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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

#4

Post by dang »

travisporter wrote: post #2

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...

We've changed to that from 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

Re: Jet propulsion by microwave air plasma in the atmosphere

#5

Post by qchris »

gnusty_gnurc wrote: post #3

What batteries are going to work for this?

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.

Re: Jet propulsion by microwave air plasma in the atmosphere

#7

Post by 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)

Re: Jet propulsion by microwave air plasma in the atmosphere

#8

Post by petschge »

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)

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.

Re: Jet propulsion by microwave air plasma in the atmosphere

#9

Post by tgflynn »

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 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.

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