Pressure calculations.

Since I've got a number of PVC-pipes in different dimensions at home already I thought it would be wise to start with the smallest first (same thickness as the medium sized one). It should be the strongest of the three kinds of pipes I have. It is a 16mm VP-pipe for electrical installations.
















I use the following equation derived by the DARK society of amateur rocketry:
Using the following physical data for PVC plastics, a 16mm european VP-pipe has the following allowed maximum chamber pressure:

Just for comparison, using the same formula for Richard Nakka's PVC chamber pressure test with a 1" pipe:

Which can be compared with 1330 psi by Nakka. As you can see, the ultimate strength predicted by the equation is too high. If it is as I suspect, the yield strength I'm using (7500 psi) is too high.

It is actually quite simple. The yield strength is not a constant but vaires with temperature. Since it's a plastic, it will become even more plastic when exposed to heat. Hence the tensile yield strength must decrease with temperature.

By using the pressure at the instant of failure provided by Nakka, we can deduce an empirical value for the tensile strength of the material that works better for these higher temperatures:
Note that this is highly dependent upon both temperature and burn time. The burn time for his experiment lasted between 0.5 and 1.0 seconds (probably more like 0.65 to 0.7 s by visual interpolation). However, the temperature is unknown (it might be deduced somehow though). But this is as close as we can get to the "real" yield stress.

So readjusting the first calculation (2) we get the following maximum chamber pressure for the 16mm PV-pipe:
This will be used in the calculations for estimating minimum throat area. Stay tuned!

Please read the safety guidelines before attempting anything like this on your own.