Balloon Torpedo (moar balloons = moar faster)

Ten years ago, I saw this article about a balloon-powered torpedo and have always wanted to test it out for myself. I really liked the idea of the acceleration not only coming from the volume of water contained in the balloon but also the same volume being displaced from in front of it at the same time.

A few days ago, I realised how easily I could mimic this research using a printed torpedo and a single modifier that removes the perimeters (and infill anchors) from a section of the part. The trick of course is to use gyroid infill because it means all the infill is still one contiguous volume. I used 20% for the open section and 100% for everything else (I didn't chase neutral buoyancy this time, but you can if you want). I printed it from TPU just to make it super robust and foot-friendly if it happens to get stepped on. Oh, and I made the fins spinny because it looks cool and helps with stability. 

The key to making this a viable pool toy AND knockoff of the original was finding a way to refill it easily in the pool. For this I picked up a $10 550ml syringe on Amazon (it's pretty self-explanatory from there). Now, my version doesn't work as well as the one in the article, but it's not bad and I'm certain it holds up to playtime a lot better.

Speaking of playtime, here are some different experiments to try (pro tip: layer on extra balloons for more thrust as desired and pre-stretch them tight and centered so they gather any slack at the bottom for best results/straightest travel).

1. Fill with water and then fill with water from syringe (classic torpedo mode)
2. Fill with water and then fill with air from syringe (point up, release near surface)
3. Fill with air and then fill with air from syringe (release from depth while pointing up, try not to hit yourself in the face)
4. Fill with air and then fill with water from syringe (I just had to complete the permutations, I know you understand)
5. Fill with water and just toss it around (like a normal torpedo pool toy)
6. Fill with air and let it go from underwater while pointing up (similar to #3 but less risk to your face) 

I've also included a couple press-fit nozzle adapters that change the size from 12mm to 9mm or 6mm for additional experiments, but TBH I haven't found myself using them. The extra speed you get from 12mm is more exciting than having a longer duration “burn” with the more constrained options. It may also be fun to fool around with a tiny valve (like for aquarium airline) and do some really precise tuning, but I haven't gone there yet.

Anyway, I think there's still a lot of fun to be had with different designs that are made for different balloons, especially ones that are taller than my Prusa Mini can print. If you test out any other options, please post a comment so I can check them out!

Finally, if you need some pointers on how to pair the balloons with the torpedo, check out this helpful video. 🚀

Bonus video of the first test fire when I used this model instead of my own: