Lunar exploration "power decoding"! Which engines were used in the Chang'e 6 exploration mission?

On July 12, 2024, Wu Baoyuan, a researcher at the China Academy of Aerospace Propulsion Technology, gave a speech entitled "Decoding the Power of Chang'e 6" at the Shaanxi special event of the China Starry Sky Science Forum "Exploring the Starry Sky and Touching the Future".

The Chang'e-6 mission is the first time that humans have collected samples on the far side of the moon. It is the most integrated mission in my country's aerospace industry and an important milestone in the world's aerospace industry. In this mission, 107 engines were used, all of which were liquid rocket engines.

Generally speaking, transport vehicles have relatively few engines. For example, a car has one engine, and an airplane generally has no more than four engines. So why did the Chang'e-6 mission use 107 engines?

Let's first look at the propulsion principle of rocket engines.

How do rocket engines propel rockets?

The driving principles of different engines are quite different.

For example, a car moves by using its engine to drive the tires to act on the ground to generate reaction force. Its characteristic is that it acts on a third object; while the turbojet engine on an airplane provides thrust by generating reaction force through ejecting gas, and its characteristic is that it generates reaction force through ejecting gas. In this respect, a rocket is closer to an airplane.

From the perspective of action and reaction: when we walk, we use our feet to push the ground to obtain power; when we stand up from a chair, we use our hands to support the armrests of the chair to obtain pushing force; when an airplane turns, it obtains corresponding force by manipulating the ailerons, elevators and rudders to act on the high-speed air.

In these examples, there is a third object, such as the ground, the armrest of a chair, or the air, and the moving body obtains reaction force by acting on the outside world. In space, like the astronauts in the movie, there is no third object, so there is no force to exert, and the only way to obtain reaction as propulsion is by throwing out the arms of the space suit.

In general, we can understand the process of jet thrust generation in this way: if there is no object in the outside world, we create and act on such an object to obtain propulsion, and this object thrown out is called "working fluid". At present, all propulsion is propelled by "working fluid", and propulsion without "working fluid" has always been a science fiction topic.

The working fluid of a rocket engine is the high-temperature combustion gas ejected, and its energy comes from the chemical energy contained in the fuel and oxidizer, which are the propellants of the engine. The exhaust velocity of a liquid rocket engine is generally 3000~4500m/s. The faster the exhaust velocity, the higher the performance.

How is this energy gradually released and drives the task to be completed?

From the perspective of engine energy use, the Chang'e-6 mission has six main phases:

The first stage: the launch vehicle is launched into the Earth-Moon transfer orbit, with a flight time of 2210 seconds and about 99% of the energy released;

The second stage: Earth-Moon transfer and near-Moon braking, flying for 5 days, releasing about 30% of the remaining 1%;

The third stage: landing on the moon, flying for 900 seconds, releasing about 40% of the remaining;

The fourth stage: Ascend from the lunar surface, fly for 360 seconds, and release about 10% of the remaining;

The fifth stage: lunar-to-earth transfer, flying for 5 days, releasing about 20% of the remaining;

The sixth stage: re-entry and water recovery (returner), the energy consumed is negligible.

From the perspective of energy use, the most surprising thing about the Chang'e-6 mission is that 99% of the energy was used to send the probe to the Earth-Moon transfer orbit. From this perspective, it is very difficult to escape the Earth's gravity and enter space. Moreover, this 99% energy release was completed within 37 minutes, which fully demonstrates the huge power density of liquid rocket engines.

Power density is the energy released per unit mass (or volume) per unit time. It is estimated that the power of the Long March 5 at launch is equivalent to three Three Gorges Dams operating at full capacity, and the power of a single 120-ton liquid oxygen-kerosene engine is higher than that of the Gezhouba Hydropower Station and the Taishan Nuclear Power Station.

The three most distinctive engines in the Chang'e-6 mission

Here are the three most distinctive engines used in this mission.

Among them, the 120-ton liquid oxygen-kerosene engine is the engine with the largest thrust in service in China. It uses liquid oxygen and kerosene as propellants, has the advantages of high density specific impulse, good storage performance, and green environmental protection. It is the backbone of my country's new generation of launch vehicles. Its single engine propellant flow exceeds 400kg/s, which is equivalent to 140 liters of fuel flow per second, enough for an ordinary family car to refuel three times. The Long March 5 booster stage uses eight such engines, providing a thrust of 960 tons. I am also honored to have participated in the development of this engine throughout the process.

The second type, the 7500N variable thrust engine, can be said to be the meritorious engine of the Chang'e Project. It can achieve a continuous change in thrust from 7500 Newtons to 1500 Newtons. This ability is crucial for executing complex space missions and is also the key to the probe's landing on the lunar surface. This engine represents China's independent innovation capabilities in the field of aerospace technology, fills the gap in the field of variable thrust engines in China, and its comprehensive performance has reached the world's leading level, providing a solid technical guarantee for my country's deep space exploration missions. In the Chang'e-6 exploration mission, the 7500N variable thrust engine was installed on the lander and used for the soft landing of the lander/ascender combination on the lunar surface. It worked for a total of 900 seconds and consumed nearly two tons of propellant.

The third type, the 3000N engine, has the characteristics of high performance, high reliability, light weight and compactness. It has the ability to start multiple times in complex space environments, and can escort large-scale transfers between the Earth and the Moon, and between the Earth and the Moon, and takeoff and launch from the lunar surface. In the Chang'e-6 exploration mission, one 3000N engine was installed on the orbiter, and was used for orbit change and near-moon and near-Earth braking throughout the mission cycle. It was started multiple times and consumed nearly 2 tons of propellant; the other was installed on the ascender for takeoff from the lunar surface, working for a total of 360 seconds and consuming nearly 400 kilograms of propellant.

In addition, the Chang'e-6 mission also installed and applied a relatively large number of attitude control engines. In addition to the Long March 5 second-stage rocket using four 300N, four 150N and two 60N engines, the four sections of the Chang'e-6 probe (orbiter, returner, lander, ascender) are also equipped with a series of attitude control engines with thrusts of 150N, 120N, 25N and 10N.

Why does the Chang'e-6 exploration mission use hundreds of engines?

Now we can answer the original question: Why does the Chang'e-6 exploration mission use hundreds of engines?

This can be understood from two aspects:

On the one hand, the rocket is divided into three stages, and the Chang'e assembly is divided into four parts. The weight that the engine has to lift varies nearly a thousand times. Each part requires an independent engine system, and the thrust requirements and usage methods vary greatly, so there are many types of engines;

On the other hand, attitude control in different flight stages requires engines in different directions to ensure it. This can be understood by referring to the attitude control of an aircraft during flight: the weight and speed of the aircraft vary in a small range. In addition to the engine, a combination of a variety of aerodynamic structures such as ailerons, flaps, leading edge slats, spoilers, etc. are used to achieve flight attitude control. In addition to the speed control provided by the main engine, a spacecraft also needs a combination of engines to achieve flight attitude control. For example, the orbiter has 8 50N, 18 25N, and 12 10N engines, a total of 38 engines, and the ascender has 8 120N and 12 10N engines, a total of 20 engines.

It can be said that aerospace engines are destined to be diverse and changeable.

Author: Wu Baoyuan, Researcher at the Institute of Aerospace Propulsion Technology