Does aerospace also need energy conservation and emission reduction? Unveiling the "green revolution" of rockets

In recent years, the number of global launch vehicle launches has increased rapidly, from 91 in 2017 to 233 in 2023, more than doubling. With each launch, humans continue to try to write a new chapter in the field of aerospace, and the impact of launch vehicles on the earth's environment has attracted more and more attention from the public. Recently, the State Council issued the "2024-2025 Energy Conservation and Carbon Reduction Action Plan", which has made many people begin to re-understand the relationship between the aerospace boom and energy conservation and carbon reduction. How to protect this blue planet and make aerospace and environmental protection go hand in hand?

01 What impact does rocket launch have on the environment?

The toxic gases, exhaust gases, and waste produced by rockets, especially those using room temperature fuel, during launch do have a certain impact on the environment . At the moment of ignition and launch, the blazing flames produced by the combustion of rocket propellants will produce many different emissions.

Among these substances, nitrogen oxides, sulfides and carbon monoxide are directly harmful to the human body; chlorine, aluminum oxide and nitrogen oxides will destroy the ozone layer and have been strictly monitored and restricted since 1996. Carbon dioxide and nitrogen oxides absorb more heat and are considered greenhouse gases. In addition, sulfides and nitrogen oxides can also cause acid rain, which is harmful to various organisms.

The exhaust gas produced by solid rocket engines contains chlorine and aluminum oxide, which are unique pollutants produced by space activities . Because they are known "ozone killers", they were the first to attract attention from the aerospace community.

The smoke from rocket launch exhaust also has a significant impact on the ozone layer . Rockets pass through the stratosphere, where their exhaust smoke stays for up to 2 to 3 years. This is the only way for humans to directly produce aerosol pollution in the stratosphere.

Emissions are not the only impact of launch vehicles on the environment. Since most rockets are disposable, greenhouse gas emissions from their production process also have an impact on the environment, and discarded parts such as solid fuel debris and rocket fragments that fall to the surface of the earth can also cause pollution in the ocean or on land.

02 The environmentally friendly path of rocket launches

Of course, aerospace researchers will not sit idly by.

If you have watched the live broadcast of a rocket launch, you may have noticed that a large amount of white water mist will gush out of the launch platform within tens of seconds at the moment of liftoff. The use of water is not only for engine cooling, protecting the launch platform and reducing noise, but also to meet environmental protection needs. After the rocket is launched, there is often a lot of dust and chemicals left on the launch platform. Using a large amount of water to flush and clean it can prevent further pollution to the environment and also help the next launch to go smoothly.

Researchers also use catalytic reduction, adsorption and other methods to treat the waste gas generated by rocket launches. When Shenzhou III was launched in 2002, the Jiuquan Satellite Launch Center put the space launch site propellant waste gas treatment system into use for the first time, transporting the waste gas generated when rocket propellant was filled to the purification device, where it was mixed with the combustion agent to purify the toxic waste gas.

In order to completely eliminate the toxic gas produced by normal temperature propellant rockets during launch, my country is replacing normal temperature propellant rockets with a new generation of rockets that use clean cryogenic propellants. For example, the Long March 7 is based on the Long March 2F rocket, but the nitrogen tetroxide/undimethylhydrazine normal temperature propellant engine on the rocket has been replaced with a liquid oxygen/kerosene cryogenic propellant engine. The benefits of replacement not only bring green propellants, but also improve rocket carrying capacity, reduce propellant costs, and pave the way for future reusability.

On May 29, 2021, the Long March 7 Y3 carrier rocket carrying the Tianzhou 2 cargo spacecraft ignited and launched at the Wenchang Space Launch Center in Hainan. Careful people will find that at the moment of rocket ignition, the tail of the "Popeye" Long March 7 did not have red "smoke" formed by incomplete combustion of nitrogen tetroxide, but instead was replaced by white "smoke" like cotton candy.

