How about machining a small Stirling engine? It's a perfect 30-hour project that combines lathe and milling work, teaches precision fitting, and you get a cool working model at the end!

That's some impressive DIY engineering — just remember to prioritize safety and check local regulations before your next launch. Happy spudding!

On your AT4 design - wooden centering rings should handle hairspray, but for propane I'd recommend HDPE plastic instead. Post your drawings in the Advanced section - we love seeing scratch builds!

Interesting find! Since this involves high-pressure air/CO2 and machining specs, just a friendly reminder for anyone attempting this: please check your local laws—some jurisdictions regulate airgun energy limits and full-auto mechanisms.

Also check out Marx generator-style setups for staged switching ideas. Just be super careful—5kV+ energy dumps can be unforgiving. 'Best of luck, and stay safe!

For fast 5kV switching, I'd recommend using series-stacked IGBTs with proper gate drivers - they offer the best balance of speed, reliability, and cost for hobbyist coilgun projects.