This article is from WeChat official account:The elementary school monk at the foot of the Laohe Mountain (ID: gh_586564bb7b66)< span class = "text-remarks">, author: old school and mountain monk, from the title figure: the movie “wandering the Earth”

Ion engine, photon engine, Hall thruster… Don’t worry, let’s draw a boundary for these sci-fi concepts.

As of today, all human rocket engines rely on the reaction force generated by throwing things out to obtain power. The academic point is the law of conservation of momentum.

The existing physical theories have the same level of understanding of gravitational space as the magic stick, and there is no engine principle other than the conservation of momentum.

Therefore, for a long time, the engine has to generate power by throwing things. The faster you throw, the greater the power.

I’m very sorry, it’s Newton’s third law after all these years.

Rocket limit

So, any engine that wants to go to the sky is designed around “how to throw fuel faster”.

For example, in an airplane engine, in order to throw it faster, it desperately dumps fuel and requires air for combustion, so an exhaust fan is installed in the engine. However, the oxygen content of the air is only 20%. Even if the air is compressed into a liquid by the exhaust fan and stuffed into the combustion chamber, there is not much oxygen, so the fuel injection speed is not fast, only a few hundred meters per second, which is simply for the rocket. Just stuff your teeth.

The rocket engine comes with pure oxygen, and of course it burns more fully, and the fuel injection speed can reach 2~5km/s. Unfortunately, after so many years, the best fuel is still Liquid hydrogen and liquid oxygen, if you want to be faster, you have to use explosives.

The detonation speed of explosives is generally 7km/s, but using explosives as fuel is too much for most people to eat.

In general, if you don’t consider the not yet practical total nitrogen anion salt, this is probably the limit of chemical fuels.

Speaking a bit abstract, this monk will show off hard-core computing. Assuming that 20 tons of cargo are sent to low-Earth orbit, the air resistance, potential energy and dry weight of the rocket are not considered:

If the fuel injection speed is 600m/s, 10 million tons of fuel are required. This is the level of aircraft engine.

If the fuel injection speed is 2.5km/s, 451 tons of fuel are required. This is the pass line of the current mainstream rockets.

If the fuel injection speed is 5km/s, 77 tons of fuel are required. Close to theoretical limit of current chemical fuels.

If the fuel injection speed is 30km/s, 6 tons of fuel are required. Hall thruster jet velocity.

If the fuel injection speed is 300km/s, only 534 kg of fuel is needed. It is said that this is the full potential of the ion engine that can be squeezed out by existing technology alone.

In theory, as long as the voltage is large enough, ions can approach the speed of light infinitely. For example, the collider can accelerate particles to more than 99% of the speed of light. If this is regarded as the speed at which the engine sprays fuel… It only needs 532 grams of fuel to send to low Earth orbit.

The prospects are pretty good, and it’s time for bitterness.

Ion engine

You may not have imagined that a sci-fi device like the ion engine was actually made by NASA’s Kaufman in 1959.

Kaufman and his ion thruster

The general principle is similar to that of an electromagnetic gun, as shown in the figure below: First, inject mercury into the ionization chamber, then inject electrons into the electron gun, and bombard the mercury atoms with the coil to accelerate the electrons to produce mercury ions. The electric field generated by the grid accelerates the ejection to generate thrust.

An electron gun is installed near the exit to inject electrons into the ion beam to neutralize it and restore it to atoms. Otherwise, charged ions will cover the entire spacecraft, and electrons will accumulate inside the engine to generate static electricity, which will cause various troubles.

To put it simply, Ionized with an electron gun in the front and accelerated with an electric field. The principle is clear at a glance. However, considering the propulsion efficiency and corrosivity, most of the propellants used today use the noble gas xenon.

In 1998, the US “Deep Space 1” comet probe used the ion engine as the main propulsion system to fly in deep space for the first time. The engine weighs only 8 kilograms and carried only 82 kilograms of xenon for 20 months of flight.

Ionization does not necessarily have to be bombarded with electrons. For example, you can get chloride and sodium ions by throwing salt into water. For example, microwave can also directly ionize gas.

