Jet propulsion by microwave air plasma in the atmosphere

[Lo.M]

Look how interesting...

And an airplane engine that uses microwave-induced air plasma. Such a jet engine simply uses only air and electricity to produce high temperature and pressurized plasma for jet propulsion. Demonstrated that, with the same energy consumption, its propulsion pressure is comparable to that of conventional jet engines of fossil fuel aircraft. Therefore, this carbon-free propellant could potentially be used as a jet propellant in the atmosphere.

Fig1- Schematic diagram of a prototype microwave air plasma jet thruster. A flattened waveguide was used to increase the electric field strength of air ionization inside the air ionization chamber.

Fig2- Images of the microwave air plasma jet at different power settings (in a unit of W). The length, temperature, and brightness of the flame increase with an increase in the microwave power.

The flame temperature can reach higher than 1000 °C , 1832°F , 1273,15 K

Fig3- Schematic diagram of a simple homemade heat-resistant device for the propulsion pressure measurements, consisting of a hollow steel ball on top of the quartz tube. The device has a small hole at the top for inserting smaller steel beads in order to adjust the threshold weight at which the ball starts to rattle due to the effect of the plasma jet. (b) The device used in the experiment, the point at which the hollow steel ball began to vibrate 

Fig4- Threshold propulsion force at various air flow settings as a function of the microwave power (in a unit of W). Linear fits were obtained with m representing the slope and c representing the y-axis intercept. I represents the air flow rate (in a unit of m3/h). (b) Similar to (a), but with the x-axis representing the air flow rate.

Fig5- Net jet pressure (excluding the contribution from the injected air but with no microwave power) at various air flow settings as a function of the microwave power (in a unit of W). Linear fits were obtained with m representing the slope and c representing the y-axis intercept. I represents the air flow rate (in a unit of m3/h). (b) Similar to (a), but with the x-axis representing the air flow rate.

Link: https://aip.scitation.org/doi/full/10.1063/5.0005814

Image for illustrative purposes only

Edited March 26, 2021 by Lo.M

[lrd.Helmet]

Still need a way to store all that electricity. Or generate it on the fly.

[sevenperforce]

The question will be power and performance in comparison to conventional engines. We could easily build an "electric jet" now with ducted fans; the problem is how to get power. Batteries are heavy and they don't get lighter when you use them up, like fuel tanks.

1 minute ago, lrd.Helmet said:

Still need a way to store all that electricity. Or generate it on the fly.

They claim linear extrapolation would give 8.5 kN with a 310 kW power source like the Tesla Model S battery pack. Where will that get us? Well, let's look at the cruising characteristics of a commuter airliner like the Dornier 328JET, powered by a pair of Pratt & Whitney PW306B turbofan engines. Cruise thrust is 5 kN at 236 m/s with a specific fuel consumption of 69.9 kg/kN/h (takeoff thrust is much higher but we're not worried about that right now).

I'm not sure which of the various Model S battery packs they were discussing, but the 100-kWh Model S battery has a power output of 311-451 kW, so let's go with that. The 85-kWh battery has a mass of 540 kg, so a little allowable linear extrapolation would guesstimate the mass of the 100-kWh battery at 636 kg. At a power output of 310 kW, you'd need 1,972 kg worth of battery to get a single hour of continuous 8.5-kN thrust. That's a specific fuel "consumption" of 232 kg/kN/h. So this very rough BOE estimate suggests that such a microwave-air-plasma-powered jet would only have 30% the cruising range of a comparable hydrocarbon-based business jet.

Plus, range would be even lower because a jet gets lighter as it burns fuel, but this would not.

Also you have to deal with the increased thrust requirements for takeoff.

Unless there's some way that the microwave plasma thruster could be VASTLY more efficient than a conventional engine in other, unique ways, there's simply no way to get the kind of energy density we need for electric planes right now.

[magnemoe]

Very cool, i always imagined air did not heat with microwaves but the microwave oven frequency is optimized to interact with water. 

However as Ird.Helmet says, this don't solve the power issue who is the main one. Long distance planes are fuel limited even with jet fuel and you can not make batteries who come close to diesel. 
Reason for this is that the diesel is just chemical energy in an tank an battery can not be pure chemical energy this way. 
Electrical cars works since the electrical engine is so effective, however here you are just creating heat something diesel does very well. 
Propeller planes with electrical engines, yes it works for shorter trips and you get an plane who is environmental friendly and easy to maintain. 
On an plane swapping batteries also makes sense since airplanes are serviced by professional ground crew who is used to handling planes with heavy equipment and battery swap would be like adding bombs on jet fighters. 

Am nuclear reactor is not an good option for many reasons, beamed power might work but see this as something for orbital launches only, you will need reaction mass in vacuum but you could skim in the atmosphere at hypersonic speed before reaching it. 
 

