This tutorial describes how to splice two bottles together when you only have bottles of the same size. Splicing bottles allows you to create larger capacity and longer water rockets. Although this tutorial shows how to splice 2 liter bottles the same technique can be applied to other bottles with straight sides. The same technique can also be used for multiple splices in the same pressure chamber. At the end of the tutorial we add reinforcing to the bottles in order for them to withstand higher pressures. If only lower pressures are required then the reinforcing is not needed.
- 4 identical 2L bottles with straight sides.
- Sikaflex 11FC glue
- PL Premium glue
- Glass Strapping tape (optional)
- Hobby knife
- Electrical tape - used as masking tape because it is flexible and stronger than regular masking tape.
- Mineral Turpentine
- Skewer stick
You can use the edge of the straight section as a guide to cut the bottle evenly. Make sure you don't leave a small flange around the edge when you cut the bottom off.
NOTE: Make sure you let go of the bottle so that it sits evenly on the bottom of the saucepan. This ensures the bottle is shrunken evenly to the correct depth.
This will shrink the bottle slightly. The rate of shrinkage will depend on the temperature. You may need to submerge the bottle for 2-10 seconds. Make sure the bottle is open on top. This takes a little practice, but is easy to master.
If the bottle is too loose, throw it away and try again with another bottle.
Lightly sand about 2cm of the inside of the second bottle.
A quick way to test is to spin them between your fingers and see if there is a wobble. If there is then just tap the bottles on a flat surface a few times while turning them. You may need to repeat a couple of times.
Do this step carefully so the bottles don't move. You want the tape to be as close to the edge as possible, as it is used as a stop when pushing the bottles together during gluing.
NOTE: You can use the PL Premium glue instead. We use the Sikaflex glue as it is non-porous and is quite flexible which prevents leaks when the bottles stretch under pressure.
On it's own the Sikaflex isn't strong enough to safely hold the splice together. It will separate beyond around 60psi.
We use a block of wood with two caps screwed into it to hold the bottles upright while applying the glue.
This splice is important for the integrity of the seal so you want to make sure there are no bubbles or channels where air could escape.
This should leave a nice clean edge.
Cut the sleeve open since being the same diameter it will not fit over the existing bottles.
Cut a 5 x 5cm section from the bottom of bottle #4. This patch will be used to cover the gap left in the sleeve. (Save the rest of the bottle.)
Wrap a couple more pieces of tape on the bottles along the edges of the sleeve. Again this is just to make the splice neater.
Mark on the outside tapes the location of where the split in the sleeve is to help you align it.
We like to stretch out and tape the sleeve to the table to make it easier to apply the glue.
Push out any air bubbles, but it is not critical to get all of them out. Since the airtight seal is provided by the Sikaflex glue.
Pull the sleeve tight with a piece of tape over the cut in the sleeve.
Let the glue cure for 2-3 days. This will again depend on the humidity of your location.
The splice is safe to handle usually within 24 - 48 hours.
Cut off the bottom sections bottles #3 and #4 including the ridge mid way up the bottle (on some bottles). These become the reinforcing sleeves that fit over the ends of the spliced bottles.
Take care when doing this step with a knife.
Now do a hydrostatic pressure test to make sure the splice is constructed properly and does not leak. Make sure you test at slightly above the pressure you intend to launch the rocket at.
You can insert a fairing between the bottles to give the rocket a streamlined shape.
A tutorial video of the whole splicing technique is available here:
* Make the splices in batches. It is a lot easier to process several bottles at the same time when you have the right tools on hand.
* This procedure may look long and complicated but it is reliable when done correctly.
* We now use this technique on 1.25L bottles as well.
* Wear gloves when working with the PU glues as they contain harmful chemicals.
* An all Sikaflex 11FC splice will not hold the stated pressures. PL premium is almost 3 times stronger than Sikaflex.
* The technique presented here is based on previous work done by others:
* Our previous symmetrical splicing technique is described here: Symmetrical Splice
* Asymmetric splice #5 development
* Asymmetric splice #5 burst tests