Image source: Xinhua News Agency (Photo by Ju Zhenhua)

To launch a rocket, high-quality fuel is needed to provide power. In order to further green the way of producing fuel, it is also necessary to be "renewable". In current aerospace technology, rockets usually use hydrocarbon fuel as liquid propellant, which is produced by steam reforming. Its principle is to react hydrocarbons such as natural gas and oil with water vapor to produce hydrogen and carbon dioxide, and also produce by-products such as carbon monoxide. If the electrolysis method is used to use electricity generated by solar energy and wind energy to decompose water to produce hydrogen, the "green hydrogen" produced will be cleaner . Some companies in Europe and Japan even use liquefied biomethane extracted from cow dung as rocket engine fuel.

03 Zhuque-2 takes the lead: Commercial space travel moves towards green and sustainable development

Rocket propellant is a key factor in determining the rocket's capabilities. Propellant burns rapidly in the rocket engine, helping the rocket to take off, accelerate and change its orbit. The performance of the propellant directly affects the rocket's thrust and speed.

At present, the mainstream rockets are mostly liquid oxygen/kerosene propellants, which are low in cost and high in specific impulse, but are prone to carbon deposition and coking. Although liquid hydrogen/liquid oxygen propellants have high specific impulse and clean combustion products, their low density makes the rocket volume too large, and it is currently difficult to use them alone on a large scale at the basic level.

Liquid oxygen/methane propellant is gradually being recognized by countries around the world as one of the ideal materials for rocket engine propellants due to its high combustion efficiency, green environmental protection, low cost and easy preparation.

Liquid oxygen is the liquid form of oxygen, which can provide sufficient oxidant for rocket engines; methane is the second cleanest compound after hydrogen. It not only burns stably but also produces almost no carbon deposits, which makes it easier to recycle and reuse the engine.

However, the development of liquid oxygen/methane rockets is not an easy task, as it involves technical challenges in engine design, material selection, fuel storage, etc. Globally, liquid oxygen/methane rockets have entered a period of rapid development, and Chinese researchers are also constantly making breakthroughs.

On July 12, 2023, China's Zhuque-2 Yao-2 carrier rocket successfully entered orbit. This is the world's first liquid oxygen/methane rocket to successfully enter orbit , and it is also the first carrier rocket in China's civilian aerospace industry to successfully enter orbit based on an independently developed liquid engine. This milestone event marks a more solid step forward in China's green aerospace industry.

The Suzaku-2 Yao-2 carrier rocket was launched successfully.

Image source: Xinhua News Agency (Photo by Wang Jiangbo)

As mentioned above, the manufacture of rockets itself also produces a lot of carbon dioxide and pollution. If the rocket can be reused, these manufacturing emissions can be dispersed over the life cycle of the rocket and its cost of going to space can be greatly reduced. As the next step for Suzaku-2, the reusable Suzaku-3 rocket, which also uses liquid oxygen/methane propellants, is also progressing steadily. On January 19, 2024, the Suzaku-3 reusable rocket completed its first large-scale vertical take-off and landing flight test, and the rocket is scheduled to have its first flight capability in 2025.

In order to further support the needs of China's future large-scale space missions, China has also carried out the development of a 200-ton full-flow regenerative cycle liquid oxygen/methane engine . At present, the engine scheme demonstration has been completed, and the torch igniter hot test, gas generator scaled part hot test and thrust chamber scaled part hot test assessment have been successfully completed.

Humans yearn for the vast sky and are eager to uncover the mysteries of the deep universe. In this process, we should not only pursue the progress of science and technology, but also deeply root the green concept in every aspect of space exploration. We have reason to believe that the power of science and technology can make the road of space exploration cleaner and more sustainable. Among the stars and the sea, let us leave the mark of human wisdom and civilization, and leave an eternal commitment to respect and protect nature.

Author: Yu Yuanhang, Senior Engineer, Beijing Institute of Aerospace Systems Engineering

Reviewer: Pang Zhihao, Chief Scientific Communication Expert of National Space Exploration Technology

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.

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