[lrd.Helmet]

Could a fuel cell with H2 and O2 be a solution? I don't know what the power output of those things is and I'm not even thinking about the environmental impact of pushing water vapour straight into the atmosphere at those altitudes.

[kerbiloid]

Then why not just hydrolox engine or hydrogen jet and air?

[kerbiloid]

A power distributing orbital station with masers. Actually, a network. Like Starlink, but Powerlink.

High-altitude electroplanes with MW receivers.

If fly above the clouds, then maybe it's normal.

Edited March 26, 2021 by kerbiloid

[YNM]

38 minutes ago, sevenperforce said:

We could easily build an "electric jet" now with ducted fans

Yep, EDF is popular with hobby aircraft modelling already.

If anything, one of the problems with turbojets is because of the very high velocity of the exhaust that they make. When directly blown into otherwise "static" air it makes for less efficient transfer of momentum. Turbofans corrected this by first mixing the higher-velocity air from the core jet with intermediate-velocity air (off the fan on the front) and only after that mixing it with ambient air. When dealing with electric propulsion I think the goal is to simply have very high efficiency over either speed or raw thrust, particularly because of the very high mass per energy stored that's available for electric energy storage for now. Given advances in propeller blade design and the reduced need for very fast travel (most people would appreciate the lavishness of the service instead of being flown in a tiny dart very fast) I think propeller (or at least unducted fan) form factor might be the shape of airplane engines to come.

Also I think this was that whole "electric propulsion" thing I've heard a few months ago. Well it is interesting but the sole reason to use this thing is when you absolutely need small cross-section like fighter jets.

Edited March 26, 2021 by YNM

[sevenperforce]

14 minutes ago, YNM said:

If anything, one of the problems with turbojets is because of the very high velocity of the exhaust that they make. When directly blown into otherwise "static" air it makes for less efficient transfer of momentum. Turbofans corrected this by first mixing the higher-velocity air from the core jet with intermediate-velocity air (off the fan on the front) and only after that mixing it with ambient air.

Makes me wonder if you could use the unique properties of this microwave plasma to create a highly-efficient vortex to suck in and compress air without needing a fan in the intake. Or if you could spread the microwave plasma jets out across the wings/control surfaces to essentially use the entire body of the plane as an intake. You can't do that with a conventional turbojet because you're limited by the turbine rotation cross-section and bypass fan. It would be MUCH quieter, that's for sure.

Not sure if it would achieve enough efficiency to overcome the energy density issues.

14 minutes ago, YNM said:

When dealing with electric propulsion I think the goal is to simply have very high efficiency over either speed or raw thrust, particularly because of the very high mass per energy stored that's available for electric energy storage for now.

A lithium-ion battery has an max specific energy of about 1 MJ/kg. Jet fuel has an energy density of 43 MJ/kg, assuming it is burned in air. So electric engines need a 1-2 order of magnitude increase in thrust-specific efficiency to compete with hydrocarbons. 

It's different with cars, of course, where curb weight is not as limiting an issue. Plus, an internal combustion engine is realistically limited to a Carnot efficiency of 20-35% while electric motors can easily exceed a shaft output efficiency of 90%. Because of their efficiency they're also much smaller and lighter, which offsets the weight of the batteries.

[Spica]

And unfortunately for electric engines, there simply isn't room for an order of magnitude increase in efficiency over current hydrocarbon fueled jet engines.

It's a fascinating technology, but I'm not sure it fits into any niche not already filled by another type of engine.

In the far future something like this could conceivably be used in a fusion powered jet engine, possibly with magnetohydrodynamic compressors and "turbines".

Edited March 26, 2021 by Spica
thoughts

[sevenperforce]

8 minutes ago, Spica said:

And unfortunately for electric engines, there simply isn't room for an order of magnitude increase in efficiency over current hydrocarbon fueled jet engines.

Spoiler

It's a fascinating technology, but I'm not sure it fits into any niche not already filled by another type of engine.

Electric engines are not limited by thermal efficiency. That's the critical distinction here.

[DDE]

6 minutes ago, sevenperforce said:

Electric engines are not limited by thermal efficiency. That's the critical distinction here.

But the whole powerplant generally seems to be... unless you have a reactor with such energetic exhaust you can just point it out of the vehicle.

[Lo.M]

We have to consider the use of batteries of

• Metal-Air,
• Sodium-Ion,
• Potassium-ion,
• Lithium-Sulfur,
• Glass batteries,
• Magnesium-Ion,
• Silicon–air,
• Lithium metal,
• Molten-salt
• And bio-batteries (ORB, MFC, Polymer-based battery)

The batteries used today are pretty inefficient, of course.

In addition to the implementation of graphene.

