Why is the altitude of 10 kilometers chosen for reusable rocket tests?

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Why do everyone choose 10 kilometers? Is it because this number sounds nice?

In recent years, the development of reusable rocket technology has become a hot topic in the global aerospace field. Especially in the field of commercial aerospace, companies from various countries are competing to develop reusable rockets that can take off and land vertically. In this process, an altitude of 10 kilometers seems to have become an important "threshold" for verifying reusable rocket technology. Whether it is SpaceX in the United States, the verification arrow of the Eighth Academy, or the VTVL-1 of Suzaku-3, the altitude of 10 kilometers always appears repeatedly in tests. So, why choose this altitude? What does it mean for the verification of reusable rocket technology? After crossing this hurdle, is there really a smooth road ahead? Today, let's take some time to talk about it specifically.

The first question that needs to be answered is - what does the height of 10 kilometers mean? In fact, in the process of rocket launch, the height of 10 kilometers is not a particularly significant "milestone". It has not even broken through the atmosphere and is still in the earth's troposphere. However, for reusable rockets, this is a crucial height. First of all, the atmospheric density below 10 kilometers is relatively high and the air resistance is strong. In such an environment, the rocket will experience huge pressure and air resistance during both ascent and descent. This means that the rocket's attitude control system, stability control system, and auxiliary equipment such as grid rudders need to undergo severe tests. In addition, the rocket will also experience "maximum dynamic pressure" at this altitude, that is, the maximum pressure exerted on the rocket by aerodynamics. This is one of the most complex stages in the rocket's flight process. The rocket needs to remain stable at this stage to avoid structural damage or failure. Secondly, 10 kilometers is the altitude at which the rocket needs to experience supersonic and subsonic state changes during flight. For reusable rockets, this is also an opportunity to verify the "transonic" control capability. Transonic refers to the process of a rocket transitioning from subsonic speed, that is, less than the speed of sound, to supersonic speed exceeding the speed of sound. At this stage, changes in airflow will cause huge disturbances to the rocket's attitude and control system. Therefore, the flight altitude of 10 kilometers can not only test the rocket's tolerance, but also verify the rocket's stability and precise control capabilities at transonic speeds. This altitude was chosen as a key test point because it represents the performance test of the rocket under extreme conditions.

So, what does it mean to complete this 10-kilometer flight? It can be said that it marks a major advancement in rocket technology. First of all, the rocket's return and successful landing at such an altitude means that it has basic reusability. As emphasized by the design team of China's Suzaku-3, the success of this flight shows that the rocket's core components such as the engine and grid rudder can be precisely matched and remain stable during the return process. However, the successful completion of this high-altitude test does not mean that there are no more obstacles ahead. Although the 10-kilometer flight verified the adaptability and control ability of the rocket in some complex environments, it is still a long way from the real orbital flight. At higher altitudes, rockets need to face more severe challenges, especially the high temperature and extremely high speed when re-entering the atmosphere. For China's reusable rocket technology, the success of the 10-kilometer flight means that the verification of the technology has entered a critical stage. The rocket's vertical take-off and landing technology has achieved initial results, but what needs to be solved in the future is how to achieve vertical recovery at a higher altitude and deal with the huge heat and kinetic energy generated during the re-entry process. For example, how to effectively protect against the high temperatures generated when re-entering the atmosphere; how to further improve the landing accuracy and speed control of the rocket; and how to make the rocket structure light enough but strong enough to cope with fatigue wear from repeated use. We expect that these problems will be gradually solved as the technology matures and the number of flights increases.

When it comes to the development and current status of my country's reusable rocket technology, one point that cannot be bypassed is the achievements of SpaceX. Since the early 2000s, SpaceX has been committed to the development of reusable rockets, and successfully achieved the vertical recovery of rockets for the first time in 2015. Since then, SpaceX has completed the recovery and reuse of orbital-level rockets many times, which has completely changed the cost structure of the aerospace industry. In contrast, China's reusable rocket technology started late. Although it has made significant progress in recent years, there is still a certain gap with SpaceX. Taking the Suzaku-3 VTVL-1 as an example, China's tests are still in the vertical take-off and landing stage at an altitude of 10 kilometers, while SpaceX has already achieved sea recovery and reuse of orbital-level rockets. However, this does not mean that China's aerospace has no possibility of catching up. In recent years, China has invested a lot of resources in the research and development of aerospace technology and has made major breakthroughs in many experiments. In terms of time, it may take about 5-10 years for China's reusable rockets to reach the current level of SpaceX. However, China's huge market demand and the investment of private enterprises in technological innovation will further shorten this gap.

In short, the 10-kilometer flight test is not only an important step in technology verification, but also the cornerstone of future space exploration. Although my country started late in this field, with continuous investment and technological accumulation, it is expected to catch up with SpaceX in the near future and usher in our own era of recyclable spaceflight.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Gan Shudong, creator of popular science videos

Reviewer: Zhou Binghong, Researcher, National Space Science Center, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.