A new design of a plasma jet engine was unveiled last year by a group of researchers in China. While not a new technology in and of itself, this new design could be the secret to allowing the use of these engines in the atmosphere — not just limited to space.
While the thrust output is still pretty puny when compared to conventional atmospheric engines, once scaled, this new type of engine could prove revolutionary for the aerospace industry.
But before we take a look at this new design, let’s get the down-low on how plasma jet engines work.
What is a plasma propulsion engine?
Plasma-based thrusters are usually thought of as a potential form of spacecraft propulsion. Such engines differ from ion thruster engines, which generate thrust by extracting an ion current from its plasma source. These ions are then accelerated to high velocities using grids or anodes.
Plasma engines don’t normally require high voltage grids or anodes/cathodes to accelerate charged particles in the plasma source but use the currents and potentials that are generated internally, in the form of a high-current electric arc between the two electrodes, to accelerate the ions. This tends to result in a lower exhaust velocity as there is a limited voltage used for acceleration.
However, with little to no air friction in space, the thrust of these engines doesn’t need to be that high. If a constant acceleration can be pumped out for months or years at a time, it could be possible to eventually reach a very high velocity.
Such engines have various advantages over other forms of electrical propulsion. For example, the lack of high voltage grids of anodes reduces the risk of grid ion erosion.
Another advantage is that the plasma exhaust is what is termed “quasi-neutral”. This means that the positive ions and electrons exist in equal numbers, which means simple ion-electron recombination in the exhaust can be used to extinguish the exhaust plume, removing the need for an electron gun.
Typical examples of these engines tend to generate the source plasma using a variety of methods, including radiofrequency or microwave energy using an external antenna. Because of the nature of the design of these engines a range of propellants can be used in them including argon, carbon dioxide, or even human urine.
As you’d expect, there are also some inherent drawbacks to this technology. Chief among them is the high energy demand required to power them.
For example, the Variable Specific Impulse Magnetoplasma Rocke (VASIMR) VX-200 engine requires 200 kW electrical power to produce 1.12 pounds (5 N) of thrust or 40 kW/N. In theory, such an energy demand could be met using fission reactors on spacecraft, but the added weight might prove prohibitive for launching the craft in the first place.
Another challenge is plasma erosion. While in operation, the plasma can thermally ablate the walls of the thruster cavity and support structure, which can eventually lead to system failure.
Such engines, to date, are only really useful once the spacecraft is in space. This is because of the relatively low thrust that prohibits them from realistically being used to launch the craft into orbit. On average, these rockets provide about 2 pounds (4.45 N) of thrust. Plasma thrusters are highly efficient once in space, but do nothing to offset the orbit expense of chemical rockets.
Most space agencies have developed some form of plasma propulsion systems, including, but not limited to, the European Space Agency, Iranian Space Agency, and, of course, NASA.
Various real-life examples have been developed and used on some space missions. For example, in 2011 NASA partnered with Busek to launch the first Hall-effect thruster onboard the Tacsat-2 satellite. They are also in use on the NASA Dawn space probe.
Another example is the aforementioned Variable Specific Impulse Magnetoplasma Rocket currently under development by the Ad Astra Rocket Company.
VASIMR works by using an electric power source to ionize a propellant into a plasma. Electric fields heat and accelerate the plasma while the magnetic fields direct the plasma in the proper direction as it is ejected from the engine, creating thrust for the spacecraft. Theoretically, a 200-megawatt VASIMR engine could reduce the time to travel from Earth to Jupiter or Saturn from six years to fourteen months, and from Earth to Mars from 6 months to 39 days.
Not too shabby.
What’s so special about this new Chinese plasma engine?
A team of Chinese engineers revealed last year a working prototype of a microwave thruster. The engine, the researchers say, should be able to work in Earth’s atmosphere with comparable efficiency and thrust to that of conventional jet engines.
Normally using noble gas, like, xenon, plasma engines have not been shown to be practical in Earth’s atmosphere as generated ions tend to lose thrust force thanks to friction with the air. Another compounding problem is that existing examples produce fairly low thrust, which is fine in space but would be pathetically small on Earth.
The new design, created by researchers at the Institute of Technical Sciences at Wuhan University, uses air and electricity instead of gases like xenon. Testing has shown that the engine is capable of producing an impressive amount of thrust that may, one day, find applications in modern aircraft.
This new plasma engine works a little similar to a combustion engine, whereby plasma is generated from a source gas which is then, in turn, heated rapidly and allowed to expand to generate thrust. In the new engine, the ionized air is used to produce a low-temperature plasma that is then fed into a tube using an air compressor. As the air travels up the tube it is bombarded with microwaves, which violently shake the ions, causing them to impact other non-ionized atoms.
This process drastically increased the temperature and pressure of the plasma, thereby generating significant amounts of thrust further down the tube.