We want to take into account the environmental impact of batteries, for example the silver-cadmium batteries that I don’t even need to talk about the toxicity of that.

Edited March 26, 2021 by Lo.M

[sevenperforce]

9 minutes ago, DDE said:

But the whole powerplant generally seems to be... unless you have a reactor with such energetic exhaust you can just point it out of the vehicle.

I could be wrong here but I don't think that's accurate (and a reactor with energetic exhaust would be MORE subject to the Carnot cycle, not less).

All hydrocarbon engines -- internal combustion engines, turboprops, turbofans, and so forth -- are fundamentally heat engines. They're using the heat of combustion and converting that into mechanical work using cycles of isothermal and adiabatic expansion and compression (whether reciprocating, as in a piston engine, or linear, as in a turbine). Even a magically frictionless heat engine can only convert a certain maximum amount of the heat into work. Carnot's Theorem says that the maximum possible efficiency of a heat engine is given by: 

where T_C is the ambient temperature and T_H is the engine operating temperature. Hydrocarbons burn at 700-1400 K and the ambient operating temperature is usually around 300 K, so the maximum possible thermodynamic efficiency of a hydrocarbon heat engine is 57-79%. Friction and other engineering inefficiencies mean that the most well-designed hydrocarbon heat engines convert only 30-50% of their heat into mechanical work, depending on their internal temperature.

Electric motors use electromagnetic fields to convert energy directly into work without requiring a heat cycle, so they can go straight to 90% efficiency or higher.

This thermodynamic efficiency is distinct from the subsequent thrust efficiency. 

[Lo.M]

2 hours ago, lrd.Helmet said:

Could a fuel cell with H2 and O2 be a solution? I don't know what the power output of those things is and I'm not even thinking about the environmental impact of pushing water vapour straight into the atmosphere at those altitudes.

The problem is the production of hydrogen that comes through electrolysis (expensive, wastes a lot of energy), biomass (large areas of planting [deforestation] needed) and oil (the goal is to stop the emission of polluting gases, are not even thinking about it).

[sevenperforce]

2 hours ago, lrd.Helmet said:

Could a fuel cell with H2 and O2 be a solution? I don't know what the power output of those things is and I'm not even thinking about the environmental impact of pushing water vapour straight into the atmosphere at those altitudes.

Wouldn't be much of a difference -- jet exhaust is already 30-32% water vapor by weight.

[Spica]

1 hour ago, Lo.M said:

We have to consider the use of batteries of

• Metal-Air
• Silicon–air

Any sort of air-breathing battery has the unfortunate tendency to get heavier as its energy is depleted. This is one sticking point that makes them less useful for aviation than you would immediately suspect given their very high specific energy content.

1 hour ago, sevenperforce said:

Electric engines are not limited by thermal efficiency. That's the critical distinction here.

Yes of course, my point was that even with theoretically ideal battery chemistries the product of battery specific energy and electric motor efficiency barely if at all beats current hydrocarbon fueled engines.

Not including the mass of the oxygen picked up by the battery during operation, a perfect Lithium-Air battery can store 40 MJ/kg, on par with hydrocarbons. When you then consider the conversion efficiency from stored energy into propulsive energy the perfect battery wins handily.

The story changes when you include the mass of the oxygen picked up by the battery, which drops the battery specific energy to at most 18 MJ/kg, which is competitive with current jet engines after accounting for conversion losses, and that's for a theoretically perfect battery.

[YNM]

10 hours ago, Lo.M said:

The batteries used today are pretty inefficient, of course.

And I sadly can't see much way beyond this and keeping things safe. Li-ion batteries used on aircraft for backup power have enough problems as it is already.

The main difference between hydrocarbon fuel and batteries is that while hydrocarbon fuel requires initial activation energy to be provided before they release the internal chemical energy, batteries in general have none of this at all, they need barely anything more than a short to make the reaction go.

11 hours ago, sevenperforce said:

It's different with cars, of course, where curb weight is not as limiting an issue.

Would say that it's a slight issue, albeit not really on the car itself and more on the legal/external physical problems like curb weight limit (there's a limit defined to divide cars and larger vehicles, you can't use the same license) as well as in a crash (not really wrt the car but with the thing it hits). We're well on the way through getting past these problems however.

Edited March 27, 2021 by YNM

[mikegarrison]

The basic problem is, electrical power needs to be generated. It doesn't matter even if the use of the electricity has perfect efficiency (which it won't). It still has to be generated somewhere. You just move the efficiency problem from the engine to somewhere else.

Now, stationary powerplants that are not limited by the need to fly around have many more options than onboard power plants. They can be nuclear, or geothermal, or hydroelectric, whatever. None of those work on airplanes, but electricity generated by them would still work on an airplane.

