Paging Kichae ‎

In light of a discussion in another thread the following was said

Kichae, I’m currently an undergrad in physics looking for some projects to work on. I’ve been playing with the idea of building an ionocraft using a HV power supply. That’s a simple start and a proof of concept. Ideally what I’d like to do is make a portable one that runs off of battery power even if for a small amount of time. Drones, for example, only last 5-10 minutes but are still used widely.

The equation for thrust is F = Id/k where I is the current, d is the distance between the wire and the skirt which attracts ions, and k is a constant. Right away we run into problems where the only way to increase thrust is to increase the current but also increases power requirements.

My question is do you think there is any way to build the craft itself to more efficiently create thrust? I was thinking of multiple circles of wires leading towards the center like a spider web and having the foil skirt or whatever the skirt is converge to look like a nozzle. Would that increase the thrust at all? Or only at the cost of increase current due to the wires all being in parallel pulling the same current.

Edit: For the sake of any other readers, here’s the wiki page for ionocrafts

thelazyjaguar also explains it below, but the basic principle behind this is an extremely high voltage in a wire rips the electrons off air molecules which causes the newly made ions to be repelled away and attracted towards the aluminum(or any metal really) skirt, smacking neutral molecules in the process creating a net downward thrust.

Seems Kichae is incognito at the moment…

But hey, I’ll bite…even though I had to look up what an ionocraft was :wink:

The way the ionocraft creates thrust appears (to me) to be mainly by an induced fluid flow. The corona wire generates charge particles which are attracted to the collector. When those ions flow towards the other end, they both push air molecules in front of them out of the way as well as drag other nearby molecules with them. This is going to create a fluid flow and thus a small amount of thrust.

The main purpose of a nozzle is to increase kinetic energy of a fluid. This is usually done by letting a high pressure fluid expand through a small gap (nozzle). Placing multiple wires in the shape of a nozzle is probably not going to significantly increase the thrust of your apparatus over just having multiple wires arranged in another other configuration.

Anywho, I’m no expert, that’s just what kind of sense I seem to be able to make out of this little device. Looks like a bit of fun though, enjoy.

I actually never thought about how a nozzle works as the allowing for a rapid expansion of gases, so thank you!

When I said nozzle, I did not mean of the wires themselves, but of the conducting skirt(aka aluminum foil). But, after having read what you said I doubt that would create a significant increase in pressure to result in a higher KE.

I only mentioned extra wires in the hopes that more electrons flowing and consequently knocking into air molecules would produce more thrust. Doing that would only increase power requirements though because higher electron flow means higher current.

I guess I should have gave an explanation of what an ionocraft is and how it works first :grimacing:
Edit: Fixed that

Yup, more wires or longer wire lengths would likely increase the total thrust as you would be pushing a larger total area of air and thus more mass (F=ma). Unfortunately it would also increase the net weight of the apparatus as well as energy requirements. We just can’t get around those pesky conservation laws.

The only way I could see nozzle’s actually being used in this setup is if they were actually around the corona wire. With nozzles in the direction of the skirt. The nozzles would have to be in a choked setup whereby they limit the flow rate through them. Ideally the charged particles would build up behind the nozzle and increase the pressure and thus their velocity when they exit. Of course you would then have to have a material that didn’t catch on fire or conduct electricity or actively bond with ionic particles :smile:

Good luck with that! :wink:

Yeah, and more importantly the F equation that governs the thrust. Conservation gets ya every time!

That’s an interesting idea…I wonder if it would be possible to have an insulator cover most of the nozzle except where you let the electrons jump down to the skirt. Remember, we aren’t using the ions or charged particles directly, but using them to bump into neutral molecules, otherwise conservation of momentum would say we’re producing no net thrust unless I had an electron gun neutralizing the ions as they left the nozzle. In which case I would have just build a good old fashioned ion engine which is more complicated than I want at the moment.

What do you mean by a choked setup for the nozzles? Perhaps I am misunderstanding something.

I’m no physicist, but if anything I’d expect it to result in lower thrust. The function of a nozzle, at least as far as rockets are concerned, is to help contain a fluid and thereby increase it’s pressure (and temperature, exhaust velocity, and specific impulse).

Operative word being help. With nothing to contain the fluid at the other end, a nozzle would just scatter the molecules leaving you with less net force. Placing the nozzles between wire and skirt and containing the ions with an electromagnetic field could maybe possibly work… but it would probably be easier to just go with the ion engine.

On the subject of scattered molecules, though, what you could do is try build a ducted ionocraft.

With no tip vortices to reduce, the efficiency bonus would be minimal, only preventing molecules from entering/leaving at too large an angle relative to your thrust vector, but with extremely lightweight materials - a thin layer of aerogel, perhaps? - it just might add enough thrust to lift it’s own weight.

Oh yeah and it would significantly decrease the probability of accidental electrocution.

You would be better off putting a little aluminum fan than doing the classic ioncraft thing :wink:

Anyway let me help you with @Kichae, @Kichae the people need you. He should get a mail now.

I think you might’ve missed the point.

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Bringing my ideas into the discussion … also no physicist but highly interested.

The most straightforward way to increase net thrust would be to increase either current and/or the air-gap.

