Electric plasma jet engines: The future of air travel, or impossible dream?
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The concept of a system of propulsion that runs on electricity and air is very attractive in today’s increasingly green-thinking world. Gone would be the messy fossil fuels and noisy exhaust of conventional jet engines. Carbon-neutral commercial flight on a large scale would finally be a real possibility. Recently, the concept of the electric plasma jet engine has sparked the imaginations of aerospace innovators and environmentalists alike. However, there are some very significant problems to overcome, if this type of propulsion is to make practical inroads into the current air-travel market.
Let’s go over the basics of the electric plasma jet engine and examine some of the challenges faced by its proponents.
Plasma-based propulsion systems have already seen some success… in space
First, we must point out that the concept of harnessing the properties of plasma (a natural state of matter along with solid, liquid, and gas), has already proven successful in several experimental and practical forms. Ion thrusters, plasma propulsion engines, helicon plasma thrusters, magnetoplasmadynamic thrusters, pulsed inductive thrusters, electrodeless plasma thrusters, and the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) are several variations of plasma engine/thruster technology in various stages of development to propel satellites and/or spacecraft. The European Space Agency, Iranian Space Agency, Australian National University, Busek, and Ad Astra Rocket Company have all developed plasma propulsion systems for space.
In 2011, NASA partnered with Busek to launch the first hall effect thruster, a type of ion thruster that was the TacSat-2 satellite’s main propulsion system once in orbit. The company has since launched several hall effect thrusters that they say could deliver “a small payload in low Earth orbit… to low lunar orbit. This incredible amount of range is unachievable for any chemical propulsion system in the same weight class.”
So, plasma-based propulsion methods such as ion thrusters have shown practical (though limited) use in space, where there is no gravity and no air resistance to overcome, and cumulative (if small) amounts of thrust can produce significant velocity over time. However, developing an electric-plasma jet engine that produces enough thrust in Earth’s atmosphere to potentially replace today’s jet engines is a much loftier goal.
How does an electric plasma jet engine work?
Rather than harnessing the attraction of differently charged ions, the concept behind the electric plasma jet engine involves superheated plasma and magnetrons (like in a microwave). In 2020, Professor Jau Tang of the Institute of Technological Sciences at Wuhan University in China announced his team’s invention of a magnetron-accelerated plasma jet design.
Tang’s design ionizes compressed air by running it past electrodes, then forces it along a specially designed quartz tube. This produces a low-temperature plasma. The tube containing this plasma intersects with a wave guide, which is essentially a pipe containing magnetron-generated microwaves. The wave guide narrows at the point where it intersects with the quartz tube, and the microwaves in the narrowed portion are at their greatest intensity. The focused microwaves excite charged particles in the plasma, forcing them to oscillate wildly and generating a release of energy, including producing heat of 1,000 degrees Celsius (1,832 degrees F). This, in turn, creates thrust along the quartz tube which acts as a rudimentary jet nozzle to direct the thrust.
Tang’s experiments showed a 1-kilogram steel ball being momentarily lifted off of the 24 mm diameter tube by the expanding gases and plasma. Tang hopes his design, after further refinement, may be used to power drones, before eventually being scaled up enough to power manned aircraft.
This all sounds exciting and media outlets ate up the story at the time. However, as with many potential technological breakthroughs in their early stages, the theoretical possibilities don’t line up with current realities or technological limitations.
Power and size limitations of electric plasma-jet engines
Analysis of Tang’s experiments show that his engine produced around 28 newtons of thrust per kilowatt (kW) of power consumed. (Another source says 10 newtons of thrust at 400 watts.) Researchers have postulated that if the technology is scaled up, the amount of thrust could be comparable to conventional jet engines.
“Ay,” as the Bard wrote, “There’s the rub.” One source says that based on the original 1 kg ball being lifted off a 24 mm tube, to reach the required airflow to compete with today’s jet engines, the electric-plasma jet engine would need to be scaled up by a factor of 15,000. And of course, with increased scale comes increased weight and size, which are not conducive to efficient air travel.
