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Old 05-07-2020, 01:30 PM   #6 (permalink)
freebeard
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Quote:
Recently, freebeard posted on the demise of the Airbus/RR electric-hybrid testbed and remarked he could not find a general thread for it.
I was looking for that thread but found this one first.

arstechnica.com: Microwave thruster makes for clean-burning jet
Air-burning plasma thruster may be competitive with jet engines.


The discussion on Slashdot (here: science.slashdot.org: Scientists Create a Prototype 'Air Plasma' Engine That Works Without Fossil Fuels (sciencealert.com) ) focuses on scalability and missing data points.
Quote:
Originally Posted by q4Fry ( 1322209 )
Thanks for the link. I found the next two paragraphs to be interesting, too. Emphasis is mine.

The question is one of scaling. At the air flow rates (around 1m3/h) and microwave powers (less than 1kW) that the researchers tested, everything scaled very nicely. But the airflows are in the region of about 15,000 times lower than those for a full-sized engine. The thrust also has to scale by about four orders of magnitude (meaning the power does, too). Extrapolating linear trends over four orders of magnitude is a good way to be disappointed in life.

I also believe that the warning signs are already in the paper. If you look carefully, there are some missing data points. For instance, at the highest microwave power, only lower flow rates are tested, while for low microwave power, all flow rates are tested. That seems like an odd omission. I suspect the plasma is not stable at high flows and high powers.
Not nearly as interesting as the Navy patents.
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We are operating at an overall mechanical efficiency of only four percent… Therefore, we find that if we increase the overall mechanical efficiency to only twelve percent we can take care of everybody. That three-fold increase in the overall efficiency can only be accomplished by redesign. – R. Buckminster Fuller

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