
Jet propulsion by microwave air plasma in the atmosphere - Egregore
https://arstechnica.com/science/2020/05/microwave-thruster-makes-for-clean-burning-jet/
======
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.

~~~
tgflynn
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.

~~~
jcims
>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.

That's essentially what they are doing, but in an oscillating path rather than
a continuous path:

 _In the waveguide, the charged particles in the plasma start to oscillate
with the microwave field_ (aka: RF) _while rapidly heating. The ions, atoms,
and electrons collide with each other frequently, spreading the energy from
the ions and electrons to the neutral atoms, heating the plasma rapidly. As a
result, the researchers claim that the plasma rapidly heats to well over
1,000°C._

I don't know if you can rectify RF energy like you can with voltage. That
might be one way to create a more continous path. They might also be able to
add an electromagnetic field to create a net velocity out the provebial barn
door as they do with ion thrusters, but I think the problem there is you just
can't get enough free air path to accelerate to very high speeds...it'd be
like trying to drive an F1 car at full throttle in bumper to bumper traffic.
You could also pass a current _through_ the plasma to generate lorentz forces
like they do in plasma driver rail guns.

One huge advantage with this design is no moving parts and presumably
extremely low manufacturing costs. So it might not be awesome for commercial
airliners but it could be useful for high endurance UAVs. Pack a few dozen of
them and use just the ones you need.

~~~
tgflynn
> I don't know if you can rectify RF energy like you can with voltage.

You can in essence, because charged particles have inertia, they don't
instantly follow the EM field. That's how RF particle accelerators work. But
there you're working with a beam of charged particles, not a neutral plasma.

It still sounds to me that they are essentially using the RF energy just to
heat the plasma and in that case the parent's objections seem valid, though I
don't know much about jet engine design.

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tomalpha
> 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
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.

~~~
macawfish
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...

~~~
tgflynn
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.

~~~
macawfish
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).

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travisporter
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-m...](https://arstechnica.com/science/2020/05/microwave-thruster-
makes-for-clean-burning-jet/)

~~~
dang
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)

------
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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api
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.

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dynamite-ready
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?

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hoseja
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).

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

~~~
whatshisface
The plasma _is_ the resistive coil. Microwaves are used to carry energy to it
because wires would melt.

~~~
mrfusion
How about tungsten coils?

~~~
catalogia
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.

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gnusty_gnurc
What batteries are going to work for this?

~~~
qchris
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.

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

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hammock
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
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.

