papers on the subject of alternative thrusters with a certain degree of cynicism. But we’ve finally been given a study on microwave thrusters that doesn’t rely on impossible physics. Instead, it used a plain old plasma thruster.
Plasma thrusters have generally been thought of as a means of propulsion in space, but now one has been designed to operate under atmospheric conditions . According to the researchers involved, it’s an air plasma thruster that has the potential to produce the same thrust as a commercial jet engine.
Combustible air?
A jet engine is just a form of internal combustion engine: combine fuel and air and compress the living hell out of the mixture. The resulting ignition rapidly heats the gas (most of which is nitrogen and doesn’t burn), forcing it to expand explosively. The rapid expansion can be used to power fans that generate thrust or used directly to provide thrust. But, the key point is that the gas needs to be rapidly heated to very high temperatures so that it can expand. The fuel of a jet engine is just the energy source for heat.
The age of steam relied on the same concept, as do modern steam turbines. Heat water to a very hot gas, then allow it to expand to do work. Again, the key is getting all that energy into the gas so that it can rapidly expand. A steam engine, though, is an external combustion engine, with the combustion heating the water before the water is sent into the place where it does work.
Now, a group of researchers has demonstrated a kind of external / internal plasma combustion engine. The essential idea is that air is ionized to a plasma, which is rapidly heated and allowed to expand to generate thrust.
To do this, the researchers used a magnetron to generate relatively high-powered microwaves (about 1kW). The microwaves travel down a waveguide (a rectangular metal tube) that gets progressively thinner and then expands again (see picture). A quartz tube is placed in a hole in the waveguide at the narrowest point. Air is forced through the quartz tube, passes through a small section of waveguide, and then exits the other end of the quartz tube.
At the entry to the tube, the air passes over electrodes that are subject to a very high field. This rips electrons off some of the atoms (mostly the nitrogen and oxygen), creating a low-temperature, low-pressure plasma. The air pressure from the blower at the entry of the tube pushes the plasma further up the tube so that it enters the waveguide.
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