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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

#41

Post by himinlomax »

mannykannot wrote: post #14

"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…

> So this is just a conventional heat engine, with an electric heater

The key is that it heats very fast.

Re: Jet propulsion by microwave air plasma in the atmosphere

#42

Post by qayxc »

mrfusion wrote: post #19
tgflynn wrote: post #9

Earlier 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…

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

That comparison wouldn't be as bad you might think.

The first functional Diesel engine for example had about 13kW of power and was only about 5x larger than a modern car engine.

This was in 1896. From 1923 onward, the engine type was small and powerful enough to be used in lorries (trucks). The first petrol engines compare even better to modern engines (in terms of power-to-weight ratio).

Re: Jet propulsion by microwave air plasma in the atmosphere

#43

Post by mannykannot »

himinlomax wrote: post #41
mannykannot wrote: post #14

"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…

> So this is just a conventional heat engine, with an electric heater The key is that it heats very fast.

While it is certainly true that any device that heats slowly would be impractical on that basis alone, I do not think rapid heating provides any relief from the laws of thermodynamic efficiency. Combustion heats very rapidly, though if this device is even faster, perhaps that would be significant in hypersonic applications? I believe one of the problems of supersonic-combustion ramjets is maintaining a stable flame.

I keep coming back to hypersonic ramjets, not because I think they are the perfect application, but because I am not sure there is any application in an otherwise conventional jet engine - i.e. one involving a rotary fan or compressor - where you would not be better off using all the available electric power in an electric motor to spin a somewhat larger fan, and not bother with heating at all.

Update: Maybe, for replacing the turbojet engines of supersonic airplanes, you need heat to reaccelerate the flow above supersonic speeds, in a convergent-divergent nozzle? But supersonic wind tunnels seem to achieve those speeds without heating the flow? I have wandered well into my area of ignorance here...

Re: Jet propulsion by microwave air plasma in the atmosphere

#44

Post by jacobush »

VBprogrammer wrote: post #40
jacobush wrote: post #39

Earlier quoted context omitted.

An idea I've had is to put a small generator instead of a shaft and gearbox, and put the AC motor powered fan somewhere else. Could improve aerodynamics, stealth properties etc.

A generator is necessarily on the same order of magnitude in size as the motors it is powering.

Is your point that they must necessarily be heavy? Because I don't think so. Modern electric motors are very compact and powerful.

Also you'd not need gearbox and shafts. You can completely decouple the fan from the turbine. You could put the fan downwards and the turbine back for instance, or the turbine on the top to hide its heat signature somewhat.

Re: Jet propulsion by microwave air plasma in the atmosphere

#46

Post by tgflynn »

RachelF wrote: post #36
blurbleblurble wrote: post #16

Earlier 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...

Most high power microwave devices aren't build on semiconductors like Silicon (solid state), yet. They are normally valves of some sort, like magnetrons, klystrons or travelling wave tubes. Some radars would be in the mega-watt range, but only pulsed with low duty cycles or 1%.

It sounds like the technology hasn't evolved much in the last 20-30 years then.

Around 1995 I was involved with a project where an engineer designed and built a multi-kW CW (I don't recall the exact value) pre-amp out of transistor based modules. There was a plan to apply the same approach to a later project at much higher power levels (100's of kW's) where the conventional approach would have been to use a klystron. I left that team (and the field) though so I don't know if the larger version was ever successfully implemented.

EDIT: I should note that this wasn't in the microwave regime, it was 500 MHz.

Re: Jet propulsion by microwave air plasma in the atmosphere

#47

Post by VBprogrammer »

jacobush wrote: post #44
VBprogrammer wrote: post #40

Earlier quoted context omitted.

A generator is necessarily on the same order of magnitude in size as the motors it is powering.

Is your point that they must necessarily be heavy? Because I don't think so. Modern electric motors are very compact and powerful. Also you'd not need gearbox and shafts. You can completely decouple the fan from the turbine. You could put the fan downwards and the turbine back for instance, or the turbine on the top to hide its heat signature somewhat.

Doing some back of the envelope calculations to see if we're in the right ball park here:

A Tesla motor weighs in at 70lbs (31kg) on its own, not including the inverter or any gearbox, and generates 362hp. Or 11hp / kg.

Using a turboprop because it's easier to do a straight hp comparison, the PW150 has a dry weight of 716.9kg and produces 5000hp continuous. Or about 7hp / kg.

Of course it's absolutely not a fair comparison for a number of reasons, for a start running a Tesla motor at full power for more than a few minutes at a time is going to result in an overheated motor where as the PT150 can do that all day long. The Tesla motor weight doesn't include the inverter which would be needed (unless it's all designed to run each motor at a synchronous speed with the generator...), nor the cooling system, a gearbox (the low pressure turbine of a high bypass engine is only ~4000rpm, the Tesla motor does 18000rpm). Not to mention the elephant in the room, you still have the weight of the generator plus the prime mover (or battery...).

Re: Jet propulsion by microwave air plasma in the atmosphere

#48

Post by jacobush »

VBprogrammer wrote: post #47
jacobush wrote: post #44

Earlier quoted context omitted.

Is your point that they must necessarily be heavy? Because I don't think so. Modern electric motors are very compact and powerful. Also you'd not need gearbox and shafts. You can completely decouple the fan from the turbine. You could put the fan downwards and the turbine back for instance, or the turbine on the top to hide its heat signature somewhat.

Doing some back of the envelope calculations to see if we're in the right ball park here: A Tesla motor weighs in at 70lbs (31kg) on its own, not including the inverter or any gearbox, and generates 362hp. Or 11hp / kg. Using a turboprop because it's easier to do a straight hp comparison, the PW150 has a dry weight of 716.9kg and produces 5000hp continuous. Or about 7hp / kg. Of course it's absolutely not a fair comp…

That just makes me think it might be feasible. If off the shelf components aren’t off by an order of magnitude, that’s a good sign in my book.

Re: Jet propulsion by microwave air plasma in the atmosphere

#49

Post by VBprogrammer »

jacobush wrote: post #48
VBprogrammer wrote: post #47

Earlier quoted context omitted.

Doing some back of the envelope calculations to see if we're in the right ball park here: A Tesla motor weighs in at 70lbs (31kg) on its own, not including the inverter or any gearbox, and generates 362hp. Or 11hp / kg. Using a turboprop because it's easier to do a straight hp comparison, the PW150 has a dry weight of 716.9kg and produces 5000hp continuous. Or about 7hp / kg. Of course it's absolutely not a fair comp…

That just makes me think it might be feasible. If off the shelf components aren’t off by an order of magnitude, that’s a good sign in my book.

To be honest that surprised me. I think the Tesla motor is slightly misleading. If you've ever seen a 3hp industrial motor you'll see where I'm coming from, they are about the same size as the Tesla's motor and probably weigh twice as much. I appreciate that the tech in those motors is very old compared to the Tesla, but if they are that far out you'd think there would be profit somewhere in improving them.

Re: Jet propulsion by microwave air plasma in the atmosphere

#50

Post by morei »

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.

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