8 ms·

Jet propulsion by microwave air plasma in the atmosphere

130 points by Egregore 6y ago

travisporter 6y ago
This is awesome! But my brain froze trying to link Wuhan to jet/plasma. Ars technica has a good article on it too. https://arstechnica.com/science/2020/05/microwave-thruster-makes-for-clean-burning-jet/ https://arstechnica.com/science/2020/05/microwave-thruster-m...
- dang 6y ago
  We've changed to that from https://aip.scitation.org/doi/full/10.1063/5.0005814 https://aip.scitation.org/doi/full/10.1063/5.0005814. Thanks! Submitted title was "Wuhan scientists develop jet propulsion by microwave air plasma", which broke the site guidelines by adding linkbait. Submitters: please don't do that. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
gnusty_gnurc 6y ago
What batteries are going to work for this?
- qchris 6y ago
  I don't think that's necessarily a useful question at this point, because some of the basic thermodynamic assumptions don't work out for deciding between batteries vs. using a fuel-burning generator. Normally, it's pretty obvious that "Hey, if we're going to burn hydrocarbons to produce thrust, we might as well do it directly and not deal with necessary conversion efficiency losses between heat -> electricity -> heat." Since they're dealing with RF-generated gas plasmas, though, there might be efficiency gains with acceptable configurations and lower intermediate losses, etc. over traditional systems that makes those conversions acceptable. I've worked in a lab accelerating gas plasmas using helicon antenna, and as the article mentions, scaling those systems isn't a trivial implementation detail. It's very tough to reason about them from first principles, or even small-scale models because of the non-linearity in the relevant physics. So, I guess in summary, imho it's probably a bit too early to talking about what kind of battery would be used, since it's unclear from the current system that a usable version would actually use batteries vs. another form of energy storage.
tomalpha 6y ago
> Extrapolating linear trends over four orders of magnitude is a good way to be disappointed in life It's clearly early days - "it works in the lab, now we just need to scale it" type of progress. But still, it's progress.
- tgflynn 6y ago
  If the article is right about the scaling required then I'm very skeptical that this will ever be practical. A 1 kW magnetron is a part found in just about every microwave oven. Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy. A long time ago I used to do RF engineering for particle accelerators and the most powerful continuous wave RF amplifiers I ever heard about were on the order of 1 MW. An even bigger problem would be efficiency, which this article doesn't even mention (haven't looked at the original paper). High power RF amplifiers aren't particularly efficient, I would guess around 30%, and there would also be waveguide losses and cavity losses if any resonant effects are used to get high enough electric fields. I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.
  - catalogia 6y ago
    > Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy. Probably a dumb question, but it's it just a matter of more power and cooling for the magnetron? I'm thinking the size of the magnetron is determined by the wavelengths you're trying to produce. (I don't mean to diminish the challenge of applying this tech, a 10 MW power source would still be quite large for an airplane.)
    - tgflynn 6y ago
      Yes, I think so. If the frequency is high enough so that it's not the limiting factor the size would mainly depend on how much power you can dissipate in a given volume. The highest power RF amplifiers I've ever seen were the klystrons used for the LEP and ESRF accelerating cavities. Those were large in part because of the relatively low frequency of 350 MHz but I believe there was also a large water-cooled absorber that dissipated whatever power was left in the MW+ electron beam given the conversion efficiency.
hammock 6y ago
How is this diffferent from the antigravity patent? "a device that uses a microwave emitter to create a high-frequency electromagnetic wave through a cavity to create a polarized vacuum. This polarized vacuum, in turn, reduces the mass of the vehicle containing the device." (I know it probably is, just curious)
- petschge 6y ago
  There is a massive working medium. For massless particles such as photons the connection between energy E and momentum p is E = p *c or p = E/c. Since c is large, the momentum p you can carry per energy E is small. For a massive particle with mass m the connection is given by E = p^2 / (2 m) or p = sqrt(2 m E). As long as E is small (so small that the velocity of the particle is small compared to the speed of light, but that is required anyway for the non-relativistic approximation I am using here a) the sqrt doesn't hurt you too much and you can actually carry much more momentum per particle.
