# Troubleshooting a DIY Ion Thruster

Been following a Make-Magazine article on how to make your own ion thruster: https://makezine.com/projects/ionic-thruster/

Went through the tutorial step by step, not seeming to get any thrust out of my thruster. I've checked the conductivity between the nails and the copper wire, they all conduct. I've checked the conductivity between each copper tubing piece and the copper wire, they all conduct. The distance between the nails and the copper tubing looks about the same at it is in the tutorial.

One suggestion I had is that my nails aren't perfectly perfectly centered, but I'm not sure if there's anything else besides this that leads to this setup not putting out any thrust.

Caution: Do not try this at home! It uses a 30 kV AC neon light power supply.

I've followed the instructions in this video, including the recommendations for safety:

Here's my build following the tutorial:

• I suspect the sharpness of the points on the nails is key. This relies on field ionization of the air, and that is caused by points on the nails sharp enough so that electric fields near the tip are sufficiently high (of order volts/Angstrom) that they will rip an electron off of individual molecules or atoms in the air. – uhoh Mar 3 '18 at 21:02
• How does the 10kv transformer in the OP video compare to the 30kv in the Make video? Is 1/3rd voltage sufficient to create the necessary electron flow? – fred_dot_u Mar 3 '18 at 22:32
• @fred_dot_u Good catch! The ionization field strength is a function of both the potential and the tip radius of curvature. Consider posting an answer with it? – uhoh Mar 3 '18 at 23:38
• make your own ion thruster - nice. Where is the DIY video on a nuclear thermal rocket? That's what I want... – ArtisticPhoenix Mar 4 '18 at 4:13
• The nails in the first Vid are coper, the ones in the second are grey, and much longer. To me this seems they would have more resistance. – ArtisticPhoenix Mar 4 '18 at 4:20

The instructional video shows a 30kv transformer providing the electromotive force for the ion engine/desk fan.

The user-created make video provides a clear reading of a 10kv transformer. That appears to be a strong indicator for lack of strength in the build.

Chuck up a set of nails in an electric drill and sharpen the points as suggested by uhoh, then pump up the volume... oops, increase the voltage with a more powerful transformer.

The tutorial video suggests a one-inch spacing for the exhaust nozzles. The maker video appears to be proportionally similar, but I suspect that no harm would come from bringing the ion destination a bit closer to the emitter. Not too close, perhaps another five to six millimeters.

It's difficult to fathom science in Imperial units, but these videos show that it works too.

• @Giskard42 I think ~10 kV/cm would be the "breakdown voltage" or field actually, in air, when a spark happens, and it involves accelerating naturally occurring free electrons fast enough to start a chain reaction. It does not produce the initial ionization, only the cascade. This application instead uses field ionization which happens at the tip only, where a small radius of curvature produces extremely high fields but only very short distances, so short that a cascade can't be initiated. These are two very different things. – uhoh Mar 4 '18 at 15:16
• Due to an error in my answer, is it advisable to delete it from the system? – fred_dot_u Mar 4 '18 at 22:32
• @fred_dot_u Your answer certainly looks good to me, I don't see any error. It also has +3 up votes so far (and no down votes) so that means others find it valuable as well. I'd say it's a keeper for sure! My comment to Giskard42 was just to clarify that the concept of breakdown voltage doesn't apply here. The air is not breaking down, or there would be a giant spark. It's just calmly ionizing one atom at a time at the tip. – uhoh Mar 5 '18 at 2:41

I very strongly suspect the sharpness of the points on the nails is key. This relies on field ionization of the air, and that is caused by points on the nails sharp enough so that electric fields near the tip are sufficiently high (of order volts/Angstrom) that they will either rip an electron off of individual molecules or atoms in the air producing positive ions, or add an extra electron producing negative ions.

Field desorption (FD) is a method of ion formation used in mass spectrometry (MS) in which a high-potential electric field is applied to an emitter with a sharp surface, such as a razor blade, or more commonly, a filament from which tiny "whiskers" have formed.

