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Day 131 - Materials Challenge, Clark cable-tie launcher
17th March 2013
Location:Doonside, NSW, Australia
Conditions:Partly cloudy, high winds ~30km/h,
Team Members at Event:GK,
Paul K and John K.
This week we had a
go at the materials challenge which
is one of the
we set a while back. The challenge basically
states that you have to build a water rocket
with only PET materials. You can't use any
tape or glue or anything else.
In order to claim the top level
the rocket needs to fly at least
200 feet high.
The rocket for this challenge was built
around a single 1.25L bottle as the pressure
chamber. In order to reach the required 200 feet,
you need to achieve stable flight as a
normal bottle by itself will tumble and not fly that high.
To get the bottle to be stable we
needed to add a nose weight and a set of
Main components of the rocket
Bases used as nose weight
Fins made from 2L bottle
For the fin can we used a 1.5L bottle with
the top and bottom removed and curled at one end for rigidity.
We made 3 fins from another 2L bottle and
used a heated nail to heat weld them to the
The fin can was then heat shrunk with a
heat gun over the
bottom of the pressure chamber. To prevent
the pressure chamber from shrinking we
filled it with cold water and the
portion of the fin can and fins we didn't want to
shrink we covered with a wet towel.
For the nose weight we used the bases
of several old bottles and stacked them on top
of the main pressure chamber. To hold them
in place we used the upper half of another
1.5L bottle and heat shrunk it over the
pressure chamber the same way as the fin
can. We wanted to make sure that the two
overlapped to better hold the fin can in
place. Because of the expected large acceleration
there was the possibility the fin can would
separate from the rest of the rocket. (No
All up the rocket weighs 151 grams and
simulations predict an altitude of 300 feet
at 120psi. Since we can't use an altimeter
(not made of PET) we have to estimate the
altitude. This rocket doesn't have a
recovery system so it comes in ballistic.
This allows us to determine the altitude
approximately by the flight duration. To get
the flight time we video record the flight
and then use a video editing program to step
through frame by frame.
Because we can't use the standard 9mm
nozzle (not made of PET) we had to make a Clark
cable-tie release head for our launcher in order to launch the
rocket. The Clark cable-tie launcher was made to
clip into our Gardena launcher. We have
included a tutorial for this version of the
Cable-tie launcher in the construction
Although it was only partly cloudy there was
a strong wind blowing. Since
we weren't launching with parachutes anyway,
it didn't matter all that much. We first launched a single 1.25L bottle
without anything attached just so that we could
compare the altitude to the PETOne rocket.
This was also a first test firing of the
Clark cable-tie launcher. At 120psi the
launch was nice and quick and the bottle tumbled
We next set up the first of the three PETOne
rockets we had prepared. As we started to pressurise the
rocket it launched at perhaps 40 psi. I'm
not sure why that was as I was looking at
the gauge, but most likely either one of the
kids had pulled on the cord by accident, or
more likely the cord had been pulled prior
to pressurisation and the collar was part
way down with the end past the cable-tie
heads. At the pressure it may have then slid
down the rest of the way. I didn't have the
video going at the time so we really don't
know. The rocket went up nice and straight
and came down with a nice thump. The nose
had buckled and one of the fins had come
off. It was easy to pull the nose out but we
weren't able to re-attach the fin.
Shortly after launch
Landed without fins
On the second attempt we pressurised the
next PETOne rocket to 120psi and launched it.
It was a very fast launch and right around
burnout during maximum velocity all the fins
were ripped off the rocket. Soon after the
rocket tumbled somewhat and landed without
too much other damage. Simulation says that
the acceleration is ~150G at burnout and a
speed of 65m/s = 234km/h (145mph). I'm not surprised
the fins came off.
Modified fins - bent inwards
Launched at 80psi
So we set up the third PETOne rocket but
this time we reshaped the fins by bending
them inwards to help prevent them from being
torn off. We also only pressurised the
rocket to 80psi to reduce the acceleration
and top velocity to try to keep the fins on.
The rocket had a nice straight and stable
ballistic flight. Again on landing the nose
buckled in and all 3 fins came off. The nose
was easy to pull out again. The flight
duration was 7.6 seconds.
We ran a number of simulations to compare
the flights against to give us an idea of
the altitude attained.
Simulation results for PETOne are:
PETOne Simulation at 80psi
PETOne Simulation at 120psi
Time at Burnout: 0.1s
Time to apogee: 3.43 s
Apogee: 69.1m (226 feet)
Crash Down time: 7.59 s
Time at Burnout: 0.09s
Time to apogee: 3.81 s
Apogee: 91.87m (301 feet)
Crash Down time: 8.75 s
We can't compare the flight time of the
120psi launch to the simulation because the
rocket did not follow a ballistic path when
it became unstable due to the lost fins. The
80psi flight was nice and stable and so we
can use the simulation to roughly estimate
altitude. With an actual flight time of 7.6
seconds the simulation also roughly
predicted 7.6 seconds.
What we thought was going to be a
relatively easy rocket to build, turned out
to need a few more improvements. We are
going to re-use the existing 3 rockets and
attach new fins to them using a different
technique and have another go
next time hopefully to get over the 200 foot
mark more significantly.
For the second last flight of the day we
flew the Axion II rocket at 120psi. It flew
well and weathercocked nicely during
burnout due to the wind. Landing was well down range.
The last flight of the day was John's
"Buzzard" on an A8-3. He really didn't want
to launch it because he didn't want to loose
it. I promised him we wouldn't loose it.
Sure enough ... we didn't loose it ... but he
managed to sit on it accidentally in the
car on the way home. It will need some
repairs to one of the fins and the body
The welding by itself isn't quite strong
enough for attaching the fins. We will try a
different technique next time - most likely little tabs that go
through the fin can bent outwards and then
welded. That way the weld will not need to
hold the fins by itself. We may also try a
more streamlined fin can.
It would be nice to come up with an all
PET recovery system other than perhaps
backgliding. Something like an air flap that
deploys a parachute like structure or at
least some kind of air brakes to upset the
airflow over the rocket so it comes down
think an even trickier challenge would be to
build the rocket without using any heat at
all to shape the bits of PET and still have
it fly straight. Let
us know if you are having a go at any of
Altitude / Time
Good launch, bottle
Altitude / Time
Rocket self launched.
One fin came off.
Altitude / Time
Good launch, all 3
fins ripped off at Max-Q and then
rocket became unstable and tumbled.
Altitude / Time
Good launch, with
modified fins. Rocket remained
stable throughout flight. Landing
caused all 3 fins to separate..
ST II - 5 seconds
Altitude / Time
? / 18 seconds
Good Launch. Rocket
weathercocked significantly at
burnout but flew well. Good
deployment and good landing.
Altitude / Time
? / ? seconds
Good straight flight, with
deployment right near apogee.