Thrusters

A propeller is a device that converts any different form of energy into mechanical energy. For example, propellers in boats, helicopters, and engines in cars can be understood as propellers.

As early as centuries ago, the famous scientist Archimedes invented a device that uses a screw to transport water to irrigate farmland, called the Archimedean screw pump, which is the first time in human history invented a device similar to a propeller.

As time goes by, the level of human technology becomes more and more advanced, and the uses and types of propellers become more and more numerous. For example, there are propellers and vector thrusters on aircraft, marine thrusters on fishing boats or warships, and space thrusters on rockets. According to the different propulsion principles of thrusters, they are divided into a variety of types such as variable pitch propeller thrusters, duct propellers, jet thrusters, ion thrusters, etc.

These thrusters are different in type and principle, but they all serve the purpose of propelling the equipment into motion. China's Tiangong space station can continue to operate in orbit in outer space, all thanks to the Hall thruster on board.

Thrusters as the power to move objects, there must be enough force to push the object to move. China's Tiangong space station weighs 60 tons, but Hall thrusters on the ground thrusters can only push a piece of paper, why would we use such a "weak" thrusters?

Hall thruster that can only push a sheet of paper

The ion flow will form a Hall effect with the magnetic and electric fields under certain circumstances, after which the bound electrode ejected electron flow will form a Hall closed loop in the magnetic field. The electron flow in the magnetic field will accelerate the impact ionization, and the xenon gas produced by the impact will become the propulsion force of the Hall thruster, which is the working principle of the Hall thruster.

As early as the 1850s and 1860s, the United States and the former Soviet Union began to research and manufacture Hall thrusters, respectively. The scientific theories were not well developed at that time, so the researchers encountered many difficulties. Eventually, the Soviet Union took the lead in creating more efficient propulsion devices and also developed two types of Hall thrusters, the steady-state plasma thruster SPT, which relies on a wide acceleration zone, and the anode layer thruster TAL, which has a narrow acceleration zone. The United States, however, changed its research direction and began developing ion thrusters because the experiments did not go too well.

In the 1970s, the Soviet Union installed Hall thrusters on satellites for the first time. Afterwards, Soviet scientists found that Hall thrusters had unparalleled advantages over other thrusters, such as they did not need to carry a lot of fuel and could save a lot of money by reducing the weight. As a result, Hall thrusters were also widely used on several satellites.

Hall thrusters, one of the most advanced electric propulsion devices available today, are widely used in the space sector. For example, if a satellite wants to ensure a long period in orbit, it needs a thrust to help it counteract the yaw caused by gravity. Since Hall thrusters are relatively simple in structure, they are not prone to failure. Combined with its high specific gravity and high efficiency, it has become one of the preferred propulsion devices for space vehicles.

Hall thrusters have jet speeds of up to 10-80km/s (specific gravity values of 1000 - 8000s), but the vast majority of models have jet speeds of 15-30km/s (specific gravity values of 1500 - 3000s). Although the propulsion speed is surprisingly high, but we examine whether a thruster has enough force to propel the equipment, the most important thing is its power.

In fact, the thrust of the Hall thruster has a lot to do with the power, because thrust is directly proportional to power. For example, a 1.35kw thruster can produce 83 mN of thrust, while higher power models of Hall thrusters are even able to reach 3 Nm of thrust in that laboratory environment. But on Earth, even the force of 3 Nm is only the size of the gravity of half a bottle of mineral water, which is probably about 300 grams.

In China's independent research and development of the Tiangong space station, equipped with four LHT100 Hall thrusters. The thrust of a single Hall thruster on board the Tiangong space station is only 80 mN. How much force is 80 mN on Earth? It is equivalent to pushing a piece of paper.

Tiangong space station is a 60-ton body behemoth, why on the ground can only push a piece of paper Hall thrusters can be used?

First of all, the reason why space stations in the space environment carry thrusters is because they can deviate from their established orbits due to gravitational effects or other reasons in their operational orbits. If it accidentally enters the atmosphere, there is a risk of crashing. The use of thrusters is to be able to periodically correct the trajectory of the space station to avoid crashes.

