A simple balloon powered jet car designed with Solidworks and builded with a Replicator (1.75 filament, 0.4 nozzle)built four weels, four axes (A.STL), four hat ( which should be put at the axes end) and one car and your done ! use low infill ( I use 0.17) for the car to improve thrust-to-weight ratio :) put a little balloon on the top tube, blow in the rear opening to inflate it and ...fire !