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Date: 17th March 2013
Location: Doonside, NSW, Australia
Conditions:  Partly cloudy, high winds ~30km/h, 22C
Team Members at Event:  GK, Paul K and John K.

Materials Challenge

This week we had a go at the materials challenge which is one of the 10 challenges 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 challenge the rocket needs to fly at least 200 feet high.

The Rocket

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 fins.

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 fairing.

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 glue allowed)

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 Clark Cable-tie launcher in the construction section.

Launch Day

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 as expected.

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 tube. :)

Lessons Learnt

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 sideways. I 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 the challenges.

Flight Details