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Bottle Splicing

posted 29 Sep 2017, 07:50 by Andy Lakin-Hall

It's time to try making a bigger capacity rocket by splicing two 2 litre bottles together.

There's some good guidance to be found online for this, but I'm following the US Water Rockets Guide.

I finally sourced some PLA adhesive to try this, which has proved difficult in the UK.

I've prepped two bottles, removing the labels and stickiness with WD40.

I cut the bottoms of both bottles as straight as I could, removing all the burrs.

One bottle needs to fit inside the other tightly, and to do this it needs shrinking in hot water. My first attempt was a disaster. I boiled some water in a saucepan and waited for it to cool down. But when I dipped the bottle in, the water was way too hot and the bottle shrank way too much. 

For my second attempt, I started from cold water and then kept dipping the bottle as the water heated up. In this way, I was able to rapidly dip the bottle and then check to see if it had shrunk just enough to fit inside the other bottle.

At least. That's what I should have done. In fact I cheated and used two different bottles. I found that a PET water bottle was already fractionally larger than the lemonade bottle, and they fitted together snugly enough. I smear of PLA glue and the bottles were fixed together.

I used a length of PVC pipe to make sure the bottles were aligned correctly and support the splice as I let it dry over a week.

In the spirit of Werner Von Braun, I decided to go for some all-up testing, pressure testing both this splice and my new 3D Printed nozzle. This also taken from a US Water Rockets Design on Thingiverse.

I forgot that failure is always an option. As I pumped up the rocket to 20psi , everything seemed to hold securely. I left it there for 5 minutes, and the pressure remained stable throughout.

Then there was a sudden loud bang. The splice had failed, sending the upper bottle chamber straight up into the ceiling, and making a neat hole.

  

The upper chamber was completely destroyed, and the lower chamber was bent back. The fins were recoverable, so I can use them again.

Now I have repaired the hole in the ceiling, I can look at what happened. Clearly the problem was the splice was not strong enough. The two different bottles obviously were too different in size for the splice to hold.

For my second attempt I am starting with two identical bottles and shrinking one very gently until the two bottles only just slide together. The slide together with just a bit of water to lubricate, and then are quite difficult to pull apart.

I've roughened up the inside edge of one bottle and the outside edge of the other, and then spread PLA adhesive around the area to be joined. Not too much - just enough for a thin consistent layer.

This is now drying as before. I have also reinforced the splice with some coloured Duck Tape. 

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