from Hacker News

Jet propulsion by microwave air plasma in the atmosphere

by Egregore on 5/6/20, 8:10 PM with 51 comments

• by mannykannot on 5/6/20, 11:39 PM

  "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.

• by tomalpha on 5/6/20, 9:53 PM

  > 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.

• by travisporter on 5/6/20, 9:05 PM

  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...

• by morei on 5/7/20, 10:51 PM

  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.

• by api on 5/7/20, 1:55 AM

  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.

• by dynamite-ready on 5/7/20, 12:02 PM

  Is the technology demonstrated in this YT video - https://www.youtube.com/watch?v=U5PYzGgHx14 - in anyway similar?

• by hoseja on 5/7/20, 8:13 AM

  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).

• by mrfusion on 5/7/20, 12:30 AM

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

• by gnusty_gnurc on 5/6/20, 9:06 PM

  What batteries are going to work for this?

• by mrfusion on 5/7/20, 12:22 AM

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

• by hammock on 5/6/20, 9:58 PM

  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)