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This experiment, which is being presented as an initial exploration of the feasibility of using microwave-generated plasmas in a sort of electic jet engine, has been attracting some attention in the general media, for example Ars Technica and Bloomberg News. To put it mildly, I am highly doubtful of the method used to estimate thrust, or even that it is demonstrating any thrust at all, but am I justified in that skepticism?

The experiment involves a vertical quartz-glass tube with an inside diameter of 24 mm. A compressor pumps air into the bottom, and immediately above that, the tube passes through the apparatus to ionize and heat the air, forming a plasma with a temperature up to 1,000 °C. The tube extends upwards beyond that for about another 23 cm, and the upper end is open to the air.   The thrust measuring device is a hollow steel sphere, diameter 75 mm, with a hole in it allowing small steel balls to be dropped into it to change its weight. In use, this sphere is balanced on the end of the quartz tube, and the weight is adjusted until the airflow causes it to wobble (there is a video in the figures section showing this.) The actual measurement of the "jet propulsion force" is the minimum weight of the sphere needed to prevent this wobbling.

On the basis of these measurements, the authors claim to have demonstrated, in this device, a "total propulsion pressure" of up to 24 kN/m², which they say is "comparable to those of a conventional jet engine of an airplane". I am not sure what "total propulsion pressure" is, but my back-of-envelope calculation suggests that they might be thinking of a jet engine's static thrust divided by the nozzle area. Regardless of whether that is so, their figure is simply the weight of the ball divided by the area of the tube, with the corresponding figure for the no-microwave situation subtracted.

Where does one begin to analyze this device? For one thing, at the point where the ball is not yet wobbling, there is presumably no flow (or just a little leakage.) Surely this is a static situation, with little or no relevance to what thrust might be generated? Furthermore, does the weight at which the ball will begin to wobble depend essentially entirely on how much pressure the compressor can reach with its outlet blocked? And pressure given here is only 1/4 atm. -- how much thrust can you hope for, from such a small expansion ratio, even with the best possible nozzle design and a 1,000 °C plasma?

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  • $\begingroup$ Oh man, if that is science, I'm Scrooge McDuck. Like they didn't have any load cells in the lab 🤣 $\endgroup$ – Jpe61 May 9 at 17:11
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    $\begingroup$ I do think this Q should be posted on physics SE... $\endgroup$ – Jpe61 May 9 at 17:12
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    $\begingroup$ @Jpe61: ...or Science Fiction & Fantasy SE. $\endgroup$ – A. I. Breveleri May 9 at 17:34
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    $\begingroup$ This is an experimental device in a laboratory. Its connection to aviation is hypothetical, at best. This is really a question for Physics $\endgroup$ – CatchAsCatchCan May 9 at 18:43
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    $\begingroup$ This is a decent question, but it's definitely one for Physics or Skeptics SE. The paper is full of holes. All I can say from the aero perspective is that they've basically invented an electric afterburner. $\endgroup$ – Therac May 10 at 6:58
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No, it's not measuring thrust. It might be measuring a proxy for thrust density (force per unit area). Anybody could use their setup to measure a ducted fan or a turbojet or even a rocket, but nobody does. They themselves could have calibrated their rattling-ball instrument with a conventional thruster of known, conventionally measured, thrust (a $20 electric ducted fan from a hobby store). (What's the threshold for rattling, anyways?)

As Jpe61 commented, it would have been much cheaper and simpler to place their thruster on a load cell (a fancy bathroom scale) and observe how downforce increases when the thruster is running. $100 instead of fancy metalwork. See, for example, fig. 6 and reference 23 of Prasad et al, "A laser-microfabricated electrohydrodynamic thruster for centimeter-scale aerial robots," https://arxiv.org/pdf/1906.10210.pdf. See also the fancier inverted-pendulum thrust stand used at Princeton to measure plasma thrusters.

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    $\begingroup$ Come on Camille, you blatantly revealed what I was trying to mystify by using "load cell", I was going to write bathroom scale, but then I though not sciency enough, gotta upgrade. 😃 $\endgroup$ – Jpe61 May 9 at 19:26
  • $\begingroup$ I am being somewhat unfair here, as I did not specifically ask this in my original question, but I seriously doubt that the rattling ball could provide a valid indication of either thrust or thrust density, regardless of how it is calibrated, given that it significantly obstructs the flow. Would you consider that to be a reasonable objection to this experiment? At the very least, I think the validity should be justified in the paper, unless it were already a well-established technique (I am trying to give the authors every benefit of the doubt here.) $\endgroup$ – A Raybould May 10 at 0:06
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    $\begingroup$ To be a valid scientific paper, it should have a hypothesis which is then studied using established methods. If this steelball -thingy is a valid established procedure, it will be stated in the paper. If not, well, you know. You can't do both at the same time, study new propulsion and measuring tech. It's either or. $\endgroup$ – Jpe61 May 10 at 12:20
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There are really two things about this experiment, one is good science (if of dubious practicality), the other bad.

To take the bad first, the experimental setup is extremely crude and the thrust measurement/calculation inaccurate. There are far better ways to measure thrust. Another answer has commented on this. From this perspective, the experiment is worthless.

The good science is, perhaps surprisingly, the use of microwave heating to create thrust. The device is basically a thermal ramjet. You ram air into one end of an asymmetrically-shaped duct, heat it in the middle and allow it to expand out the back. A conventional ramjet burns fuel vapor to do the heating. Towards the end of WWII German researchers even had some experimental success with burning powdered coal and Lippisch (of Messerschmitt Komet rocket fighter fame) studied a delta-wing plane powered in this way. Rather less romantically, the Meredith principle uses the waste heat from an aero engine by ducting-in the radiator. First used on the Spitfire, it later boosted the Mustang's performance quite significantly. So why not use an electrical heater? A perfectly good idea in principle.

But, speaking as a sometime plasma power system engineer, making the thing efficient is a huge challenge. In my day it was all meaty inductors of coiled copper pipe with oil-based coolant flowing through, air- or vacuum-gap capacitors and huge inefficiencies (hence the oil cooling). No doubt we can nowadays do clever stuff with switched-mode solid-sate digitally-controlled games, but at the high voltages needed to strike and sustain a dense plasma in air, it would still be a big challenge to get the efficiency comparable to say an array of old-fashioned heating elements.

There are other big engineering problems. With dubious efficiency, the power levels needed for aircraft flight would dump very large amounts of heat into the circuitry - keeping it all from melting would be hard. Also, a dense plasma has a high concentration of ions and free radicals (charged molecules), and hence is chemically highly reactive. It tends to eat one of the electrodes with unwelcome rapidity and, depending on the local conditions, will either eat into the duct walls or deposit oxidized electrode material on them. None of this is good for low maintenance or long-duration flights.

Finally, the thermal ramjet is inefficient at low speeds. It needs an alternative thruster to take off and get it up to a few hundred knots, and really only comes into its own at supersonic speeds. So it can never be a solution for subsonic mass air travel. A hypersonic suborbital trans-global spaceplane? Maybe one day, I won't bet on that one.

So - good science, dubious practicality.

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