limitations of the performance of the exothermic materials at the time, this technology has never been mature and could not be put into practical application. In the early 21st century, the European Space Agency and the Russian Space Agency also joined the ranks of exploration and launched projects on inflatable reentry and deceleration technology. Unfortunately, these attempts ultimately failed to achieve complete success.

If this flexible inflatable return capsule technology can be successfully applied, the cost of transporting goods between the space station and the Earth, or even between the Moon and the Earth, will be greatly reduced in the future. This means that humans can carry out space cargo at a very low cost, greatly reducing the burden of space transportation, making space resource development more economical, and helping humans take further steps in the universe.

Some information comes from: CNKI, Huanqiu.com, etc.

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