Hornet on log

Dear Evil Engineer: Can I construct a gun that fires live hornets?

Image credit: Dreamstime

This month, the Evil Engineer offers advice to a villain with an urge to reconnect with nature.

Dear Evil Engineer,

I spent my summer working remotely, coordinating ransomware attacks against vulnerable third-sector organisations. While grateful for the steady income stream during these uncertain times, I’m growing sick of staring at my computer screen for 16 hours a day.

My sister told me that I need a new hobby, preferably one that will get me back in touch with nature and/or involve handicrafts. So, I went on the darknet and ordered a nest of Asian giant hornets, which I’d like to incorporate into a custom weapon. I’d like to use them as projectiles for firing through the open windows of my awful neighbours’ homes. Do you have any advice about how to proceed?

Yours

A nature-loving villain

 

Dear Villain,

Taking time to reconnect with nature is good for our wellbeing, and highly recommended under these challenging circumstances. I encourage all those stuck working and living indoors to get some fresh air and try rustic activities such as flower arranging, mushroom foraging and swan punching. Building a hornet gun sounds like a perfect way to get in touch with nature.

The best way to fire live hornets into your neighbours’ homes would be with a low-energy air gun, which projects the insects using bursts of compressed air. When a spring-piston air gun is cocked, a piston compresses a spring. When triggered, the spring is released, compressing the air in the pump cylinder, increasing pressure, and propelling the ammunition.

This would allow you some control over the direction of the live hornets. A basic air gun like this can be built at home using non-specialist items like bike pumps, compression springs and metal tubing. The barrel should be only just wide enough to allow the hornet to slide through lengthways with its wings folded. You will need to build a small compartment in the rear of the barrel for loading live hornets; this would need to open as you fire (e.g. by having a trap pulled down mechanically as the trigger is squeezed).

The hardest part of this may be in transferring the right amount of kinetic energy to the hornet (muzzle energy) such that it is deposited into your neighbour’s house angry but alive.

For the purposes of this simple model, let’s begin by assuming that the rifle is perfectly efficient (the hornet is accelerated by the full force generated in the gun), and that from the moment the hornet leaves the barrel it begins working to stabilise itself by generating as much thrust as possible in the opposite direction to its direction of travel. So, how much thrust can a hornet produce? Unfortunately, insect flight is an extremely complex field of study. A 2003 paper in the Journal of Experimental Biology warned: “The small size, high stroke frequency, and peculiar reciprocal flapping motion of insects have combined to thwart simple back-of-the-envelope explanations.”

Well, apologies to the biologists, but until you pop some hornets in a tiny wind tunnel and publish peer-reviewed figures on their flight dynamics, it’s the back of the envelope for us here at Evil & Treachery.

Let’s make a rough estimate for the forwards thrust an Asian giant hornet can generate from the horizontal drag it opposes at its top speed. As the Reynolds number for a honeybee is 1,000, we can assume that hornets must experience turbulent drag. Drag is half the product of air density, drag area (drag coefficient × area), and velocity squared, so we first need to calculate drag coefficient and area.

If we grit our teeth and approximate a hornet as a 4:1 elliptical cylinder (coefficient of drag ~0.15) with a maximum diameter of 1.5cm, this gives us a drag area around 2.65×10-5m2. Plugging this into the equation for drag, along with the density of air (1.225kg/m3) and the top speed of an Asian giant hornet (11m/s), this gives us a drag – and therefore also maximum forwards thrust – of 0.002N. Asian giant hornets weigh approximately 1g, so using Newton’s second law, we estimate that they can accelerate at 2m/s2. This would mean a giant hornet reaching its top speed in about five seconds, which doesn’t sound too off-the-mark for an agile flying insect.

Say you want the hornet to be able to decelerate to a comfortable flying speed of 11m/s within 25m. Assuming constant deceleration, this gives us an initial speed of 15m/s. This corresponds to a muzzle energy of 0.11J.

Can I promise that this is the precise muzzle energy you will need to send a live hornet buzzing angrily around your neighbour’s home office at a range of 25m? No! There are many other factors at play, such as the drag experienced by the hornet as it is projected, the drag area of the hornet when accounting for its wings, variation in behaviour and strength between individual hornets, effects of wind, and theG-force hornets can withstand (although given the implications of the square-cube law for small animals, I would be surprised if this was an issue at these energies).

However, I can tell you that a muzzle energy of around 0.11J is a good place to begin experimenting for a range of around 25m. What’s the worst that could happen? If you fire the hornet with too little force, you’ve released a nasty piece ofwork into the world (no evil act is too small to matter). If you fire the hornet with too much force, you’ve essentially got yourself an air gun that shoots hornets rather than bullets (insert the hornet head-first to make the most of their needle-like stingers).

So put on some protective gear, experiment with your new toy, and – most importantly – have fun!

Yours,

The Evil Engineer

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