My current project

Strange question you might think!.
But this thought is the starting point for my current project.
It does not matter much how fast you throw the balloon in the air, the maximum height will not be impressive.
But if you replace the balloon by a tennis ball and throw it in the air with the same initial speed it reaches a much higher altitude.
The reason is of course obvious, the tennis ball has lower air resistance and higher kinetic starting energy (more weight, same speed) compared to the balloon.

Now back to the wrockets.
I found out with my 14 Liter rocket that adding extra sections would not add that much extra height.
The added volume and energy stored in the segment increases the apogee only by a small fraction.
With the a cross section off 87 mm diameter, together with maximum pressure of 14 bar it can hold, you will never reach heights that come close to the world record.
I needed a type of wrocket construction with:

- less drag -> smaller diameter
- higher maximum pressure -> thicker bottle walls.

An idea that has been tried several times, but as far as I know is not very successful is heat-shrinking the PET bottles.
Shrinking bottles can give smaller diameters AND thicker walls!
But up to now the non linear contraction (wrinkling) of the bottles always caused a problem.
After several experiments I found a way to overcome the main part of the wrinkling. And now i am able to shrink bottles to a diameter of 70 mm.
This does not seem much but must make quite a difference.
The drag (same aero-shape) is reduced to (70/87)^2= 0.64 of the original and the maximum pressure at best can almost be doubled (less diameter AND thicker walls) provided that the strength of the PET material is not affected by the heat treatment.

The shrinking set up consists out of a steel cylinder and a metal heater element (both from an old high volume printer).
The cylinder has thick walls and is heated evenly by placing the heater element inside and the roller is rotated so the PET bottle is also heated evenly.

After a while the PET bottle starts to shrink (and wrinkle!), but the metal roller sort off "irons" out all wrinkles because the PET bottle wants to contract further and so all folds are stretched until the surface is flat again.On one side a sort of filler ring with a thickenss of 0.5 mm is mounted so that this side shrinks only to 70.5 mm diameter. In this way the segment can be assembled more easily into a wrocket.

The result of the shrinking can be seen here below. On the right hand side the original sized bottle. The PET body is shrunken both in diameter and in lenght as might be expected.

With the shrunken segments i made a pressure container consisting of 9 elements and aqua tested it up to 240 psi / 16 bar before it started to leak. It did NOT explode!. Water was pressed between two segment halves and at that spot ripped apart the glue.
So only part of the goal was reached (smaller diameter), the pressure increase was only about 30% instead of the expected 100%.
See the rocket gallery for the shrunken body rocket I made with these segments
...............more to come