2020-07-15

Ion thruster, our goal is the star sea! The interstellar expedition is not far from us

By yqqlm yqqlm

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When Musk’s Starlink satellite launch was widely reported by the media, I noticed a sentence,”In the next few months, they will gradually enter the 342 miles above the earth with the help of airborne ion thrusters. Book a track.” Where is this ion thruster sacred?

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Starlink satellite krypton ion thruster (picture from Starlink official information)

Speaking of ion thruster , We must first talk about the traditional chemical rocket engine. As we all know, traditional rocket engines are propelled by chemical fuel. The rocket engine will be loaded with fuel and oxygen additives before it is lifted off. The rocket engine does not contact the external air, so the oxygen agent is equivalent to the oxygen in our air and can help the fuel burn. The heat released by the combustion of chemical fuel is converted into kinetic energy through the nozzle at the bottom of the rocket to propel the rocket to heaven.

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The principle of liquid rocket propulsion

However, traditional rocket engines use kerosene or liquid hydrogen as fuel , Its fuel-driven efficiency is relatively low. Even if the kinetic energy utilization rate can be improved by lengthening the bottom nozzle, it still has a permanent solution. Rockets powered by chemical fuel are only suitable for short-distance flight. In order to increase propulsion speed and endurance, the fuel carrying capacity can only be continuously increased.

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Space shuttle carrying high-load fuel

This is also the status quo of traditional rockets:low efficiency; Specific impulse is low (in seconds, refers to the duration of one kilogram of fuel to produce one kilogram of thrust); carry more fuel, low payload.

What is the difference between ion thrusters?

The working principle of the ion thruster is:first ionize the propellant, and then use the electric field to accelerate the ions to form a thrust, and at the same time spray electrons to the emitted ion beam, so as to neutralize the positive ion flow and let the spaceflight The device itself does not accumulate a large amount of charge.

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Ion thruster

The source of the concept of ion thruster can actually be traced back to a long time ago. In 1917, Goddard, an assistant professor at Clark University, filed a patent application, and this published patent is considered the earliest electric propulsion system.

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The electric propulsion system patent US1363037-0 applied by Goddard

Limited to the technical conditions of the year The ion thruster was not really developed. Later, with the development of technology, electrostatic thrusters, electromagnetic propulsion systems, Hall thrusters, etc. appeared one after another.

  • Static thruster

In 1959, American physicist Harold R. Kaufman manufactured the first practical electric propulsion device at NASA, and named it ion thruster. The principle is that a hollow cathode discharge tube releases electrons outward, and at the same time shoots the electrons into the ionization chamber, the electrons bombard the gas in the gas chamber (such as mercury vapor, xenon, etc.), and then ionize the gas into positively charged ions And ejected to generate thrust. Subsequently, the neutralizing ions do not charge the thruster. This type of ion thruster is also known as bombardment ion thruster.

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Schematic diagram of electrostatic thruster (electron bombardment ion thruster)

  • Electromagnetic propulsion system

1954 The Soviet Union’s Kurchatov Atomic Energy Research Institute developed an electromagnetic propulsion system. The electric pulses emitted by this propeller ablate the propellant into a plasma. Then the plasma is ejected backwards at high speed under the acceleration of the electromagnetic field, forming a thrust. Its propellant is selected from polytetrafluoroethylene. As a propellant, PTFE not only occupies a small volume, but also can be stored for a long time in a high vacuum and extremely low temperature environment, which is very stable and reliable. In November 1965, the Soviet Union’s Venus 2 deep space probe was equipped with an electromagnetic propulsion system.

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Early pulsed plasma thruster

  • Hall thruster

Hall thruster can be a thruster The star of the world has used it as a research object for deep space thrusters in recent years. Hall thrusters are developed with reference to the Hall effect. The principle is that in the thruster, the propellant is accelerated by the electric field. The Hall thruster confines the electrons in the magnetic field and uses the electrons to ionize the propellant to accelerate the ions to generate thrust and neutralize the ions in the plume.

In 2001, an Ariane-5 rocket put the French communications satellite STENTOR into orbit. And four Hall thrusters were installed on the satellite.

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Smart-1 equipped with Hall thrusters

The advantages of ion thrusters are obvious, the volume Small, long stroke, less fuel, and high propulsion efficiency. But the ion thruster still has obvious shortcomings, because the power is too low, so the thrust is very small, only a few tens of milli-newtons, about the weight of a piece of paper. Compared with the rocket thruster, it is really a little bit. As a result, ion thrusters are now generally used for satellite propulsion correction to push the satellite into a predetermined orbit.

Back to the beginning of this article, Musk’s star chain satellites use ion thrusters as orbital power, because each star chain satellite is only more than 200 kilograms, and it is better to be equipped with a small ion thruster. Even if the thrust is small, as long as the continuous acceleration is superimposed, the final efficiency is considerable.

For the benefit of mankind, of course our goal is not limited to surrounding the earth, it should be the sea of ​​stars! With ion thrusters, deep space navigation may no longer be an illusion.

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Imaginary spaceship equipped with ion thrusters

Now known to go to Mars, if you use traditional rocket thrusters, you need 6 month. But if you use an ion thruster, it only takes 39 days! This greatly saves costs! It’s incredible!

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Mars

So what is the limit speed of the ion thruster?

The Deep Space One probe in the United States, after a few years of launch and flight, its speed increased to an astonishing 81.58 kilometers per second! You know that Voyager 1 was launched in 1977 and has been flying for 43 years. Now it is only 17 kilometers per second. It’s unbelievable!

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Solar ion thruster on Deep Space No. 1 detector

Although 81.58 kilometers per second The speed is not a big deal for the entire universe, but at least it is a good start. Science needs exploration, technology needs innovation, and experience needs accumulation. We look forward to ion propulsion technology to a higher level and bring us to know more stories about the universe!


Reference:

1.https://en.wikipedia.org/wiki/Ion_thruster

2.https://en.wikipedia.org/wiki/Pulsed_plasma_thruster

3.https://en.wikipedia.org/wiki/Hall-effect_thruster