Another way would be to decrease the mass of the contraption. I don’t know about supporting materials, I guess lightweight wood is already quite a good trade off. Using Aluminium wire could be also good. Though you trade electric conductivity for lower density. Aluminium has a better density to electric conductivity ratio though. I guess Aluminiumfoil is already standard on those craft.

Now shaping the airflow … I found this Blog post:

I suggest building a tunnel with two sheets of lightweight non conductive material on both sides of the air gap all the way from the corona wire to the collector foil. This will probably hinder air moving out of the way in different directions then down and may increase thrust. I think the distance between those sheets is significant. The less distance the more effect you’ll probably see. Problematical could be changing pressures pulling the sheets around and closing of the passage from the corona wire to the collector foil.

In the comments of the blogpost someone suggested using magnets to force the ion flow that exits upwards from the corona wire downwards …

Mass flow rate through a nozzle can become limited “choked” when certain conditions are met. In other words, you can increase pressure all you want on one side, but the mass flow rate will no longer increase with it (meaning you can blow your craft up if you exceed material limitations).

With a bunch of like charged particles all repelling each other in a confined space, the pressure would increase. The charged particles could go from pushing other molecules around to being the propulsive force themselves. This could basically be an electrical analogy of combustion chamber and exhaust nozzle. Mind you though, the requirements to contain and sustain such a contraption would likely be impractical…especially if you wanted to produce work.

EDIT: Now that I look at the article closely, I should be saying velocity and not mass flow rate because the fluid density can increase with increasing pressure.

This actually isn’t really my field of expertise, so I’m not sure what I can really add here (plus, I’m suffering from a mighty cold or flu-like virus right now, and not exactly in a functional head space), but here goes…

Nozzles basically do two things: Constrict the passage of a fluid through a limited space so as to force the pressure of that fluid to increase, and direct that fluid in a cohesive flow, so that you maximize your change in momentum in the direction of your choice. I’m not sure how much of a difference a nozzle would make here, since I’m not familiar with the specifics of the engine design, but I suspect you’d need a semi-sealed container within which pressure can build up for a nozzle to do you any good, and I don’t know if that’s compatible with the whole ionization thing or not.

Now, if you could build a nozzle out of the magnetic field…

Wow lots of replies!

I had actually been thinking along the same lines hoping that someone else would either confirm or deny this. If anything, I would think some of the neutral molecules would actually lose some KE or bounce back up and knock other molecules moving downwards away losing thrust. If the nozzle were to successfully increase the pressure, I also thought that instead of moving through the nozzle the gas could just escape out the top or through the airgap. After thinking about it a lot today I came to the conclusion that a nozzle probably won’t work too well.

Exactly :wink:

The crafts are already built out of balsa wood sticks and aluminum foil with pretty high gauge wire. I’m not sure what more you can take away from the craft.

Thanks for the read Lomsor! That is very interesting. I wonder how you could direct that airflow more efficiently and in the same direction. Before reading the article your response to the tunnel confused me but now it makes a lot more sense. I came to a similar idea when thinking about it. When I get around to build and testing one of these, I will definitely create some sort of tunnel to see if that changes anything.

I wish you well in getting better! I’ve had a rather mean cold too for the last week and a half and it hasn’t been too fun either.

Like I said earlier, I’m not sure I could actually create an increase in pressure with a nozzle without it leaking elsewhere. So a nozzle might not be something that could help too much. I’m not sure a magnetic nozzle would help too much…the air molecules that are ejected are effectively neutral so a magnetic field wouldn’t affect them. What it could do is accelerate the ions and increase the energy imparted to the molecules…as well as maybe redirect ions that shoot upwards back downwards to create more collisions. It’s a thought.

How would you create a magnetic nozzle? I’ve also thought about somehow generating an oscillating AC current that isn’t uniform so that it would create a lopsided magnetic field forcing the charged particles downward. I’m not sure how to engineer that or how efficient that would even be compared to just adding more voltage.

I just got done speaking with my professor about this and extending the idea further. We wondered why exactly the craft are built in triangles and if that’s the most efficient way. Our guess was that it was probably the easiest to make with the cheapest materials. I also thought about changing the gas in which it’s being used in to see what difference in thrust it would make. It would be a very interesting experiment to see the thrust created in regular air compared to inside something with a lower k(ion mobilization constant) value) like xenon or argon.

I need to go buy some sticks and wire and build a cheap one and borrow a power supply from school for awhile to do some testing.

Thanks for the responses guys!

Well, the whole idea is basically that the ions are used to control the flow of the neutral particles via air friction, right? So the bulk flow of air should follow the path of the ions.

I’m assuming a sheet current travelling up a cylindrical conductor, such that the magnetic field lines wrap around the engine. Negatively charged ions, then, should spiral around the conductor, travelling “up” the outside wall, and “down” the inside cavity (if it’s the positively charged ion doing all of the work here, it all still applies, of course, with the current reversed). Without doing any calculations to verify this, my gut feeling is that if the conductor is conical, the ions should still travel parallel to the conductor, in essence making the ion engine itself a nozzle.

That’s my guess as well. A pyramidal nozzle works as well as a conical one, at any rate, I think the magnetic nozzle should still be applicable even if it were a triangle.

One would think that a more easily ionizable gas should increase thrust, right? Especially if you can doubly ionize something. If not, at the very least the heavier the atoms, the higher percentage of them that will be ionized.