- cwillu 6y ago
  In just about every way possible. In essence, this is a jet with a unique compressor, using microwaves to create high temperatures rather than burning fuel. The benefit is that the reaction mass can be entirely normal atmospheric air.
deleted 6y ago
[deleted]
mannykannot 6y ago
"The essential idea is that air is ionized to a plasma, which is rapidly heated and allowed to expand to generate thrust." So this is just a conventional heat engine, with an electric heater. This heater may be able to get the air much hotter than other methods, but the thing about a heat engine is that you cannot get useful work out unless the working fluid can expand sufficiently. An afterburner creates more thust by heating the air to a higher temperature than could be tolerated by the turbine, but the air is by then at a relatively low pressure, and so the Carnot efficiency is very poor - most of the extra fuel's energy goes into producing a hotter (and very visible) exhaust plume. So, for this to be a component of a jet engine, it will need a compressor comparable to, or with an even higher presure ratio, than in current jet engines, and that compressor will have to be powered somehow (IIRC to the tune of about 50,000 SHP in the biggest engines now in use.) For the most part, it makes no sense to use electricity to power a heat engine. In guessing where this might be useful, the only scenario I have come up with is for hypersonic ramjets, where electric motors turning fans are not an alternative, and possibly especially on worlds where the atmosphere does not support combustion.
- tgflynn 6y ago
  Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma. Then you might be able to reach much higher exhaust velocities than with a conventional jet engine. I don't think your thermodynamic objections would apply to that case since the energy would remain organized, it wouldn't just be heat. To do that though you'd need to find a way to maintain overall electrical neutrality.
  - mannykannot 6y ago
    A few months ago, there was a demonstration of a lightweight model airplane propelled by the air currents produced by a corona discharge [1], and ion rocket motors have great specific impulse, but they are both low-thrust devices. I believe you are right, that these are not heat engines, any more than an electric motor is: in all these cases, the moving part is directly accelerated in the direction we want it to go. [1] https://cosmosmagazine.com/technology/researchers-successfully-fly-ion-drive-model-aircraft https://cosmosmagazine.com/technology/researchers-successful...
stevespang 6y ago
Adding steam or water to the mix would generate H2 and O2 to recombine with explosive thrust ?
mrfusion 6y ago
If we’re making plasma why not push it out the back with an electric field? Why is heating and expelling it more efficient?
mrfusion 6y ago
If it’s just using the microwaves to heat the air why not use resistive coils instead?
- whatshisface 6y ago
  The plasma is the resistive coil. Microwaves are used to carry energy to it because wires would melt.
  - mrfusion 6y ago
    How about tungsten coils?
    - catalogia 6y ago
      I think those would burn, since these engines are meant to breath air (with oxygen in it.) Tungsten filaments work in lightbulbs because those are filled with an inert gas, or vacuum. The filaments burn quick when the bulb is broken.
    - whatshisface 6y ago
      There's really no solid that can handle thermal plasma. We're talking 10,000K to 100,000K temperatures here.
api 6y ago
This makes me think of all those UFO sightings of weird craft that flash multicolored lights that look like emission spectra you would get from plasma. I wonder if at least a few of these might have been classified experimental propulsion systems similar to this. The basic physics of this is not new and it's not like these programs have lacked the funding to experiment with crazy tech.
hoseja 6y ago
Correct me if I'm wrong but would this not spew massive amounts of nitrogen oxides? You know, the nasty pollutants you get when superheating air, such as in fuel-efficient diesel engines (see Dieselgate).
dynamite-ready 6y ago
Is the technology demonstrated in this YT video - https://www.youtube.com/watch?v=U5PYzGgHx14 https://www.youtube.com/watch?v=U5PYzGgHx14 - in anyway similar?
morei 6y ago
How is this better than an electric motor turning a fan? Modern high-bypass jet engines get the vast majority of their thrust from turning a fan. The turbine part is (mostly) just used to generate torque to drive the fan. Modern electric motors also have ridiculously high efficiencies (> 97% isn't uncommon). So how would using electricity to heat the air be better than using the same electricity to turn a fan? The only place I can think of is high supersonic where fan efficiency starts to drop.

