The team created a microwave-jet plasma engine, wishing to replace the existing aircraft engine

Based on physicist Chris Lee's post, published on Arstechnica.

I often read scientific reports about rocket engines replacing combustion engines with a certain degree of skepticism. But finally, there appeared a study of microwave propulsion that did not lay the foundations of impossible physical concepts. Instead, it applies old plasma propulsion technology.

Plasma propulsion is still often considered a way to propel a spaceship in a hybrid, but now there is a device designed to operate in atmospheric environments. According to the researchers involved, this air plasma propulsion has the potential to generate thrust comparable to conventional jet engines.

Flammable air?

In short, jet engines are just a form of internal combustion engine: tightly compressing the combination of fuel and air. The ignition activity will ignite the combination of gas (with most of the component is nitro, which can not burn gas), forcing it to expand at a terrifying speed. The increase in gas volume will cause the fan to run, creating thrust or directly providing thrust.

But the key is to increase the temperature of the gas to be both fast and high so it can expand at the required speed. Jet engine fuel is only an energy source that generates heat to heat gas.

The technology of the steam engine era is also based on the same concept, and so does the modern steam turbine. The bottom line is still to put all the energy gained into the gas so they can expand quickly. However, the steam engine is an external combustion engine, the water is heated before flowing into the system.

In the new research report, the team describes a type of internal / external plasma combustion engine. The basic idea is this: ionizing the air to get plasma, it heats up and expands rapidly, creating thrust.

To do this, the researchers used a magnetron to create high-power microwaves (about 1kW). The microwave travels along a waveguide (a square metal tube) that narrows and then widens. The researchers placed a quartz tube into the hole at the narrow end of the waveguide. Air is forced through the quartz tube, through the narrow section of the waveguide, and drains at the other end of the quartz tube.

At the inlet of the quartz tube, air passes through the electrodes, causing electrons in some atoms (most of which are nitro and oxygen) to splash out, creating plasma of low temperature and pressure. Then, the air pressure from the point of entry of the quartz tube pushes the plasma into the waveguide.

In the waveguide, charged particles inside the plasma begin to oscillate, interact with the microwave field and rapidly heat up. The ions, atoms and electrons continuously collide, spreading energy from ions and electrons to neutral atoms, the plasma temperature rising rapidly.

Results: the researchers claimed plasma increased to a temperature of 1,000 degrees Celsius.

And thrust appeared

Hot plasma creates a flame like a difficult lamp; hot gas escapes the waveguide and creates thrust. Measuring gas pressure (ie thrust) turned out to be unbelievably difficult; Most pressure sensors and barometers do not like the high temperature emanating from the plasma at all.

Difficulties forced researchers to be creative. They closed the quartz tube with a hollow steel sphere with a small hole in the surface containing a steel ball. If the plasma repulsion is high enough, it will cause the sphere to vibrate on the top of the quartz tube. When the sphere is heavy enough, it stays still on the top of the tube. The researchers estimate the total thrust created by the gas by balancing it with the gravitational force acting on the sphere. 

Finally, the team measured the thrust of about 28N / kW, which is the same as the turbofan turbines (I calculated the thrust of modern engines at about 15N / kW). 

The question is: how big can the system scale be? With a steady stream of air (about 1m ³ / h) and microwave power (about 1kW) as in the test, the scale is pretty good. But the air flow in the system is about 15,000 times lower than the standard size engine, the thrust must also increase the scale to about 4 times (ie the capacity must also increase). 

I also believe that the researchers have also issued warning signs in the scientific report, and after reading carefully, there are some missing data. For example, with the highest microwave power, only a low flow rate was measured, when the microwave power was low, the team measured all air flow ratios. I think the plasma is unstable at a state of strong current and high power.

You might think that this technology will reduce engine mass, but I'm not sure. If the plasma propulsion engine were to be part of the turbofan turbojet system, I think it would make the engine system heavier. 

Anyway, this is a very interesting research project, and I hope it will come true.