This description here is the way how I do it now. It works the best for me at the moment. I don't know if it's the best there is, if you are familiar with a better way, i'm open to suggestions.
The highest pressure i was able to launch a rocket with was at 140 psi / 9.7 bar.
With the standard 87 mm diameter of the bottle that is equivalent to a pulling force of over 5500N on the glued binding!.
This binding i make with a two component resin from polyurethane, sold in the Netherlands under the brand BISON, named: Kombi power, polyurethane. On the package of the glue is stated a maximal strength of 160kg/cm^2 and also ............ that PET material can't be glued with it!! In fact, according to the glue wizard, there is no glue suited for PET material. Puzzled why it can hold that amount of force?. The strength comes, as far I can see, out of the hardened polyurethane that fills the grooves on both sides of the bottle walls. The grooves are made before the parts are glued together with a coarse file for wood.
So only a very small cross section of the polyurethane is loaded with the seperation force, hmmmm, there must be a better way, but how?. Well i will ask the people at Bison if they have an advice.
Cross sectional drawing of the glue area with the working forces on it at 10bar
The forces given here, is for bottles with a diameter of 8.7 cm and an overlap of the glued area of 4 cm. The total pressure on the inner wall on the glued area is 10600N. If both bottle walls have the same thickness and material properties, this pressure will be divided equally over the two bottle halves. The inner part expands (i measured about 3%)so the outher bottle exerts about half the total pressure on the inner bottle
Instructions for joining two bottle parts
Cut of the top and bottom part of a PET bottle, so that only the straight cylindric part is left. Do this with several bottles and try which combination(s) fit the best in each other the tighter the fit the better.
Here, in Holland, we have PET bottles with a multi layer (somewhat thicker) wall, these bottles are made for recycling a couple of times and withstand higher pressures compared with the thinner wall, one way bottles. I found that most of the times a best fit is reached with a (Pepsi / Fanta / Coca ) bottle on the outside and a Spa bottle on the inside. Sizes vary a bit, so if you look around, you will find good combinations
The spa bottle has somewhat thinner walls and is has the smaller diameter. Here on the right you see the cut-off cola bottle. The inner side is rougend up with a very coarse grained type of sanding paper and grooves are made with a coarse file for woodworking.
The edge of the other bottle is rounded off, this is done with a hot air gun / paint stripper. If you heat the rim it will contract. Heat the rim carefull and slow, otherwise it will wrinkle and your rocket piece will be useless.
The rounded side will push the glue / epoxy between the two bottle parts when you fit them together. The outer side is roughend in the same way as the inner side. The glue I use is a polyurethane two component resin, available in do-it-yourself stores. Reaction time is about 24hr.
Clean both pieces and make sure no grease is present. Then put a ring of polyurethane resin on the roughned inner side of the largest bottle-ring. And press the the two pieces together. Roll the two halves along their axis to check if they are in a straight line.
When both parts are pressed together you must see a full coverage off glue around the whole surface of contact off both bottles. A rim of at least 3 cm is needed, preferably more if your going for the high pressure range. If the epoxy is put on correct then you won't have a leak. Even with a small leak it is almost impossible to reach higher pressures.