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Jet propulsion by microwave air plasma in the atmosphere

arstechnica.com

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Re: Jet propulsion by microwave air plasma in the atmosphere

#11

Post by catalogia »

tgflynn wrote: post #9
tomalpha wrote: post #6

> 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.

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 tech, a 10 MW power source would still be quite large for an airplane.)

Re: Jet propulsion by microwave air plasma in the atmosphere

#12

Post by cwillu »

hammock wrote: post #7

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)

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.

Re: Jet propulsion by microwave air plasma in the atmosphere

#13

Post by neltnerb »

tgflynn wrote: post #9
tomalpha wrote: post #6

> 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.

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…

I'm sure it will be a long road to practicality, but that it ionizes all of the air and drives it out is fascinating. Of course it also then requires strong electric drivers since it's basically driving the engine with the magnetron.

I can kind of imagine improved transistors and other technology making scaling the magnetron easier, hopefully all that work on fusion containment has helped us understand plasma better here too!

Re: Jet propulsion by microwave air plasma in the atmosphere

#14

Post by mannykannot »

"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.

Re: Jet propulsion by microwave air plasma in the atmosphere

#16

Post by blurbleblurble »

tgflynn wrote: post #9
tomalpha wrote: post #6

> 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.

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...

Re: Jet propulsion by microwave air plasma in the atmosphere

#17

Post by flukus »

tgflynn wrote: post #9
tomalpha wrote: post #6

> 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.

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…

> I'm pretty sure those don't exist and if they did they'd be very large and heavy

I'm clueless on these things, but what order of magnitude for "heavy" are we talking about here? If we take a 1Kg microwave and add 4 orders of magnitude we're at 10,000Kg and a 747 weighs in at ~200,000Kg, so by that measures it seems achievable. I'd also assume existing terrestrial ones aren't optimized for weight in any way.

> I would be surprised if there's much hope of that competing with conventional jet engines on efficiency.

Efficiency isn't the only measure, there if energy can be cheaper than fuel then less efficient can win out. There may also be applications for long running, low weight flight powered by solar like starlink.

TIL I learned of this handy site: https://whatthingsweigh.com/how-much-does-a-boeing-747-weigh...

Re: Jet propulsion by microwave air plasma in the atmosphere

#19

Post by mrfusion »

tgflynn wrote: post #9
tomalpha wrote: post #6

> 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.

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…

Imagine the first internal combustion engine compared to your 2015 Honda Civic though.
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