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Burst Test Report 01.01.2001

The unintentional burst of a water rocket is always a danger to every water rocketeer. Quite a while ago I thought about the necessity to have qualified data on the burst pressures of PET bottles. So here it is: a burst test of some PET bottles with minimum risk of damage to things or persons.

Before anybody reads any further, make sure that if you want to do similar experiments, do them ONLY AND ONLY if you take at least the same or higher security precautions as I did them. You MUST have a good standing in "normal" water rocketeering so that you really know what you are doing. This is pretty serious stuff, no playgame!! Don't make me liable if anything happens when you try to do something similar!!

Security precautions:

  1. The test arrangement was placed on our outside balcony, making sure that no persons were standing on surrounding balconies.
  2. The window adjacent to the test area was protected by a heavy shutter.
  3. The bottles were all marked with a knifed scratch to ensure an early burst. This was done to simulate the scratches that every water rocket has after some use.
  4. Bottles were pressurized from the inside with a long pressure hose connection.
  5. Tightly fitting earplugs were used in spite of an almost closed door to the balcony.
  6. Wear professional eye protection (sharp shrapnel!).

Test Area for Burst Test Test area on my balcony:
  1. One of two heavy flowerpots for anchoring the launcher (2) on the ground
  2. Copper tube launcher
  3. Trigger ring, taped to the bottle
  4. Bottle in test, covered with some talc powder to increase effect
  5. cover of a draining pipe in the floor
  6. saucer for flowerpot
  7. balcony window, protected with shutter
  8. broom, leaning at a concrete wall
Scratch in the bottle A small scratch with a sharp knife (marked black here for better visibility); this should have weakened the bottle so that the bursting would start here.

This knifed scratch was added to each bottle to get "safe" burst test values. The knifed scratch should simulate the scratches every bottle gets after some use as a Water Rocket.

Note the groove above the label.

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Notes:
  1. at about 5.5 bar the groove above the paper label expanded, forming a "straight" outside
  2. at about 9 bar the bottle started growing in diameter, forming some sort of a 360° belly
  3. bottle burst is VERY rapid at 9.5 bar
  4. the trigger ring was fired backward
  5. the drain pipe cover (5), 105 grams, was elevated out of its tight fit in the floor
  6. saucer (6), 315 grams, was elevated
  7. broom (8) was toppled over - fortunately  it did not fall into the camera
  8. sorry that I cannot deliver the sound - but it was each time impressively LOUD!
Bursted bottle The remnants of the 1.5 liter bottle after the burst test.

Notes:

  • the PET was 0,20 mm (+) thick
  • burst pressure 9,5 bar = 138 psi
  • left front: the black marked scratch did not rip!
  • center: bottle mouth with o-ring. All material was ripped off completely. This was probably some sort of a "peeling back" motion, firing the trigger ring away from the bottle.
  • Areas with thick material (bottle mouth and bottom) emerged small sharp shrapnel, not shown here.
Blue bursted bottle Remnants of another 1.5 liter bottle:
  • top left: the neck part
  • center: bottle mouth
  • bottom: the bottom of the bottle plus some of the shrapnel
  • the mouthpiece (sharp ends!) was catapulted out each time; taping the neck of the bottle to the launcher did not help. The mouthpiece is particularly dangerous due to its higher mass compared to the other shrapnel.

I do not use these 0.20 mm mineral water bottles for rockets. They break too early.

Sprite bottle This is the last bottle in my test series.
  • 0.5 liter Sprite
  • PET 0,45 mm thick
  • it ripped along the knifed scratch
  • burst pressure 13.8 bar = 200 psi
  • the torque produced from the escaping air bent the bottle sideways
Fireworks celebrating  the new year. The video was taken on 1st Jan 2001 in the morning (better light conditions).

The other burst tests were, as a side effect, carried out around midnight to celebrate the New Year 2001. Here you see what other people did.

We (my brother and me) had a clear conscience for NOT polluting the air while welcoming the New Year (except a little more CO2 than usual while pumping ;-)

On May 9th, 2002, Roderick McGuire tried an explanation, why the rupture lines appear mostly parallel to the longitudinal axis of a bottle:

"A PET bottle is made from a small preform (e.g. see
      http://www.ball.com/bhome/pet/preform/prehome.html )
that is heated up, stretched out long with an internal metal piston, and inflated with ~ 400 psi = 27.6 bar air into a mold. The stretching phase orients the long plastic molecular chains longitudinally. Thus when a bottle bursts a longitudinal split is favored because a latitudinal split would have to break molecular chains. When a longitudinal split starts it is rapidly propagated (by the force of the escaping water or gas) ...".

In my case, the rupture lines along the "peeling back" PET stripes, driven by the escaping air, were  probably stopped at the neck by the supporting strength of the copper launcher's head. The thick PET material there could not bend as well as the thin one on the bottle sides, and therefore broke. While bending back, theses stripes pushed back the trigger ring, eventually assisted by the pressure wave. Compressed air, from the pressure hose and the launcher, fired out the mouthpiece, which was now not hold back by the cable ties any more. In retrospect, the mouthpiece must have been fired against the wall where the broom was leaning (see video), being reflected from there into the garden, where I was lucky to find it, about 15 m from the test spot.

Roderick made his own pressure test. Not using a copper launcher with it's support to the neck, the propagating longitudinal split  "... can travel up into the relatively thick and strong sections of the neck.", which it did.

 

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