But that electricity has to somehow be either transmitted to the airplane or stored on board the airplane, and that's where you run into your next big problem.

[kerbiloid]

33 minutes ago, mikegarrison said:

But that electricity has to somehow be either transmitted to the airplane or stored on board the airplane, and that's where you run into your next big problem.

That's another greatest challenge before the humanity after the invention of food for slimming.

[YNM]

4 hours ago, mikegarrison said:

But that electricity has to somehow be either transmitted to the airplane

This is actually another interesting aspect. Even without directly beaming power to the airplane, we'd need to recharge the batteries on ground as current jetplanes need to refuel. Given that I think we're settling for cars that could be driven for maybe 6 hours then being recharged for 30 minutes (which is actually a good time for you to rest and relax), wonder how we'd deal with airplanes, would we have more charging points (and cables) and more battery groups or what ? Airplane fueling currently is much more simpler, right (as in you don't need to go around and refuel the different tanks separately) ?

EDIT : On a further thought, currently we have fuel farms to store fuel at airports... would we need battery banks the same size/capacity as a fuel farm ? Or even if we opt instead to replace the batteries and charge them slowly on the ground, how much batteries can we save compared to doing it like we do fuel today ? Although in any case it's still a ton of batteries to have around.

There's a lot of interesting aspect when it comes to using electric power for vehicles I guess.  While those battery banks would be expensive, it would be a good news for grid stability...

Edited March 27, 2021 by YNM

[Spica]

In my opinion at least  it seems hard to beat liquid chemical fuels for the role of "extension cord" between a power plant on the ground and an aircraft, at least for high speed or long range use. The power conversion method aboard the aircraft could be a heat engine and generator, fuel cell, or really anything that meets the specific power and efficiency requirements you need.

Although if you're already making hydrogen, methane, or a heavier hydrocarbon from mass and energy on the ground, it seems like a pretty good idea to just burn the fuel in a traditional aero engine rather than converting  all its energy into electricity first. Then you can have most of the "environmental cleanliness" (this depends on the source of the electricity of course) of an electric aircraft while still making use of the high energy content and easy handling of hydrocarbon fuels.

[mikegarrison]

42 minutes ago, Spica said:

In my opinion at least  it seems hard to beat liquid chemical fuels for the role of "extension cord" between a power plant on the ground and an aircraft, at least for high speed or long range use. The power conversion method aboard the aircraft could be a heat engine and generator, fuel cell, or really anything that meets the specific power and efficiency requirements you need.

Although if you're already making hydrogen, methane, or a heavier hydrocarbon from mass and energy on the ground, it seems like a pretty good idea to just burn the fuel in a traditional aero engine rather than converting  all its energy into electricity first. Then you can have most of the "environmental cleanliness" (this depends on the source of the electricity of course) of an electric aircraft while still making use of the high energy content and easy handling of hydrocarbon fuels.

And thus you have just described SAF. (Sustainable Aviation Fuel.)

While everybody wants to talk about electric airplanes and hydrogen and whatever else, generally speaking most of the real focus of mitigating aviation climate effects are going into SAF efforts.

[YNM]

1 hour ago, mikegarrison said:

And thus you have just described SAF. (Sustainable Aviation Fuel.)

While I agree that they're possible in theory my question then fall on whether that is actually what's being done. Like we still have forests just being opened up for new palm oil plantations. I mean once the damage is done and the conversion is finished there really isn't anything much one can do but use it though I guess we do need some way to straighten up the source first before moving to completely utilize it.

[mikegarrison]

1 hour ago, YNM said:

While I agree that they're possible in theory my question then fall on whether that is actually what's being done. Like we still have forests just being opened up for new palm oil plantations. I mean once the damage is done and the conversion is finished there really isn't anything much one can do but use it though I guess we do need some way to straighten up the source first before moving to completely utilize it.

I can tell you that this is where a LOT of focus is going. Whether or not it will ultimately be successful, who knows? But cutting down rainforests for airplane fuel is not included in what is considered "sustainable". The goal is to not compete for food crops or contribute to land use change climate impact.

(Also, SAF does not necessarily mean "biofuel".)

https://www.iata.org/en/programs/environment/sustainable-aviation-fuels/

https://www.shell.com/business-customers/aviation/the-future-of-energy/sustainable-aviation-fuel.html

https://skynrg.com/sustainable-aviation-fuel/saf/

https://www.energy.gov/sites/prod/files/2020/09/f78/beto-sust-aviation-fuel-sep-2020.pdf

https://www.icao.int/environmental-protection/pages/SAF.aspx

Etc. Etc.

You may also hear about a "basket of measures", which is ICAO-speak for saying they are working on a bunch of different pathways toward reducing aviation climate impact rather than waiting for one magic bullet that will solve everything at once.

Edited March 27, 2021 by mikegarrison