There is a big news in 2019. The Japanese Hayabusa 2 probe successfully sampled an asteroid 300 million kilometers away, using a microwave ion engine and microwave ionization.

Landing, sampling, and ascending on the asteroid all in one go.

The principle and structure of ion thrusters are not complicated, but there are many problems. For example, the acceleration grid of the positive and negative electrodes at the back is very in the way. Not only is it not efficient, but also has to withstand the impact corrosion of high-speed ions, which requires high levels of material.

So everyone thought of Hall.

Hall thruster

Compared with the ion thruster with a simple principle, the principle of the Hall thruster is a bit more brainstorming.

Let’s talk about the Hall effect: When a current passes through a magnetic field, electrons or ions will move laterally, causing a lateral potential difference in the conductor. It is as if the Yangtze River has a horizontal force that makes the water level on the south bank higher than that on the north bank.

The advantage of this is that when the electronic (negative) and the ion ( Positive electricity) When mixed together, the Hall effect can be used to separate, so as to accelerate while ionizing. In other words, the Hall thruster combines the ionization part and the acceleration part, that is, the magnetic field and the electric field are combined, and the acceleration grid that was originally in the way is abandoned.

In order to allow the mixed electrons and ions to perform their duties, the design of the magnetic field and electric field is obviously more sophisticated, and the nozzle has changed from a sieve to a ring structure.

The Eight Immortals Cross the Sea

Ion thrusters and Hall thrusters are the two most mainstream applications, and the difference between the two is clear at a glance.

The principle is adjusted again, and there are many types, such as: pulse plasma thruster, magnetic plasma thruster, arc heating thruster and so on.

The basic idea is to ionize atoms first, And then blast it out with an electric field. As for the principle, the monk is only responsible for completing this group of German pictures. It depends on your good fortune to understand how much you can understand:

But to be honest, because they are essentially tossing ions, there is nothing wrong with calling them “ion thrusters” in general. Also, engines, thrusters, and propellers are just called differently, and they all mean the same thing.

Ion thrusters have the advantage that they are not picky, and easily ionized atoms can be used as propellants. Xenon is the current mainstream propellant, but it is a rare gas after all, which is very precious. Therefore, other propellants are also very popular, such as zinc, nitrogen, iodine, magnesium, bismuth, etc., with different spectra, which is really beautiful.

It looks like the situation is pretty good. Why haven’t I heard of ion thrusters before? Of course this is because you are ignorant!

Accumulate to a thousand miles

Although the nozzle speed of the ion thruster is an order of magnitude faster than that of the chemical engine, the ejection is too little, and the final thrust is really small, so they all use “minew” as a unit, which is similar to the force of farting.

So from the ground to space, we still have to rely on chemical rockets, and the stage of ion thrusters is in space.

In 1998, on the US Deep Space 1, the ion engine nozzle speed was close to 30km/s, but it could only spray 100 grams of fuel for a whole day of work, which only increased the speed of the satellite by 10m/s, and the thrust was only 90 mN. , Equivalent to 9 grams of weight.

Fortunately, this thing is really long-lasting. I worked for 14,000 hours and accelerated and decelerated variously during the period. I came and went with the asteroids and made great achievements.

Ion thrusters are not really black technology, and deep space flight has been widely adopted. Generally speaking, 2 million kilometers away from the earth is considered “deep space.” In such a place, heavy chemical rockets are just waste.

In 2007, the US Dawn asteroid probe, 3 ion thrusters, thrust 92 mN, and accumulated 6.9 billion kilometers for 11 years.

BepiColom launched by European Space Agency in 2018The bo Mercury probe, 4 ion thrusters, with a total thrust of 290 mN, plans to fly 9 billion kilometers for 7 years.

Even though the European SMART-1 is only for detecting the moon, it also uses ion engines as its main thruster.

There is also the Hayabusa, which took samples from an asteroid 300 million kilometers away. The four ion thrusters on the bottom are particularly eye-catching.

In addition to deep-space flight, orbit change climbing near the earth, there are also ion thrusters. Because ion thrusters rely on electricity, they are also called electric thrusters.

In 2015, SpaceX launched two all-electrically propelled communication satellites, and installed four XIPS-25 ion thrusters with a thrust of 165 mN.

