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/The area around the nozzle throat seems to be the trickiest part of the print due to the very short layer times. Some things that have worked for me:Increase layer time (and thus cooling time) by printing multiple nozzles at once. I find that printing two nozzles, with one placed upside down, works well for me.Use extra cooling -- point a small fan, or even a stream of compressed air, at the extruder during the layers around the throat.