The hardest obstacle I've had to overcome in the development of PVC solid and hybrid rocket engines is in the design and manufacture of good reliable nozzles.

Most rocket aficionados like me don't have access to specialized tools or the skills to make nicely machined steel or graphite nozzles and so we look for technique that can be worked with every day tools and readily available materials. Such is the case of the ceramic nozzle for PVC rockets here presented. A practical, easy and cheap way of making very efficient nozzles out of modeling clay, shower pan liner and the same PVC tubing with which the engines are made of.

DESIGN

This technique can be used to make ceramic nozzle inserts for different diameter engines and is basically a guide of what I have found to work. There is no one size rule so I recommend to experiment with different molding tube dimensions and angles till you come up with what works best for your specific needs.

The first thing needed to make a nozzle is to find the design parameters based on the rest of the rocket engine for which the nozzle is to be made. I'm not going to go over this topic here but will recommend where and with what to find such parameters. The most important parameter in a nozzle is the throat diameter, these are some programs you can use to calculate throat diameters:

SRM.XLS - Solid Rocket Motor Design Written by Richard Nakka.
SRM.ZIP Version: 1.00 536 kbytes Zipped EXCEL 2000 spreadsheet - Freeware.

John Bolene nozzle calculator - web based applications very easy and reliable. Don't remember where I got this from but all the credit is to the master.

HDP - Hybrid Design Program by Rational Technology - Emil Johnsen
Hybrid Design Program helps you design hybrid rocket motors by calculating all the essentials you need to build your motor.

Other parameters important in nozzle design are the Convergence / Divergence angles and the Expansion Ratio