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*This article is from the 15th issue of Banyuetan in 2024

The Chinese have been yearning for space for thousands of years, and many flying myths bear witness to this dream. Today, Chinese rockets have already flown into space, but these rockets that have flown into space have not yet had the experience of "flying home" - rocket recovery is still a dangerous hurdle for Chinese rocketeers.

In fact, the exploration of recycling domestic large rockets has accelerated this year. Recently, the new technology verification rocket for reusable launch vehicles developed by the Eighth Academy of China Aerospace Science and Technology Corporation successfully conducted a 10-kilometer flight test at the Jiuquan Satellite Launch Center. Blue Arrow Aerospace is about to carry out a 10-kilometer vertical take-off and landing flight test, and Deep Blue Aerospace is also about to carry out a 5-kilometer vertical take-off and landing test of the first stage of the "Nebula-1" rocket... Almost every month, China's rockets have a "big move" to recycle.

So, why do we need to recycle rockets like we recycle Coke bottles? What is the difficulty in recycling rockets? How many steps does it take for a rocket to go from "leaving home" to "returning home"?

Why recover rockets?

With the command "3, 2, 1, ignition!", the rocket soars into the sky. This is the impression most people have of rocket launches. The rocket flies into space, and a beautiful parabola is all that everyone remembers. Few people care whether it will come back or not.

On January 19, 2024, LandSpace successfully completed the vertical take-off and landing test of its test rocket at the Jiuquan Satellite Launch Center.

Indeed, before 2015, rockets were disposable and consumable, and once launched, they would never come back. However, the successful recovery of the Falcon 9 in 2015 made history: it turns out that rockets can also be reused, and "flight-like launches" are not impossible.

In fact, Qian Xuesen, the "Father of Chinese Spaceflight," had envisioned this in his 1963 book, An Introduction to Interstellar Navigation: "A transport rocket, whether a large first-stage rocket or a small second-stage rocket, can fly back to the ground as long as it is equipped with wings, so a transport rocket can be used many times."

What are the benefits of letting the rocket "fly home"? Three sentences - reduce costs, increase efficiency, and improve quality.

The most direct way is cost control - recycling and reuse will naturally reduce costs. "Rocket recycling is the best way to directly reduce launch costs. In the past, rocket launches were one-time, just like 'taking a one-time plane'. Once a rocket is recycled, it can take off and land multiple times like an airplane, spreading the cost more and more," said Bai Guolong, a member of the Chinese Astronautical Association and aerospace science blogger known as "Captain Shenxian".

Secondly, the reusability of rockets means that the frequency of launches has increased, which is the hope for the expansion of the industry. Traditional rocket launches must be prepared for at least two months, and they must keep an eye on the time window and "depend on the weather". "Unlike reusable rockets, they can be launched again after simple repairs and maintenance. The substantial increase in rocket utilization, launch frequency, and launch flexibility will greatly improve the ability to quickly enter space, making large-scale space exploration and development possible," said Huo Liang, founder of Deep Blue Aerospace.

In addition, the upgrade of the transport capacity of reusable rockets will help improve the overall quality of space exploration. "At present, large-scale constellation networking requires large-capacity, low-cost, and high-frequency rocket launches, and large reusable rockets can support my country's Internet constellation engineering strategy," said Dai Zheng, general manager of the rocket research and development department of Blue Arrow Aerospace and commander-in-chief of Suzaku-3.

How difficult is it to recover a rocket?

The roadmap is simple and clear, but turning it into reality is extremely difficult.

A traditional rocket consists of two stages, the first stage at the bottom and the second stage at the top. The current rocket recovery refers to the process in which the first stage and the second stage separate after the rocket flies to an altitude of more than 100 kilometers, the second stage enters the predetermined orbit, and the first stage, which was originally only self-destructive, returns to the launch site. In short, rocket "recovery" at this stage means the "return home" of the first stage.

Its "return home" is roughly divided into three steps. The first step is the separation of the rocket body and attitude adjustment. After the rocket is ignited and launched, it rises to an altitude of more than 100 kilometers, the engine of the first stage is shut down, and the second stage is separated from the first stage. For the first stage to return to the ground, it must first "turn around", that is, adjust its attitude. The second step is to slow down. After turning around, the rocket enters the return process at a very fast speed. At this time, the engine needs to be ignited for the second time to slow down the speed. The third step is landing and hovering. Since the ideal posture for the rocket to return to its original position is with the tail down and the head up, the engine needs to be ignited for the third time to adjust the posture and give the rocket a reverse thrust so that its acceleration and speed are reduced to 0 at the same time when landing.

Why is it said that it is more difficult for a rocket to "return home" than to launch it?

The most difficult part is to "return accurately". "During the descent of the rocket, as the fuel is consumed and the speed changes, different thrusts need to be output to ensure smooth deceleration. This requires the rocket engine to accurately and dynamically adjust the thrust and have the function of multiple starts." Huang Shuai, chief designer of the Gravity-2 rocket of Oriental Space, emphasized that a set of precise algorithms must be carefully guided for the rocket to return smoothly.

"Landing steadily" is not easy either. During the rocket's return flight, the descent attitude and landing angle must be controlled with high precision. Once the landing angle is wrong, the rocket may overturn, causing fuel leakage and explosion. This difficulty is like throwing chopsticks into a bottle, not to mention that this is a giant "chopstick" with a descent speed of more than 1,000 meters per second. In fact, even if effective deceleration is achieved before landing, the huge inertia brought by the rocket weighing dozens of tons must be taken into account. You know, there are a large number of sophisticated electronic components in the rocket. To ensure that these "treasures" can be used again, their buffering skill values ​​must be maxed out.

On July 21, the Nebula-1 rocket independently developed by Deep Blue Aerospace completed a simulated high-altitude recovery flight test.

Another difficulty is "long-term use". Zheng Ze, deputy general manager of Deep Blue Aerospace, said that reusable rockets need to withstand the test of repeated use and multiple re-entry into the atmosphere, so they need to use new composite materials with high strength, high temperature resistance and ultra-light weight, and overcome a series of material performance difficulties. The reliability requirements for components such as engines are particularly high. In addition, reusable rockets must be easy to maintain, and simple inspection and maintenance after recovery can meet the requirements for re-launching. There are also many technical gaps in evaluation and inspection to be filled in this regard.

The exploration never stops

What else needs to be done for Chinese rockets to achieve the “go and return” function?

"On the one hand, we need to optimize the management and approval process for rocket launches and balance safety and efficiency. On the other hand, we need to build more launch sites and launch positions, including land and sea recovery launch platforms, to accommodate the need for higher-frequency launches." Bai Guolong said that only by creating an environment where technology can iterate rapidly and processes and costs are continuously optimized, will this mountain climbing route hopefully be smoother.

Rocket recovery is difficult, but the confidence of astronauts is unshakable. In the Qian Xuesen Library of Shanghai Jiaotong University, there is an envelope on display. In 1941, when Qian Xuesen completed his famous paper "Axial Compression Buckling of Cylindrical Shells", he wrote "Final" in red on the envelope containing the manuscript. When he put down his pen, he suddenly realized that "Final" also means "end": "How can the pursuit of truth end?" Qian Xuesen took the envelope and wrote "Nothing is final" in black.

Huang Shuai said: "Our exploration of rocket recovery technology will never end."

Original title: "How many steps are there to get the rocket home?"

Banyuetan reporter: Zhang Manzi/ editor: Fan Zhongxiu

Editor: Zhang Ziqing / Proofreader: Qin Daixin