Separating the electron from a neutral atom or molecule is hard work and usually has low efficiency. In real ion thrusters for space application, the majority of the weight and power consumption are a result of the ionizing aspects of the design, rather than the acceleration aspects.

You'll need to find a way to either sharpen the nails, or produce some whiskers on them. I think this was omitted from the tutorial you are following.

below: Slide 52 from Ion sources Ionization and desorption methods explains that the extremely strong field of order 1E+10 V/m needed to ionize atoms is obtained by a needle at 10 kV potential when the radius of curvature of the tip is decreased to 10 microns. As long as the distance to ground is large, it matters very little if it's 1cm or 10cm. From the point of view of the field at the sharp tip, that's nearly infinity. Almost all of the potential drop happens in the first millimeter or so, and the field is only high enough to ionize atoms or molecules at the very tip.

## Field Emission Electric Propulsion (FEEP)

This principle is also used in some designs for electric thrusters for small spacecraft. The implementation is a little different though. Instead of a metal point in a gas, the needle is hollow and a liquid propellant is introduced into the capillary space. When there is a high field, the liquid naturally forms a cusp or point called a Taylor cone. Individual atoms will become ionized and leave the surface.

When operated differently, the Taylor cone will reach a mechanical instability and ionized droplets will be formed. This is called Electrospray ionization and is distinct from a field ionization ion source.

Here's one example of an electrospray thruster with an array of microfabricated sharp points.

Field-emission electric propulsion (FEEP) is an advanced electrostatic space propulsion concept, a form of ion thruster, that uses liquid metal (usually either caesium, indium or mercury) as a propellant. A FEEP device consists of an emitter and an accelerator electrode. A potential difference of the order of 10 kV is applied between the two, which generates a strong electric field at the tip of the metal surface. The interplay of electric force and surface tension generates surface instabilities which give rise to Taylor cones on the liquid surface. At sufficiently high values of the applied field, ions are extracted from the cone tip by field evaporation or similar mechanisms, which then are accelerated to high velocities (typically 100 km/s or more).

• Dear lord I didn’t realize how much more there was to getting this drive right that wasn’t explained in the video. Deep-diving on all your feedback tonight, thank you all so much for the great insight. – Colin Warn Mar 4 '18 at 20:31
• @ColinWarn I added some more material. You can try to use a file to sharpen your nails, or replace them with very sharp needles. There may also be chemical methods to grow whiskers on the nails. Sharpness is the key to making this work. – uhoh Mar 5 '18 at 2:37
• really appreciate the help, thank you so much. Any suggestions in regards to search queries/websites to look at if I'm interested in deep diving on all the intricacies of optimizing an ion thruster? – Colin Warn Mar 6 '18 at 18:32
• @ColinWarn it's a deep subject and I'm only familliar with some of the basic physics, rather than the implementation. I would recommend you as least skim through all of the questions and answers in this site related to ion thrusters either by searching text or searching by the tags. – uhoh Mar 6 '18 at 19:06

Final answer to this question: Talked to a professor at Cal Poly who specializes in electric propulsion about this project, she tells me this Make Magazine project shouldn't work at all. She stated that there needed to be way more than 7 cathode "holes" (i.e copper piping), and that this should only be able to work in a vacuum.

I did notice in this project, when I had a HUGE voltage spike from my power supply being turned on, that there was a little movement from the napkin placed up front. My guess is that the project from this Make Magazine only worked because it utilized extremely high voltage. In theory however, from my year of talking to people and looking around on the internet, this shouldn't be able to work.

• I had a similar experience with Make's Stirilng engine project. – Chris B. Behrens Feb 6 '19 at 23:17
• "this should only be able to work in a vacuum" doesn't it use air as the propellant? – Organic Marble Feb 7 '19 at 16:19
• It turns out your professor is not 100% right, or that you've at least incopletely quoted them. See this answer which links to this which eventually links to the Nature video Ion drive: The first flight which does indeed fly using the principle of field-ioniziong air molecules and electrostatically accelerating them to produce thrust at sea level. – uhoh Sep 16 '19 at 2:23