Every Rocket needs a nozzle, and converging-diverging de Laval nozzles can accelerate their exhaust gases to supersonic speeds.

This Thing allows you to change parameters to design your own optimal rocket nozzle -- just click on the "Customize" button and the rest should be self explanatory.

You can find the OpenSCAD source code in the download section, and learn more about the geometry and trigonometry behind the nozzle at:
http://www.waters.to/blog/rocket-nozzles-part-1-the-math/