130 points by Egregore 6y ago

This is awesome! But my brain froze trying to link Wuhan to jet/plasma. Ars technica has a good article on it too. https://arstechnica.com/science/2020/05/microwave-thruster-makes-for-clean-burning-jet/ https://arstechnica.com/science/2020/05/microwave-thruster-m...

dang 6y ago
We've changed to that from https://aip.scitation.org/doi/full/10.1063/5.0005814 https://aip.scitation.org/doi/full/10.1063/5.0005814. Thanks! Submitted title was "Wuhan scientists develop jet propulsion by microwave air plasma", which broke the site guidelines by adding linkbait. Submitters: please don't do that. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html

gnusty_gnurc 6y ago

What batteries are going to work for this?

qchris 6y ago
I don't think that's necessarily a useful question at this point, because some of the basic thermodynamic assumptions don't work out for deciding between batteries vs. using a fuel-burning generator. Normally, it's pretty obvious that "Hey, if we're going to burn hydrocarbons to produce thrust, we might as well do it directly and not deal with necessary conversion efficiency losses between heat -> electricity -> heat." Since they're dealing with RF-generated gas plasmas, though, there might be efficiency gains with acceptable configurations and lower intermediate losses, etc. over traditional systems that makes those conversions acceptable. I've worked in a lab accelerating gas plasmas using helicon antenna, and as the article mentions, scaling those systems isn't a trivial implementation detail. It's very tough to reason about them from first principles, or even small-scale models because of the non-linearity in the relevant physics. So, I guess in summary, imho it's probably a bit too early to talking about what kind of battery would be used, since it's unclear from the current system that a usable version would actually use batteries vs. another form of energy storage.

tomalpha 6y ago

> Extrapolating linear trends over four orders of magnitude is a good way to be disappointed in life It's clearly early days - "it works in the lab, now we just need to scale it" type of progress. But still, it's progress.

tgflynn 6y ago
If the article is right about the scaling required then I'm very skeptical that this will ever be practical. A 1 kW magnetron is a part found in just about every microwave oven. Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy. A long time ago I used to do RF engineering for particle accelerators and the most powerful continuous wave RF amplifiers I ever heard about were on the order of 1 MW. An even bigger problem would be efficiency, which this article doesn't even mention (haven't looked at the original paper). High power RF amplifiers aren't particularly efficient, I would guess around 30%, and there would also be waveguide losses and cavity losses if any resonant effects are used to get high enough electric fields. I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.
- catalogia 6y ago
  > Scaling that up by 4 orders of magnitude would need a 10 MW RF source or amplifier. I'm pretty sure those don't exist and if they did they'd be very large and heavy. Probably a dumb question, but it's it just a matter of more power and cooling for the magnetron? I'm thinking the size of the magnetron is determined by the wavelengths you're trying to produce. (I don't mean to diminish the challenge of applying this tech, a 10 MW power source would still be quite large for an airplane.)
  - tgflynn 6y ago
    Yes, I think so. If the frequency is high enough so that it's not the limiting factor the size would mainly depend on how much power you can dissipate in a given volume. The highest power RF amplifiers I've ever seen were the klystrons used for the LEP and ESRF accelerating cavities. Those were large in part because of the relatively low frequency of 350 MHz but I believe there was also a large water-cooled absorber that dissipated whatever power was left in the MW+ electron beam given the conversion efficiency.

hammock 6y ago

How is this diffferent from the antigravity patent? "a device that uses a microwave emitter to create a high-frequency electromagnetic wave through a cavity to create a polarized vacuum. This polarized vacuum, in turn, reduces the mass of the vehicle containing the device." (I know it probably is, just curious)

petschge 6y ago
There is a massive working medium. For massless particles such as photons the connection between energy E and momentum p is E = p *c or p = E/c. Since c is large, the momentum p you can carry per energy E is small. For a massive particle with mass m the connection is given by E = p^2 / (2 m) or p = sqrt(2 m E). As long as E is small (so small that the velocity of the particle is small compared to the speed of light, but that is required anyway for the non-relativistic approximation I am using here a) the sqrt doesn't hurt you too much and you can actually carry much more momentum per particle.
cwillu 6y ago
In just about every way possible. In essence, this is a jet with a unique compressor, using microwaves to create high temperatures rather than burning fuel. The benefit is that the reaction mass can be entirely normal atmospheric air.

