water rocket vessel pressure testing




When finishing the construction of the pressure vessel of your rocket, there are two major questions to be answered:

- What about leakage?
- Can it hold the pressure?

Just filling the vessel with compressed air is a very bad idea and not the way to go. It is dangerous, a pressurized vessel can contain lots of energy that, at rupture, is released with a big bang and the pieces of PET material are scattered with very high velocity.
Locating a leak with pressurized air is sometimes hard and dangerous. You don't want to hold your ear close to the pressurized vessel with a chance of explosion!.

There is a better way. Fill the vessel of your rocket COMPLETELY with water. Then slowly pressurize the vessel, because the water is almost incompressible the total stored energy is much, much less. Also finding a leak is easy, just look where the water comes squirting out. The maximum test pressure should always be at least 1 bar or 15 psi above the intended maximum launch pressure. Leave this pressure on for about 1 minute. Stay alert, although the stored energy is much less compared with an air filled container, you'll be surprised from the kick the little compressed air can give at rupture.

To demonstrate this the picture here to the right shows a rupture of the 9L rocket during pressurization at around 10 bar / 145 psi.
Notice the milky section in the vessel one bottle section to the right from the splash of water. The velocity of the expelled water is so high that a vacuum is formed in the remaining water.










This vacuum caused the glued interconnection to come loose. Part of the bottle was deformed inwards (The glued interconnection is good under pressure, but behaves poorly under vacuum). Air was sucked in together with some grass.