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Is there any (current, or planned, or hypothesized) practical application for engines of low thrust - like, below 40N - in the atmosphere?

I'm asking this in relation to the Non-vacuum ion propulsion question: it can be used as an educational tool or technology demonstrator, or a hobby model, but I can't imagine a scenario where someone would find a ion engine or similar actually useful.

Am I missing something or are they simply a thing destined for space and will never find actual use down on Earth?

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Yes. They are being considered as a smaller, more quiet, no moving parts alternative to fans for cooling electronic devices.

Source: http://www.patentlyapple.com/patently-apple/2012/01/apple-reinvents-the-ionic-wind-generator-cooling-system.html

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This is perhaps not exactly what you are asking about, but one possible use is to counteract drag on the ISS. In order to experience aero-dynamical drag, you technically have to be inside an atmosphere, and so an ISS orbital-decay-fighting thruster is not in a complete vacuum, thereby qualifying. This scheme is proposed for the VASIMR ion thruster. This has a few advantages, firstly a lot smaller use of fuel due to a higher specific impulse than chemical rockets, and secondly it is much gentler to microgravity experiments due to the much lower accelerations. (Actually it cancels out most of the acceleration caused by drag.)

For real atmospheric use, the power consumption is the main reason to not us an ion thruster. The NSTAR uses almost 24 kW per N, and then you are much better off pushing air with a propeller.

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    $\begingroup$ nitpicking a bit, the ion propulsion would actually improve the microgravity conditions, as the currently present decelerating force would be canceled out. But yes, not what I meant - I thought of pressures much closer to sea level. $\endgroup$ – SF. Feb 27 '16 at 21:20
  • $\begingroup$ @SF of course, I thought of it when writing, but it was much easier to write "lower acceleration". It covers the same point $\endgroup$ – Hohmannfan Feb 27 '16 at 21:23
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JP Aerospace, a company which provides airships for advertising and who says they hold the world altitude record for airships, is working on using electric propulsion in the atmosphere as part of their Airhip-To-Orbit project. As high up in the atmosphere as buoyancy can get, a solar powered low thrust rocket engine would slowly accelerate an airship to orbital velocity, they propose.

enter image description here

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    $\begingroup$ I wonder if it's just a money grab scam... surely if the density allows the buoyancy to keep it afloat, the drag would prevent it from speeding up... $\endgroup$ – SF. Feb 28 '16 at 2:48
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    $\begingroup$ It seems to be an honest business, and this project is to a large degree run by volunteers AFAIK. It looks stupid, but it doesn't look evil. I suppose it comes down to drag versus thrust up there, which is the essence of the question here. In any case, it is a lot of effort to put a small payload in orbit, compared with for example a secondary payload cubesat on the Atlas V, now even offered for free to universities. $\endgroup$ – LocalFluff Feb 28 '16 at 3:31
  • $\begingroup$ Founder and owner John Powell has discussed it on the space show, more in the earlier appearances. I'm still none the wiser about the physics. $\endgroup$ – LocalFluff Feb 28 '16 at 3:39
  • $\begingroup$ @LocalFluff are you "the wiser" yet? ;-) I've just asked What are the major technical challenges to JP Aerospace's Airship to Orbit? $\endgroup$ – uhoh Oct 2 '18 at 11:23

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