These two satellites climbed from the transfer orbit to the 36,000 kilometers high synchronous orbit by ion thrusters:

OriginalThese kinds of orbit changes near the earth are all chemical rockets, which can be completed in a few hours, but require several tons of fuel. The ion thruster is the opposite, small and exquisite and low in cost, but the process of changing orbit takes several months.

Obviously, the patience of everyone has been getting better in recent years. The proportion of commercial satellites using all-electric propulsion has risen sharply, and it is estimated that it has exceeded half.

The threshold of ion thrusters is about to be broken: XIPS ion series from L-3 in the United States, BHT Hall series from Busek, T Hall series from AMPAC-ISP, and μ microwave from Japan Series, British T ion series, German RIT radio frequency series, Russian SPT Hall series……

If the previous businesses were robbed of Chemical Rockets, then the following activities are based on their own ability.

At the end of the autumn leaves, the world will be determined

Everyone may not have thought that the small thrust actually has an advantage: precision!

If you think about it, a chemical rocket will be stormy with a light breath. It’s too difficult to control the intensity. The thrust is small, but it can be precisely controlled, especially for those satellites that require extremely high precision.

The GOCE probe launched by the European Space Agency in 2009 mapped the Earth’s gravitational field with unprecedented accuracy. Due to the high measurement accuracy requirements, the satellite must fly in a low orbit of about 250 kilometers, and there are many air molecules at this altitude, and the slight air resistance will cause the satellite’s altitude to gradually drop.

So, GOCE installed ion thrusters to offset the slight air resistance. This product continued to work for 2 years, allowing the satellite to always maintain a precise orbit, showing unparalleled advantages.

Nowadays, the orbit positioning and attitude control are completely the world of ion thrusters. But this capability is actually nothing, the real uniqueness of the ion thruster is here:

Remember how the ground gravitational wave was detected. The gravitational wave caused a displacement of 0.00000000000000001 meters on the mirrors separated by 4 kilometers. If you want the displacement to be more significant, you have to increase the distance between the mirrors. This distance on the earth can no longer meet the requirements, so everyone thought of space.

Laser interference space antenna LISA, the space gravitational wave detection program jointly developed by the U.S. and Ou Meng, is planned to be completed in the 2030s. Three satellites will form a triangle with a side length of 2.5 million kilometers, and the laser will take 25 seconds to complete the distance. Interested friends, give you an official website:

Now the problem is coming. The essence of gravitational wave detection is to measure tiny movements. If the satellite keeps shaking, then it can’t be done.

Therefore, the satellite must be truly “stationary” in space and become an ultra-quiet and ultra-stable platform. The academic point is “towing-free control” technology, and the popular point is to eliminate various interference forces on the satellite.

For example, the light pressure generated by the sun shining on the satellite, the tiny thrust generated by the cosmic rays hitting the satellite, the drag caused by scattered air molecules escaping into space, and so on.

Needless to say, this kind of activity is none other than a non-ion engine.

End of 2019The Long March 4 sent China’s “Tianqin One” to the sky, and the ion engine thrust accuracy reached 0.0000001 N. It successfully completed the flight verification without towing control, becoming the second country in the world to master this technology.

It is estimated that in 2035, 3 Lyra satellites will be used to form an equilateral triangle with an arm length of 170,000 kilometers at an altitude of 100,000 kilometers to build China’s space gravitational wave detection system.

From the point of view of parameters, Lyra is still a little bit behind LISA, but who makes us more money? The “Tai Chi Project” with a distance of 3 million kilometers between the stars is already on the agenda…

Where are the stars and the sea

Although there are not many jobs currently available for ion engines, they are more worth looking forward to than chemical engines. We might as well look forward to it.

The little friends who watch a lot of science fiction movies usually have a misunderstanding, thinking that the spacecraft drives the engine all the way to the moon and to Mars. In fact, whether to go to the moon or to Mars, the spacecraft accelerates to a certain speed near the earth and then floats past without power. Like an intercontinental missile, it is thrown away.

For example, it took Apollo to land on the moon for 3 days from the earth to the moon, but the engine only worked for 1010 seconds, ten minutes.

Hurry up here