Ion propulsion is a promising method of aeroplane propulsion with the potential, given advances in technology and changes in electricity sources, to make flying carbon neutral. However, it is currently beset by many serious problems, such as the relatively low energy density of batteries compared to jet fuel, and the low amount of thrust generated by ion propulsion engines.
Ionic propulsion is the result of the Biefeld-Brown effect, where a voltage applied to two terminals of a capacitor causes a force if the size of the terminals is not equal. This force accelerates the particles between the terminals, regardless of whether they are charged or not, causing them to exit, leading to a reaction force in the opposite direction on the propulsion engine, causing the craft to be propelled forward. Brown himself believed that the electrodes were somehow interacting with the Earth's gravitational field, and claimed that he had invented a new field of science called electrogravitics. While widely discredited, Brown's theory is still occasionally cited by UFO enthusiasts as a potential form of propulsion for alien spacecraft. He also believed that his thruster would still work in a vacuum, though tests in near-vacuum conditions have since disproved this.
These new tests are consistent with the current theory explaining the Biefeld-Brown effect, where the small electrode ionises the air around it and then repels the ions towards the large electrode, where they are neutralised. The movement of ions is the reason behind the force generated. In addition to propulsion engines, there is also the potential for this to be applied to pumps, both for air and water.
Another type of engine, more commonly used in spacecraft as it is capable of operating in a vacuum, is the ion thruster. For example, the Dawn spacecraft, which was launched in 2007 to explore Ceres and Vesta, used an ion thrust engine. In the first stage, a neutral atom is ionised by a high-energy electron, causing it to lose an electron and become a positive ion. The Dawn spacecraft used xenon, as its high relative atomic mass of 131.29 meant that a larger reaction force was exerted on the spacecraft for every ion accelerated, allowing it to conserve atoms (a large concern as the spacecraft could not be refuelled). The walls of the chamber are positively charged, attracting the free electrons before they have a chance to rejoin the xenon ions. The xenon ions then drift between a positive and negative grid at the end of the engine, which exerts a very strong electrostatic force accelerating the ions out of the engine, and therefore accelerating the spacecraft. As the xenon ions are far less massive than the spacecraft, they must be accelerated to a very high speed, of up to 150,000km/hr. Even with this, the thrust generated by this engine is still very low, at roughly 92mN, which is far too low for aircraft applications. This is suitable for spacecraft only due to the lack of air resistance in space, which allows the craft to accelerate to very high speeds over time. On Earth, air resistance makes this sort of flight impossible.
Luckily, the thrust generated by the engine can also be increased on Earth by using the air instead of a stored supply of xenon atoms. As a result, the engine has been redesigned at MIT, such that it consists of a thin wire electrode at one end, and an aerodynamically shaped larger electrode at the other end, coated in aluminium foil. The air is ionised at the wire, and the ions flow towards the other electrode, colliding many other air molecules on their way, increasing the thrust produced. One of the advantages of ion propulsion over conventional jet fuel engines is that while jet engines have a thrust to energy ratio of approximately 3N/kW, ion propulsion engines can achieve up to 6.26N/kW. In addition, ion propulsion is almost silent, while a standard jet engine at 100ft produces a sound with a magnitude of 140dB, where the CDC (an American regulatory body) states that sounds of above 120dB can cause immediate damage to ears. It's clear from this that engines could benefit from being quieter. The quieter engines could also have military applications, for stealth planes and bombers, and if these engines can be made viable, then it's entirely possible that many of the world's militaries would swap out jet engines in favour of quieter engines.
However, there are many shortcomings of current ion propulsion technology. Perhaps the most restrictive is that currently, ion propulsion engines only have a thrust density of 3N/m^2. For reference, the CF6-80C2-A1, a jet engine which is sometimes fitted on the Boeing 747-400, has a thrust density of 60,000N/m^2. As a result of this, ion propulsion engines are not currently a feasible alternative to jet engines, as the space required for the engines would be completely impossible on current aeroplane designs. Furthermore, due to this, such planes would lack sufficient thrust, because of the smaller engines, and would therefore only be able to travel at slow speeds. Using the drag equation d = Cd*((p*V^2)/2)*A, we can see that the air resistance that the aircraft will face is proportional to the square of the velocity, meaning that the thrust requirement increases exponentially. As the air resistance is also proportional to the area, increasing the number of engines will also increase air resistance, leading to an even higher thrust requirement.
There is also the problem of the energy density of batteries. The MIT team used lithium polymer batteries, which have a much higher energy density than regular batteries, of up to 2.63MJ/L. However, this pales in comparison to jet fuel, which has an energy density of 34.7MJ/L. This severely limits the capabilities of electric ion propulsion planes, which will need to wait for advances in battery technology before they become feasible. There is an alternative solution, where a "hybrid" plane is built, which stores energy as petrol, and then burns it to convert it to electricity, which is then used by the engine. However, this will negate some of the advantages of ion propulsion planes, such as their quietness and very low emissions.
Overall, ion propulsion engines are not currently near becoming a feasible replacement for jet engines, and advances in battery technology and the thrust density of engines are needed for them to be a viable alternative. However, if these goals are reached, ion propulsion engines could and would easily become an industry standard, due to greater fuel efficiency and quietness which would allow airports to operate 24/7 without disturbing locals.
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