Mainly this is an exercise for me to learn openscad. I found that I had to re-learn a great deal of trigonometry, and a couple completely new ideas such as bezier curves.There is too much to explain here about rocket nozzles, but in brief: in the combustion chamber, the flow is subsonic (Mach<1); the constriction of the throat is designed to increases the pressure and temperature, and therefore the speed of the flow, to Mach =1; In the nozzle extension, the hot gas is allowed to expand, decreasing the pressure but further increasing the velocity to supersonic speeds. Ideally, when the gas exits the nozzle extension, the exhaust pressure is the same as the ambient air pressure.This design is an approximation of the ideal geometry, using the formula of G.V.R Rao as described in a paper from 1958 .(http://arc.aiaa.org/doi/abs/10.2514/8.7324)  You can't actually read that paper, though, since it's pay-walled, so I learned the basics here: http://www.braeunig.us/space/propuls.htmThere are many parameters in the file, so it's not easy to read. It also won't calculate the optimum shape based on your propellant choice and flight regime; you have to figure out that stuff yourself. but you can define the ideal throat radius, and expansion ratio, and the geometry of the throat and exit nozzle size and shape will be correct for those numbers. These would look great on a model of the space shuttle or saturn V. You could also have fun playing with them with compressed air or even steam (Be Careful!). It could be fun to cast one in metal and try it out with actual flames, but that truly is rocket science, and you're likely to blow something up in the process.This is an ongoing project. the most up-to-date files will always be at my Github:https://github.com/Buback/ParametricDeLavalModel