deleted 6y ago

[deleted]

mannykannot 6y ago

"The essential idea is that air is ionized to a plasma, which is rapidly heated and allowed to expand to generate thrust." So this is just a conventional heat engine, with an electric heater. This heater may be able to get the air much hotter than other methods, but the thing about a heat engine is that you cannot get useful work out unless the working fluid can expand sufficiently. An afterburner creates more thust by heating the air to a higher temperature than could be tolerated by the turbine, but the air is by then at a relatively low pressure, and so the Carnot efficiency is very poor - most of the extra fuel's energy goes into producing a hotter (and very visible) exhaust plume. So, for this to be a component of a jet engine, it will need a compressor comparable to, or with an even higher presure ratio, than in current jet engines, and that compressor will have to be powered somehow (IIRC to the tune of about 50,000 SHP in the biggest engines now in use.) For the most part, it makes no sense to use electricity to power a heat engine. In guessing where this might be useful, the only scenario I have come up with is for hypersonic ramjets, where electric motors turning fans are not an alternative, and possibly especially on worlds where the atmosphere does not support combustion.

tgflynn 6y ago
Yeah, it seems like it might make more sense if they could figure out a way to use the RF fields to actually accelerate some of the ions in the plasma. Then you might be able to reach much higher exhaust velocities than with a conventional jet engine. I don't think your thermodynamic objections would apply to that case since the energy would remain organized, it wouldn't just be heat. To do that though you'd need to find a way to maintain overall electrical neutrality.
- mannykannot 6y ago
  A few months ago, there was a demonstration of a lightweight model airplane propelled by the air currents produced by a corona discharge [1], and ion rocket motors have great specific impulse, but they are both low-thrust devices. I believe you are right, that these are not heat engines, any more than an electric motor is: in all these cases, the moving part is directly accelerated in the direction we want it to go. [1] https://cosmosmagazine.com/technology/researchers-successfully-fly-ion-drive-model-aircraft https://cosmosmagazine.com/technology/researchers-successful...

stevespang 6y ago

Adding steam or water to the mix would generate H2 and O2 to recombine with explosive thrust ?

mrfusion 6y ago

If we’re making plasma why not push it out the back with an electric field? Why is heating and expelling it more efficient?

mrfusion 6y ago

If it’s just using the microwaves to heat the air why not use resistive coils instead?

whatshisface 6y ago
The plasma is the resistive coil. Microwaves are used to carry energy to it because wires would melt.
- mrfusion 6y ago
  How about tungsten coils?
  - catalogia 6y ago
    I think those would burn, since these engines are meant to breath air (with oxygen in it.) Tungsten filaments work in lightbulbs because those are filled with an inert gas, or vacuum. The filaments burn quick when the bulb is broken.
  - whatshisface 6y ago
    There's really no solid that can handle thermal plasma. We're talking 10,000K to 100,000K temperatures here.

api 6y ago

This makes me think of all those UFO sightings of weird craft that flash multicolored lights that look like emission spectra you would get from plasma. I wonder if at least a few of these might have been classified experimental propulsion systems similar to this. The basic physics of this is not new and it's not like these programs have lacked the funding to experiment with crazy tech.

hoseja 6y ago

Correct me if I'm wrong but would this not spew massive amounts of nitrogen oxides? You know, the nasty pollutants you get when superheating air, such as in fuel-efficient diesel engines (see Dieselgate).

dynamite-ready 6y ago

Is the technology demonstrated in this YT video - https://www.youtube.com/watch?v=U5PYzGgHx14 https://www.youtube.com/watch?v=U5PYzGgHx14 - in anyway similar?

morei 6y ago

How is this better than an electric motor turning a fan? Modern high-bypass jet engines get the vast majority of their thrust from turning a fan. The turbine part is (mostly) just used to generate torque to drive the fan. Modern electric motors also have ridiculously high efficiencies (> 97% isn't uncommon). So how would using electricity to heat the air be better than using the same electricity to turn a fan? The only place I can think of is high supersonic where fan efficiency starts to drop.