Everything that still runs on fossil fuels has an electric future. Cars and trucks have been headed in this direction for a long time. In fact, some of the first horseless carriages were electric. The technology needed more than a century to mature, but we’re to the point where it’s not uncommon to see electric cars on the road. If you keep an eye out, Tesla cars are easy to find motoring around any big city. It’s only a matter of time before aircraft are propelled by electricity, too.
Aeronautical engineers have dreamt of electric aircraft engines for decades. One part of the challenge is, “How does an electric aircraft engine produce thrust to move the aircraft forward?” Most designs concepts of this nature have, in the past, been based on the conventional jet engine, but with an electric motor turning the fan blades to produce thrust instead of using aviation jet fuel. This approach has been unsatisfactory for a number of reasons. An outside-of-the-box paradigm shift was in order.
Scientists at the Institute of Technological Sciences, Wuhan University, in China, have built a prototype plasma jet thruster in their lab that may be the electric jet engine of the future. Dan Ye, Jun Li, and Jau Tang collaborated on the research and published their findings in, “Jet propulsion by microwave air plasma in the atmosphere” (published online: 05 May 2020).
While plasma jet thrusters are not new (NASA used them on the Dawn space probe) the research team built theirs to use high-pressure air, sourced from Earth’s atmosphere rather than from an onboard tank of gas such as Xenon (which is what the Dawn spacecraft relied on for fuel). Before explaining how their plasma jet thruster works, we need to understand what plasma is.
Plasma is often called the fourth state of matter, the first three states being; solid, liquid, and gas. Matter can change states. For example when heated above the freezing point, ice cubes change from a solid to a liquid (water). When water is heated above the boiling point, it changes from a liquid to a gas (water vapor). When heated a great deal more, certain gases will change state and become a plasma. As enough heat (energy) is added to a gas, the energy absorbed by individual molecules breaks the bonds between electrons and atoms.
The result is a ‘soup’ of free electrons and ions (atoms without their outer electrons). The electrons have a negative charge, while the ions have a positive charge. Overall, the plasma ‘soup’, or ‘cloud’, is neutral, i.e., no net charge (neither positive nor negative). Interestingly, plasmas are excellent conductors of electricity thanks to all those free electrons and ions.
Plasma is abundant in nature, for example; stars (including our Sun), lightning, auroras, sprites and elves. Plasma is also common in man-made goods such as fluorescent bulbs and neon lights. If it’s still not clear what a plasma is, this simple video (4:11) may help you visualize it:
Now, how does the air plasma jet work? There are four essential systems: (1) an air compressor to pressurize atmospheric air and direct it into a chamber, (2) a magnetron which generates microwaves, (3) a waveguide (shown in yellow/gold in the illustration below) to direct the microwaves at the high-pressure air in the chamber, and (4) electric power supplies. The experimental prototype also sports a high-temperature quartz tube (which would not be necessary on an aircraft) and a cooling system for the microwave components.
The compressed air, which is already under high-pressure when it enters the chamber, is heated by the microwaves to a very high temperature to the point that the air is ionized: electrons are liberated from their atoms as the atmospheric gases become a plasma. The high-temperature, high-pressure ‘soup’ escapes at high velocity out the quartz tube (like an exhaust nozzle of a rocket) creating thrust. The more power applied via microwave energy, the more thrust created. The next picture shows the different amounts of thrust generated at different power levels (Watts).
The prototype system demonstrated by this experiment would need to be scaled up to provide the horsepower required by a passenger airliner. When the pilots of such an aircraft increase engine throttle, microwave power increases resulting in greater forward thrust. A decrease of the throttle decreases microwave power, reducing forward thrust.
Two questions quickly come to mind. First, in a real aircraft, where is all this electricity coming from to power the magnetron and the air compressor as well as incidental onboard electronics and electrical systems? The most mature power generation system I can think of that would fit this niche is fuel cell technology, but someone may come up with a better idea before air plasma jets are ready to fly.
Second, what effect would lightning have on an air plasma jet engine? Recall that plasma excels at conducting electricity. If such an aircraft flew through a thunderstorm, what kind of damage, if any, might a lightning strike cause?
These questions will have to be addressed. Whatever the answers may be, I’m optimistic that there’s an electric future for air transportation.
Question of the night: Do you enjoy traveling by air?
