A few days ago, the 502 Institute of China Aerospace Science and Technology Corporation successfully developed China's first magnetic focusing Hall thruster. It adopts a new generation of Hall thruster technology, which represents the current mainstream development direction in the world. Compared with similar foreign products, performance indicators in specific impulse and efficiency can be increased by more than 20%. The Hall thruster, whose name sounds very cold, is actually one of the electric propulsion systems. The electric propulsion system is also a type of rocket. It has a very popular name called an electric rocket.
In space, the electric rocket has its own advantages
Electric rockets are different from the chemical rockets currently used to launch spacecraft. Chemical rockets generate high temperatures by propellant catalytic reaction or oxidant combustion agent reaction. The propellant is heated and ejected. The electric rocket does not use chemical fuel, but uses electric energy to heat or ionize the propellant to accelerate the jet to generate thrust.
Or to put it this way, a chemical rocket is a propellant that reacts chemically to generate energy and uses it to propel the rocket. The electric rocket, the propellant itself does not produce energy, it is the use of external electrical energy into propellant jet kinetic energy, and promote the rocket.
Both rockets consume propellant, but the consumption is very different. For the same space mission, the electric rocket is highly efficient, and it consumes about 1/10 or less of the propellant of the chemical rocket. Its electrical energy is provided by the aircraft, which is generally obtained by converting solar energy, nuclear energy or chemical energy. Currently, solar energy is mainly used, so it is also called a solar electric rocket. Some countries are developing nuclear-powered electric rockets. Propellants commonly used hydrogen, nitrogen, argon or alkali metal vapors.
There is also a concept related to this, called impulse, which is the impulse generated by the consumption of unit mass propellant. The greater the specific impulse, the higher the technical performance of the propulsion, and the size of which depends mainly on the ejection speed of the propellant. As mentioned before, the specific impulse of an electric rocket is much larger than that of a chemical energy rocket. In addition, it has a series of advantages such as high efficiency, low fuel consumption, low vibration, low cost, long life, and safety. The electric rocket engine has the advantage of long service life, which can be started thousands of times and accumulated work for tens of thousands of hours.
Electric rockets have so many advantages, but they also have a huge disadvantage: they have a small thrust of less than 100 cows. Therefore, if you expect to use an electric rocket to launch a spacecraft from the ground into space, it is absolutely impossible. At present, the launch of a spacecraft can only be a place where chemical rockets can display their abilities. However, once it is in space, the situation is very different. For spacecraft attitude control, position maintenance, orbit change, and interplanetary navigation, electric rockets have advantages that cannot be matched by chemical rockets. However, when electric rockets are used for orbital changes, they need to be used. The time is much longer than the chemical rocket.
R & D for several decades, all-electric propulsion has matured applications
Actually, the theory of electric propulsion was introduced as early as the early 20th century. However, due to the relatively complicated technology, it was not until the late 1950s that some countries began to study the engineering of electric thrust systems. To date, there are about hundreds of man-made earth satellites and space detectors in the world, and electric propulsion systems have been used or are in use.
The kinetic energy of the chemical rocket is limited by the chemical internal energy of the propellant, and the propellant ejection speed is limited, which is about 2 to 4 km/s. The electric rocket breaks this constraint and the ejection speed can reach 15 to 80 kilometers. In seconds, it is easy to achieve one-step higher specific impulse performance than chemical propulsion. Theory and practice have proved that the electric rocket is 10 times more efficient than the current standard chemical rocket, and it requires less propellant to accomplish the same task, which can increase the payload of the spacecraft, or increase the amount of propellant to extend the spacecraft. The service life of the device. It can increase the effective quality of spacecraft to about 90%. It is currently the most advanced space propulsion technology in the world.
At present, a 15-year-long high-orbiting communications satellite weighs about 5 tons, of which the chemical fuel weight is up to 3 tons. If all electric propulsion schemes are adopted, the satellite can be "slimmed down" to 2.5 tons, making communication satellites even smaller. Cheaper rocket launches. In addition, all-electric push technology can greatly extend the lifespan of communications satellites. Fuel carrying capacity will no longer be a constraint on the lifespan of satellites. The design life of communications satellites will generally exceed the current upper limit of 15 years and reach 18 to 20 years.
In recent years, the electric propulsion system has developed rapidly. For 2015 alone, the cumulative operating time of the first ground-borne 200-mm ion propulsion system developed by the 510th Institute of China Aerospace Science and Technology Corporation has exceeded 11,000 hours. With the ability of satellites to operate reliably in orbit for 15 years, this indicates that China's self-developed electric propulsion system has reached the international advanced level and will fully enter the stage of engineering application, and it can meet China's communication satellite series platform, high-rail remote sensing platform and deep space. The development needs of the detector.
