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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:
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The test arrangement was placed on our outside balcony, making sure that
no persons were standing on surrounding balconies.
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The window adjacent to the test area was protected by a heavy shutter.
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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.
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Bottles were pressurized from the inside with a long pressure hose connection.
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Tightly fitting earplugs were used in spite of an almost closed door to the
balcony.
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Wear professional eye protection (sharp shrapnel!).
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Test area on my balcony:
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One of two heavy flowerpots for anchoring the launcher (2) on the ground
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Copper tube launcher
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Trigger ring, taped to the bottle
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Bottle in test, covered with some talc powder to increase effect
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cover of a draining pipe in the floor
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saucer for flowerpot
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balcony window, protected with shutter
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broom, leaning at a concrete wall
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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. |
CLICK HERE (465 KB) to download a video of a bursting bottle
4352 downloads
since 1.1.2001
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Notes:
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at about 5.5 bar the groove above the paper label expanded, forming a "straight"
outside
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at about 9 bar the bottle started growing in diameter, forming some sort
of a 360° belly
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bottle burst is VERY rapid at 9.5 bar
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the trigger ring was fired backward
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the drain pipe cover (5), 105 grams, was elevated out of its tight fit in
the floor
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saucer (6), 315 grams, was elevated
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broom (8) was toppled over - fortunately it did not fall into the camera
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sorry that I cannot deliver the sound - but it was each time impressively
LOUD!
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The remnants of the 1.5 liter bottle after the burst test.
Notes:
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the PET was 0,20 mm (+) thick
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burst pressure 9,5 bar = 138 psi
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left front: the black marked scratch did not rip!
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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.
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Areas with thick material (bottle mouth and bottom) emerged small sharp shrapnel,
not shown here.
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Remnants of another 1.5 liter bottle:
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top left: the neck part
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center: bottle mouth
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bottom: the bottom of the bottle plus some of the shrapnel
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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. |
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This is the last bottle in my test series.
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0.5 liter Sprite
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PET 0,45 mm thick
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it ripped along the knifed scratch
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burst pressure 13.8 bar = 200 psi
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the torque produced from the escaping air bent the bottle sideways
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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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