If it’s just using the microwaves to heat the air why not use resistive coils instead?
The plasma is the resistive coil. Microwaves are used to carry energy to it because wires would melt.
Jet propulsion by microwave air plasma in the atmosphere
21–30 of 54 posts
Re: Jet propulsion by microwave air plasma in the atmosphere
#22Earlier quoted context omitted.
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 th…
Thank you for this interesting comment! When you say "continuous", would you consider some very high frequency solid state switching amplifier to be "continuous" enough for this application? I realize it's a different order of magnitude, but those GaN/Si transformers make me wonder if we aren't far off from some kind of megawatt scale solid state amplifier shakeup...
It's conceivable. I've seen solid state RF amplifier modules successfully combined into some quite high power units and that was over 20 years ago. I haven't followed the technology since so I'm not up to date on that at all. In any case I think a 10 MW amplifier and associated power supplies and cooling equipment would need to be large and heavy.
A single jet engine generates about 20 MW of power and I'd be very surprised if you could pack 10 MW of RF amplifier into that kind of size and weight envelope.
Re: Jet propulsion by microwave air plasma in the atmosphere
#23Re: Jet propulsion by microwave air plasma in the atmosphere
#24Earlier quoted context omitted.
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 th…
> 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…
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.
Re: Jet propulsion by microwave air plasma in the atmosphere
#25Earlier quoted context omitted.
The plasma is the resistive coil. Microwaves are used to carry energy to it because wires would melt.
How about tungsten coils?
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.
Re: Jet propulsion by microwave air plasma in the atmosphere
#26Re: Jet propulsion by microwave air plasma in the atmosphere
#27Earlier quoted context omitted.
Thank you for this interesting comment! When you say "continuous", would you consider some very high frequency solid state switching amplifier to be "continuous" enough for this application? I realize it's a different order of magnitude, but those GaN/Si transformers make me wonder if we aren't far off from some kind of megawatt scale solid state amplifier shakeup...
It's conceivable. I've seen solid state RF amplifier modules successfully combined into some quite high power units and that was over 20 years ago. I haven't followed the technology since so I'm not up to date on that at all. In any case I think a 10 MW amplifier and associated power supplies and cooling equipment would need to be large and heavy. A single jet engine generates about 20 MW of power and I'd be very sur…
Makes sense. Maybe we'll see a fusion plasma thruster before the power/energy storage shrunk enough to make this viable as is (in the atmosphere and gravity of Earth at least).
Re: Jet propulsion by microwave air plasma in the atmosphere
#28"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…
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.
Re: Jet propulsion by microwave air plasma in the atmosphere
#29This 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.
Re: Jet propulsion by microwave air plasma in the atmosphere
#30"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…
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…
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-successful...