An even larger problem (literally) is the issue of how to achieve the required electrical power supply on a moving/flying craft without a connection to the local power grid. As noted, Tang’s experimental engine produced around 28 newtons of thrust. By comparison, the Airbus A320’s CFM56-5B engine produces between 98,000 and 147,000 newtons of thrust, and the aircraft requires two engines to achieve its performance goals. One analysis showed that, assuming the same thrust requirements, an electric/plasma jet engine (if one could even be scaled appropriately to become airborne) would require about 7,800 kW of power. This equates to 570 complete Tesla-sized battery power units for a single hour of flight. The current A320 can theoretically only accommodate 130 of these power units as its total payload. And that, of course, wouldn’t leave any surplus for, say… passengers and luggage, not to mention Diet Dr. Pepper and salted peanuts.
In short, there is currently no existing battery technology with the effective power-to-weight ratio to get such a large propulsion unit off the ground. Jet fuel contains far more energy than batteries can manage at the same weight (up to 43 times more). Weight is always the primary problem to overcome in any flying craft, and current battery technology can’t support the power needs of even a single theoretically upscaled electric plasma jet engine, let alone a pair of them. Detractors also point out that getting that much electrical power from the onboard power source to the engines is another problem that is currently insurmountable, requiring the use of superconducting materials that don’t exist yet.
Proponents of the electric plasma-jet engine claim that it would utilize no fossil fuels, but this is similar to all arguments in favor of electric vehicle use, in that it assumes a clean/renewable source of electric power. Today, only 20% percent of electricity generated in the US is considered clean or renewable. The remainder still comes from burning fossil fuels or from nuclear reactors. Additionally, all of the fossil fuels burned by all of the world’s airliners only account for around 13% of carbon emissions generated annually. So it could be argued that there are larger and easier “green” targets to hit than pie-in-the-sky visions of electric/plasma jet-powered air travel.
In attempting to solve the electric power requirement problem, some bolder researchers point out that there are now conventional nuclear fission reactors small enough that they could theoretically be placed on a large passenger aircraft and generate enough electrical power to drive future upscaled electric/plasma engines. Whether or not passengers and governments will tolerate nuclear-powered commercial flight is a question for the future, but we’re betting it will be a hard sell. Stationary nuclear reactors are enough of a problem already, and society isn’t going to want to deal with the effects of any crashes of nuclear-powered airliners.
Another sci-fi-type solution might be to use high-powered lasers or directed-energy generators to beam power to an airborne vehicle. This is theoretically doable with highly accurate tracking and navigation technology, but the amount of power that would need to be sent along the beam to sustain a flying airliner is currently not even in the realm of possibility.
Furthermore, all of this (admittedly fun) speculation assumes that Professor Tang’s claims of newtons of thrust per kW are accurate. Steven Barrett, MIT professor of aerospace engineering and designer of the first ion-powered aircraft that flies without any moving parts, was very skeptical of the Wuhan group’s claims of thrust to begin with. Barrett tweeted, “it’s flawed on both the physics and the measurements. They’ve built a pressure cooker with heating from microwaves, with a valve that rattles when the air in the tube is heated enough, then interpreted the transient air escaping as sustained thrust.”
Them’s fightin’ words, but regardless, Tang’s hopes of an electric-plasma jet powered drone haven’t yet come to fruition, and we’ve seen no further progress on any attempts to scale up an electric plasma jet engine.
So, as all-electric flight continues to evolve in some limited markets (eVTOL, air taxis, and even regional jet routes by 2030), it seems unlikely that there will be enough global interest in electric-plasma jet engine development to spur the kind of advancements necessary to make it a reality. At least for the time being.
–By Jeff Davis, Intergalactic Scribe
Sources:
https://technology.nasa.gov/patent/LEW-TOPS-34
https://www.nasa.gov/general/the-potential-for-ambient-plasma-wave-propulsion/
https://www.busek.com/hall-thrusters
https://youtu.be/hiXuHjyxW14?si=ewZr7PYMkDrYJI3C