Second, space is a vacuum environment, where the object movement is completely free of resistance. The force of a milli-Niu unit on Earth can only push a piece of paper because the Earth resistance is too large, but the vacuum environment in space will not have such trouble. So even milli-Niu units of Hall thrusters can also push 60 tons of the Tiangong space station.

It can be said that the space station is sent into orbit under normal operation is no other boosting force, but need a "steering wheel" to correct the trajectory. So the station's thrusters do not need too much power, but need to use more precise force to adjust their position.

Hall thrusters are perfectly suited to the needs of the space station. Because the large thrust is likely to be a slight force to push the space station to a hundred thousand miles away, only a small thrust to achieve precise fine-tuning in space without resistance.

What's more, Hall thrusters have a longer life span, despite their low thrust. The Tiangong space station, as a small base for a long stay in outer space, needs a thruster with stamina. If you use chemically-fueled thrusters, not only do they need to be refueled frequently. It is more likely to "retire" earlier than the space station because of the life span problem.

So our weak Hall thruster is like the little turtle in the race of the tortoise and the hare, which runs slowly but is more suitable for the long-term development of the space station.

Thruster to the rescue

There have been historical Hall thruster rescue emergencies, and in 2003, Japan launched an asteroid probe into space. The asteroid probe, called Hayabusa, was preparing to return to Earth at the end of its mission.

But the main thrusters, which were powered by chemical fuel, suddenly failed. Japanese experts on the ground were anxious but could not do anything about it, because if the unmanned probe failed in outer space, it would basically be abandoned.

But at that time, the Hayabusa asteroid probe also carried a Hall thruster as an auxiliary thruster. In order not to waste the successful probe launched by the country at great expense, experts had to activate the Hall probe to propel the Hayabusa asteroid probe back. Later, the Hall thruster finally brought back the probe, although it took 3 years longer than the chemical fuel thruster, but this is enough to prove that the Hall thruster has a longer life and is more suitable for long time work.

Unlike other thrusters with conventional chemical fuel as propulsion, Hall thrusters have a fairly high specific impulse. While conventional engines, even the highest specific impulse hydrogen-oxygen machines, have a specific impulse of 450s, the Hall thrusters on the Tiangong space station have a value of 1500 - 3000s, indicating that Hall thrusters can provide more speed increments than fuel thrusters with the same fuel consumption.

Therefore, the Hall thruster, which can only push a piece of paper on the ground, has become an important propulsion force for China's Tiangong space station. Although the Hall thruster carried by China in the space station is more advanced than the propulsion method of the International Space Station, it is not long since China developed the Hall thruster.

Hall thrusters help mankind reach for the stars

The thrust is so weak because the accelerated ions of Hall thrusters all operate in very small units and the mass of ions ejected is very limited. However, as one of the important thrusters in space, there are at least 200 or more Hall thrusters operating in space today. So Hall thrusters have an unshakable position in the field of human spaceflight development, which helps mankind to move towards the sea of stars.

Now, our use of Hall thrusters in the Tiangong space station has become a space equipment that the whole world is watching. China has also taken the initiative to open the right to use the Tiangong space station, free of charge to the world. More than twenty countries have already taken the initiative to apply for the qualification to enter the Tiangong space station, hoping to use the unique environment of the space station to develop more results about the space field.

The U.S. Starlink satellite, which also carries a Hall thruster, weighs 227 kg and has a propellant weight of about 81 kg. The Hall thruster it carries is the first time humans have used Krypton as a propulsion tool.

In fact, most people have a misconception that when humans are driving a vehicle in outer space, they think that there will always be power jets to boost the movement of the vehicle. But in fact, the vehicle will only move by propulsion when it is out of the range of gravity.

For example, the Apollo moon vehicle into space after no other thrusters to provide it with thrust. Thrusters only after pressing the launch button to run for just ten minutes and then did not continue to work, the vehicle in space "floating" for three days before reaching the moon.

If we later manufacture Hall thrusters with greater thrust, then the human-made vehicles in space navigation speed will be greatly increased. The various advantages of Hall thrusters themselves will also amplify the scope of the human journey and help mankind to reach the starry sea.