This amazing feat is achieved, in part, through the use of a flattened waveguide (a rectangular metal tube) through which the microwaves are focussed. Generated by a specially designed 1KW, 2.45-Gh magnetron, the microwaves are sent down the guide that tapers down to half its initial size as it approaches the plasma, and then expands again. This process boosts the electric field strength and impacts as much heat and pressure to the plasma as possible.
A quartz tube is also placed in a hole in the waveguide at its narrowest point. Air is forced through this tube, then passes through a small section of the waveguide, and then exits the other end of the quartz tube.
As air enters the tube, it passes over electrodes that are subject to a very high field. This treatment strips electrons off some of the air/gas atoms (mostly nitrogen and oxygen), which creates a low-temperature and low-pressure plasma. Air pressure from the device’s blower at the entry to the tube then ushes the plasma further up the tube until it enters the waveguide.
Once the plasma is in the waveguide, the charged particles start to oscillate within the microwave field — causing rapid heating. In doing so, the soup of atoms, ions, and electrons collide with one another frequently, spreading the energy from the ions and electrons to the neutral atoms, heating the plasma rapidly.
As a result, the researchers claim that the plasma rapidly heats to well over 1,000°C. The exhausted hot plasma creates a torch-like flame as the hot gas exits the waveguide, thus generating thrust.
How powerful is the new plasma engine?
If the airflow in the compressor is kept finely tuned, the flame jet produced in the tube, the researchers noticed, appeared to lengthen in response to an increase in microwave power. Based on this observation, the researchers attempted to quantify how much thrust was being produced.
While this sounds relatively simple on the surface, it came with one serious catch. The thousand-degree plasma jet produced by the engine would destroy a regular barometer.
To overcome this, the team decided to think a little outside the box. They devised a way to balance a hollow steel ball on top of the tube. This ball was filled with smaller steel beads to change its weight as and when required. At a certain weight, the thrust would be such that it would counteract the gravitational forces acting on the ball downwards on the exhaust end of the tube, allowing it to be elevated at a certain height above the tube.
You can check out the real footage of the engine in action here.
Using this measured distance and by subtracting the thrust added by the compressor, the team was able to indirectly get an estimate of the thrust from the plasma jet.
Using this innovative, if unconventional, technique, the team was able to test the device over a range of power levels and airflow rates. As it turns out, they managed to find a linear relationship between propulsive force and both microwave power and airflow.
What’s more, the technology appears to be pretty efficient too. It is able to pump out a propulsive force at 400 W electrical input, and 1.45 cubic meters of air per hour, was 2.45 pounds of thrust (11 N), representing a conversion of power into thrust at a rate of 6.29 pounds of thrust (28 N)/kW.
Assuming the linear relationship between microwave power (and airflow) and thurst output, it should be possible to use a Tesla Model S battery capable of outputting 310 kW and turn that into something like 1,911 pounds (8.5 kN) of propulsive thrust force.
To put that into perspective, the now-cancelled Airbus E-Fan two-seater electric aircraft used a pair of 30kW electric ducted fans that combine to produce a total static engine thrust of around 1.5 kN. Using some back of cigarette packet calculations would mean efficiency of around 5.62 pounds (25 N)/kW. Not bad, but it’s not as good as the prototype produced by the researchers.
Moving forward, the team is already looking for ways to use a more sophisticated, and reliable, method to test the thrust output of the technology. They are also looking at ways to further refine and improve the efficiency of the engine.
That being said, things are certainly looking up to this innovative plasma thruster concept. But, if only things were that simple. There are, of course, some important caveats with such an innovation.
As exciting as this technology is, it probably won’t be able to find many buyers in the up-and-coming eVTOL market. While quieter than props of ducted fans, its thousand-degree exhaust could cause some serious problems. Another problem is that, as Ars Technica points out, that “the airflows are in the region of about 15,000 times lower than those for a full-sized engine. The thrust also has to scale by about four orders of magnitude (meaning the power does, too). Extrapolating linear trends over four orders of magnitude is a good way to be disappointed in life.”
Some people looking over the data have also pointed out some strange omissions from currently available data released from the team. For whatever reason, and none is given, the data points do not show the highest microwave power levels at the highest airspeeds of the prototype.
While this mist simply is a matter of the rig not being tested at such levels, it could also indicate that there are some serious problems with the engine at these power levels.
Yet another problem for the future of such an engine is its power supply. While it is at least, if not more, efficient than regular Airbus engines given a like-for-like energy supply, the fact remains that aviation fuel is a very energy-packed fuel source. This is especially the case when compared with batteries (in fact somewhere in the region of 43 times more).
Compare the new engines’ 28 N/kW, to the engines on a commercial Airbus A320, which produce around 220,000 N of thrust combined. This means that for the new engine to power a comparably-sized jet plane would require more than 7,800 kilowatts of power — about the same as that produced by 570 Tesla Powerwall 2 units.
That being said, this is a very interesting technological innovation. If this new plasma-based thruster design does prove to be the real McCoy and is scalable, not to mention efficient, it could mark something of quantum-leap in non-fossil fuel powered aviation propulsion.
Watch this space, well, sky.