Ion engines are also called ion thrusters, ion drives, or ion propulsion. In Japan, it attracted attention for its role in the asteroid explorer Hayabusa2. Although not high in thrust, engines that are efficient in the thrust they provide from fuel energy are an essential technology for deep space exploration. This article describes the structure, applications, and construction of the ion engine.
An ion engine is an electrostatic acceleration propulsion system that obtains thrust by electrostatically accelerating plasma. They are mainly used to provide propulsion for space probes and satellites.
The propulsion system of spacecraft, such as space probes and satellites, basically uses the reaction force generated when gas or other substances are injected for propulsion. The speed of this injection is called the exhaust velocity.
Spacecraft propulsion systems can be broadly classified into two types: chemical propulsion systems (chemical rockets) and electric propulsion systems. A chemical propulsion system obtains propulsion by combustion, a type of chemical reaction. While chemical propulsion systems have very high thrust, they are characterized by low specific impulse, which corresponds to the fuel consumption of the engine.
Specific impulse is the number of seconds that one kilogram of propellant can continue to produce one kilogram of thrust. Solid fuels used in chemical propulsion systems last 300 seconds, and liquid fuels last about 500 seconds, while ion engines can last 2,500 to 4,000 seconds.
Chemical propulsion systems have low specific impulse but very high thrust, so they are also used for launching satellites and space shuttles.
An electric propulsion system is an engine that uses electrical energy to heat or accelerate propellant and exhaust it to obtain thrust. Although it has less thrust than chemical propulsion systems, it can go longer with less fuel because of its higher specific impulse.
This makes it possible to reach distant destinations and is used for deep space exploration and satellite orbit correction.
The Ion engine is one of the electric propulsion system engines. With ion engines, the exhaust velocity can be increased by a factor of about 10 compared to the injection of chemically propelled engines.
The ion engine first turns the fuel into plasma and extracts only ions from it. The mechanism is to accelerate those ions to obtain thrust. Therefore, the ion engine consists of three main areas.
This part ionizes fuel (propellant) to produce ions. Propellant (mainly noble gases) is heated by arc discharge or microwaves to ionize it into plasma. There are three types of plasma generation methods: DC discharge, RF (Radio Frequency) induced discharge, and microwave discharge.
The microwave discharge type does not require electrodes and is not limited by their life span. It also has features such as no need for protection against atmospheric exposure when testing on the ground. Furthermore, no special procedures or additional equipment are required for plasma ignition, simplifying and extending the life of the entire ion propulsion system.
This is the part that accelerates the ions produced by the Ionization process to obtain thrust.
Multiple (2-3) aperture grids are installed and high voltages of several hundred to 1000 V are applied to them. This potential difference draws out and accelerates only the ions in the plasma.
The ion engine extracts only ions. Therefore, if left as is, the satellite will become negatively charged in a short period of time. If this happens, the ion beam will flow backwards and will not be able to generate thrust.
A neutralizer is then used to neutralize the ion beam by emitting electrons. This prevents the satellite from becoming electrically charged and allows the engine to continue generating propulsion.
Ion engine features include the following
- High specific impulse
- Can be operated for extended periods of time.
Especially in deep space exploration, spacecraft travel through space for several years. Therefore, the operating time of the spacecraft's engines can reach 10,000 hours. In other words, it is necessary to obtain propulsive power for a long time with less fuel.
A chemical propulsion rocket, like the ones we often see in satellite launches, is not suitable for deep space exploration when the majority of its weight is accounted for by fuel. This requires engines like ion engines that have high specific impulse and can run for long periods of time on little fuel.
In addition, although ion engines have lower thrust, when operated for extended periods of time, their final arrival speed in space is also higher than that of chemical propulsion rockets.
On the other hand, ion engines can only operate in a vacuum, so they cannot be used for launches from Earth. It is mainly used in space.
Ion engines were originally developed for space exploration. Therefore, it is basically installed on spacecraft and used as the main propulsion system for satellite orbit correction and space probes.
The first ion engine was placed on NASA's SERT I in 1964 for operational testing in satellite orbit. It has since been used in the space probes Hayabusa, SMART-1, and Hayabusa-2, as well as the satellite Kiku No. 8.
In recent years, commercial communication satellites and nano-satellites have also been equipped with ion engines.