The most important result of the electric propulsion system in 2015 was the successful launch of the world’s first all-electric commercial communications satellites made in the United States on March 2. Satellites used ion thrusters to complete all missions such as orbit transfer and position maintenance. The full application of electric propulsion technology to achieve mature applications also means that satellite technology development and application space are undergoing major changes.
Electrical energy rotation, two types of propulsion are more favored
According to different accelerating methods for converting electric energy into propellant kinetic energy, electric rockets can be classified into three types: electrothermal type, electrostatic type, and electromagnetic type. Each type can also be divided into multiple types.
Electrothermal thrusters use electric energy to heat propellants (such as helium, ammonia, hydrogen, etc.) to vaporize them, and they are accelerated by the expansion of the nozzles to generate thrust. Its specific impulse is 700 to 1000 seconds.
Electrostatic thrusters are used to ionize propellants (such as mercury, helium, hydrogen, etc.) from the tank through the ionization chamber, accelerate the formation of beams under the action of the electrostatic field force, and generate thrust. Its specific impulse is 8500 to 20000 seconds.
Electromagnetic thrusters use the principle of the electromagnetic field to generate Lorentz force on the plasma, which accelerates the propellant in a neutral plasma state to generate thrust. Its specific impulse is 5000-25000 seconds.
At present, there are two kinds of electric propulsion systems that are widely used and mature, namely Hall thrusters in electromagnetic thrusters and electron impact ion thrusters in electrostatic thrusters. The two thrusters are essentially the same. They use electrical energy to ionize the inert gas, helium, to form a plasma of ions and electrons. The ions are accelerated by the electric field to produce thrust.
The difference is that the ionization thruster is separated from the ionization zone and the acceleration zone, so the thruster has higher efficiency, higher specific impulse, and less consumption of propellant. The disadvantage is that the technology is complicated, the types of power supply are numerous, and the size and weight are relatively large; The ionization zone and the acceleration zone of the thruster are combined, so the technology is simple, the type of power supply is small, the size and weight are smaller, and the reliability is higher. The disadvantage is that the impulse is lower. Both have their own advantages.
At present, Boeing and Japanese satellites mainly use ion thrusters, and Laura, Loma, and Thales-Alenia, Airbus and other companies use Hall thrusters.
Extended reading
The future of nuclear rockets
At present, the electric propulsion system has been widely used. There are about 100 spacecrafts that are currently in orbit using electric propulsion systems. The cumulative working time of ion thrusters is about 200,000 hours, and the cumulative working time of Hall thrusters is about 100,000 hours. They are mainly applied to the maintenance of the position of geostationary satellites and the orbital change of space probes.
In July 2001, the launch of the “Artemis†satellite in Europe caused the failure of its launch vehicle. The satellite did not enter the scheduled orbit. Finally, the electronic rocket on the satellite eventually reached the scheduled orbit after 18 months of orbital shift. In August 2010, the U.S. advanced ultra-high-frequency satellite No. 1 was also launched. Due to the failure of the satellite chemical rocket, it finally relied on the electric rocket on the satellite to complete almost all orbital shift missions within 14 months.
In May 2003, Japan launched the "Starling" asteroid sampling probe in the case of a chemical rocket failure, using four ion thrusters to complete the sampling return of a near-Earth asteroid.
In September 2007, the United States launched the "Dawn" asteroid probe with three ion thrusters as its main engine, and probed Vesta and Ceres.
China began research on electric rockets since 1967. In 1978, the LIPS-80 ion thruster developed by the 510th Institute of China Aerospace Science and Technology Corporation won the first prize of the National Science and Technology Progress Award. In October 2012, China launched the launch of the No. 9 satellite and successfully validated the accuracy of various electric propulsion technology solutions, on-orbit work performance, compatibility with spacecraft, and long-term on-orbit work capabilities. This means that China's all-electric propulsion system has initially had in-orbit application capabilities.
In the future, our country's electric rockets can be used for geostationary orbit satellite position maintenance and orbital transfer, space probes and main advancement for manned deep space exploration, orbit maintenance for low earth orbit satellites, attitude and orbit control of spacecrafts and many other aspects. Foreign countries are currently developing nuclear-powered manned Mars spacecraft using nuclear-powered electric rockets to reduce manned rockets’ time to and from Mars, reduce the impact of cosmic radiation and long-term weight loss on the body, and at the same time reduce the number of life-supporting substances.
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