This is a brushless propeller launcher mostly reusing miniquad parts. The gearbox or pullcord launchers are neat but hard to aim. Pull the trigger to spin the propeller up, release the trigger to launch. 

You will need: 

• A 22 mm diameter quadcopter type brushless motor with M5 threaded shaft. Mine has a 2205 2633kv motor but other lengths and speeds should be fine.
• A BLHELI_32 or S ESC, Dshot600 compatible. I have used a Wraith32 plus.
• A small 3s or 4s battery. I'm using 4s 850mAh
• A ZMR250 carbon fiber arm
• Some M3 and M2.5 hardware
• A microswitch for the trigger
• A raspberry pi pico board
• An XT60 connector
• A Velcro battery strap
• A voltage regulator or BEC circuit to power the pi pico. 5V output
• Protective eyewear. Seriously. It likes to make the propellers explode. 

Instructions:

• Print all parts, and use an M5 tap to cut threads into the propeller adapter. screw the adapter onto the shaft of the motor.
• hot glue votlage regulator and the Pi Pico into the housing.
• install the trigger switch with M2.5 screws. wire it to Pico pin 9 and a ground pin.
• wire the ESC throttle to Pico pin 5 and a ground pin.
• wire power from the ESC to the voltage regulator and the ESC.
• wire 5V and gnd from the voltage regulator to pico VSYS pin and a gnd pin. 
• flash the raspberry pi pico using the UF2 file in the zip archive here. If you would like to build the firmware yourself or customize it, you can find it here: 
  https://github.com/earlynerd/brushless-propeller-launcher
   
• screw the housing together, strap a battery on top, load some propellers. let em rip. carefully. 

Credit to @Nikorasu for the propeller hub design, it works really well. the prop adapter used in this project is a modification of a part from his ultimate launcher. the rest of it is my creation. 

This launcher is fully capable of spinning propellers until they explode into plastic shrapnel. I've design ed the housing to be a decent knuckle guard in case that happens, but its your responsibility to wear eye protection and use the thing responsibly. take care that nobody is standing in the plane of the propeller in case it does fail.

It can send propellers well over a hundred feet, take care where you point it. I've included a customized slightly sturdier propeller in this project that i recommend be used with it. It can use the other props on my profile also but not at top speed. I've limited throttle to 55% in the firmare image included, you can unlimit it if you build the firmware project on my github. 

if you connect the USB cable or battery while the trigger is pulled, it will enter ESC passthrough for BLHELI_32 ESCs. Should be able to configure them that way if you need to.