Summary
Here's a video of the String Launcher in action. It was inspired by a video I saw by Bruce Yeany. His string launcher is beautiful and I knew I lacked the woodworking skills to replicate it, so I just 3D printed what I would need.
Unfortunately I can't share all the details. I don't know were the motors came from. They were in a box of left over stuff from a physics make and take workshop from 10 years ago or so. However, I made this in Tinkercad and you can easily go in and edit my model to fit what ever motors you happen to find.
Post-Printing
Print without supports. Mount the motors at wheels. I used a little masking tape around my motors to make them fit snuggly in the holes. Then glue the string guide to the main body as shown in the picture.
I put a layer of EZ Fuse silicone tape around one of my wheels. This helped increase friction and also close the gap between the wheels a tad.
Then wire the motors so that they will spin to pull the string through. I know you could get away with one motor and one wheel that spins freely. I decided to go with two motors to make mounting the second wheel easier/more reliable.
The only other thing to remember is to feed the string through the string guide before you tie it into a big loop.
Phun Physics Questions
How is it possible to push a string?
Can you use the arc to determine the velocity of the string as it leaves the launcher?
Does the string fit a model of projectile motion? If so, when does it stop being a projectile?
Is this related to the Chain Fountain?