While writing my r ecent piece on aviation, I was exploring more experimental forms of propulsion. Surely there is something more exciting to be had for aerospace than just spitting out burning, liquidified, dinosaur remains through a 100-year-old engine design?

During the time I was writing that piece I had gone out for dinner with a good friend of mine who had studied aeronautical engineering. It sparked a long conversation about ambitious propulsion technologies, among them ionic wind thrusters; something that would technically be called an electrohydrodynamic thruster (EHD).

I got back that evening and started researching avidly. I watched a series of YouTube videos ranging from the technical to the fantastical, by scientists and amateurs alike. There's something hypnotising about the purple glow of a corona discharge, the continuous hissing noise, that makes it worthy of a sci-fi spacecraft.

I was awash with experience in DIY electrical engineering (ie, endless failures, undiagnosed problems, stacks of unreadable code) having tinkered in robotics for about a year. Yet my progress there had hit a series of walls that I was figuring how to manoeuvre around. In the meantime I needed another, easier and finite project to renew my enthusiasm.

I opened up my battered notepad of design sketches and got to work with a preliminary design. Firstly, I wanted a skeletal design that allowed for easy maneuvering of components to understand trade-offs involved in finding an optimal set up. That would then allow a second final design that would be embedded within a more rigid and aesthetic structure if I wanted to.

I got onto AliExpress and ordered all the necessary components to at least facilitate a prototype. Military work was to take me offline on exercise for the following 10 days which aligned perfectly for me to return to a stack of parcels covered with stickers written in Mandarin. A few dozen pounds spent and I went to bed in the early hours.

How does it work?

The basic principle of an EHD thruster is to generate sufficiently high electric fields such that it will produce a corona discharge near a sharp electrode, ionising the surrounding air (ie knock electrons off air molecules so they become charged). The resulting ions are accelerated by the electric field and transfer momentum to neutral air molecules through collisions, creating an airflow, ie ionic wind, that generates thrust.

What did I order?

A power supply (variable current and voltage output)
A high voltage generator
Various diameter tube brackets
Copper rings for the anode
Various cathodes for testing: aluminium sheets varying by thickness, copper foil
And I used Lego blocks as a modular, reconfigurable basis for the chassis mount to easily test for different electrode geometry.

The variables I played around with:

Distance between the cathode and anode
Material for the cathode
Power levels

The Results:

Bonus random videos. A plasma arc I created when testing the HVDC generator.

I found anything higher than a 6V input into the HVDC generator produced arcs that interrupted the corona discharge. The optimal level for the components I had was at 4A and 5.5V at the low-voltage input side of the HVDC generartor, which was at the limit of what it would tolerate. (I burned through a cheap power supply box which had inadequate protection against backfeed surges).

The best cathode proved to be the copper foil cut out into a series of spikes poking back towards the anode. It was mechanically very flimsy and would bend much easier than household aluminium foil would. However its sharper, thinner, more malleable form made it a better discharge point. Additionally its higher conductivity (60 MS/m vs 37 for Al) helped but this is less important at these milliamp levels.

Given the anode copper rings had a diameter of 38mm, and cathode copper foil had a diameter of 53 mmm, the optimal distance in this set up was 22mm.

I measured its thrust by placing it downwards onto a set of scales. Crudely-speaking, it produced a thrust of 10g, or 0.1N! So with a setup weight of around 900g consisting of the power supply box, HVDC generator, Lego chassis, electrodes and wiring, (ignoring the need for a separate batter), it could generate a thrust-to-weight ratio of around 0.01. Safe to say this isn’t going to the stars anytime soon.

What did I learn?

Building workable prototypes is surprisingly easy. The hard bits? Reliability, economies of scale, refining solutions that approach physical limitations.

As fascinating as some technologies are, think critically about the laws of physics you are engineering against. I didn't expect to invent anything useful in my room but when the thrust from your near-1kg setup is 10g (and physically limited to not much more than that) it's humbling.

Don't listen to AI chatbots too much on safety, they have to cover their backs. If you do listen to them as gospel you'll never push the limits of your designs. Listen to keen human amateurs instead - they have as much desire for preservation as you and have actually done it before.