This is a work in progress
It's tricky to print a balloon rocket car due to the wheels and axels. So I figured "why not use the LEGO wheels we already have?"
Basically this is the mechanism for a typical balloon car but embedded into a custom LEGO-compatible body, so you can just snap it on to whatever LEGO vehicle you want.
Place a standard balloon over the lip of the "pipe" on the top. Blow through the rear nozzle to inflate.
I'm still working on variations - my goal is to showcase the differences in nozzle size for instance, so they can be used to help kids understand Newton's laws etc.
The bottom of the model is LEGO compatible, so there is some bridging. I'm still zeroing my best settings, please feel free to share best practices in comments.
Be cautious of your bed temp, if it's too high the bottom will warp enough to prevent it from clutching LEGO studs correctly. I'm using 45 and it's okish.
Infill is a balance - you want this lightweight, but it also needs to be infilled enough to be airtight. If you're using as a teaching tool you could also make a bunch of different infills to demonstrate the weight impact.
No supports should be needed, the interior piping is angled enough. I have a small support built in to the tube model that will snap off easily.