Tiangong already has Hall thrusters, so why is it necessary to transport chemical fuel up there?

At 14:22:22 on July 24, 2022, the Long March 5B Yao-3 carrier rocket carrying the Wentian laboratory module was successfully launched into space.

The total launch weight of the Wentian experimental module exceeded 23 tons, of which the dry weight of the entire module was about 21.5 tons, and there was another 1,550 kilograms of hydrazine fuel. The so-called hydrazine fuel is actually what we often call unsymmetrical dimethyl hydrazine and nitrogen tetroxide, both of which are chemical fuels. This has caused some confusion. As we all know, chemical fuels are heavy, take up a lot of space, and have high transportation costs. They are not ideal aerospace power sources. The Tiangong space station uses the LHT-100 Hall electric thruster, which uses element 54 xenon as fuel and does not need chemical fuel at all. Why do we need to transport so much chemical fuel to the space station?

What is a Hall Thruster?

Hall thrusters are a type of electrostatic thrusters. In addition to electrostatic thrusters, the electric thruster family also includes electromagnetic thrusters and electrothermal thrusters. As the name suggests, Hall thrusters utilize the Hall effect, which was discovered by physicist Hall in 1879. Simply put, when an electric current passes through a semiconductor perpendicular to the external magnetic field, an additional electric field will be generated in a direction perpendicular to the current and magnetic field, so a potential difference will be generated at both ends of the semiconductor. The Hall thruster consists of a device that can produce the Hall effect and an electron source. In the Hall thruster, the electrons emitted by the electron source will form a ring-shaped electron beam due to the Hall effect, which makes the Hall thruster look quite sci-fi.

When we input xenon gas into the Hall thruster, the high-speed electrons in the circular electron beam will ionize the xenon gas. The ionized xenon ions will be ejected backwards at high speed, thus generating thrust.

Hall thrusters look very high-tech, but after understanding the principle, we will find that there is no essential difference between Hall thrusters and traditional chemical thrusters. They both rely on the reaction force generated by throwing things backwards to propel forward. The only difference is that chemical thrusters throw out high-speed jets generated by the combustion of chemical fuel, while Hall thrusters throw out ionized xenon ions. This is also the advantage of Hall thrusters, because they only throw out ions, so the fuel consumption is very small, and electricity is something that is readily available in space.

If there is any disadvantage of the Hall thruster, it is the lack of thrust.

The action force is equal to the reaction force. Since only some ions are thrown out, the thrust that can be generated is naturally not that great, usually only a few Newtons. A few Newtons of thrust can basically not push anything on Earth, but it is different in space. There is no resistance in space, so only a little thrust is needed to make the spacecraft move. Therefore, it is very suitable to install Hall thrusters on spacecraft, and there is still room for development in the future. Not long ago, my country's HET-450 single-channel Hall thruster achieved a thrust of 4.6 Newtons in ground experiments. Since Hall thrusters are so suitable for installation on spacecraft, and the Tiangong space station has indeed used Hall thrusters, why do we need to transport so much chemical fuel up there?

Although the Hall thruster is good, it is not omnipotent. Its biggest drawback is that its thrust is too low, which makes it seem a little powerless when facing some special situations.

As we all know, there is a lot of space debris in space, and the space station is likely to overlap with the orbits of these space debris during its operation. If these space debris hit the space station, it will cause immeasurable losses to the space station. Previously, the robotic arm of the International Space Station was hit by a very small space debris, resulting in an extremely obvious hole. Although space debris is terrible, its trajectory can be predicted in advance, but it is not enough to predict in advance, and it must be able to avoid it in time. The thrust of the Hall thruster is very low. If the space debris is discovered a month in advance, it can be avoided, but if it is only a few days in advance, the Hall thruster will not work, and chemical fuels must be used at this time.

In addition to avoiding space debris, chemical fuels are also needed when docking with spacecraft.

There is a difference between the orbits of the launched spacecraft and the Tiangong space station. In order to dock successfully, the spacecraft must gradually adjust its position and catch up with the space station, which greatly prolongs the docking time. However, if the space station uses chemical fuel as power and adjusts its posture and position by itself, the docking time can be greatly shortened. This kind of task that requires rapid adjustment in a short period of time is definitely not possible with the tiny thrust of the Hall thruster. However, all this is only temporary. With the continuous breakthroughs in high-power Hall thruster technology, it is still possible for our spacecraft to completely get rid of chemical fuels in the future.

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