Introduction: Water Rocket Bottle Splicing

Picture of Water Rocket Bottle Splicing

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
  • Scissors
  • Electrical tape - used as masking tape because it is flexible and stronger than regular masking tape.
  • Sandpaper
  • Mineral Turpentine
  • Thermometer
  • Skewer stick

Step 1:

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Clean all 4 bottles with soapy water and thoroughly rinse out.

Step 2:

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Remove the cap rings and labels on all the bottles. Make sure you don't accidentally cut the bottle when removing the cap ring.

Step 3:

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Remove the label glue residue from the bottle with mineral turpentine. Wipe the bottle dry with a clean paper towel.

Step 4:

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Cut off the base of all 4 bottles and trim the edge evenly and square.

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.

Step 5:

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Heat about 3cm of water in a flat bottomed saucepan. Use a thermometer to measure the water temperature as it is easy to overheat the water.

Step 6:

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When the temperature reaches around 72C, place the bottle briefly in the water and quickly remove.

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.

Step 7:

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Test to see if it fits in the other bottle. If not, submerge it again for a little longer. But don't make it too loose. The bottle should snugly but easily slide in.

If the bottle is too loose, throw it away and try again with another bottle.

Step 8:

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Dry the bottle before the next step. If there are water droplets on the bottle while curling the edge then the edge won't curl evenly.

Step 9:

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Curl the shrunken edge of the bottle on warm frying pan. You can use a ring made of cardboard to help keep the bottle sitting square on the frying pan. Turn the bottle continuously to make an even curl.

Step 10:

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Lightly sand the outside shrunken section of the bottle.

Lightly sand about 2cm of the inside of the second bottle.

Step 11:

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Fit the two sections together so that the bottles are aligned.

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.

Step 12:

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Apply electrical tape around both bottles where the joint meets. This will keep the edge clean, for the next gluing phase.

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.

Step 13:

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Mark the two sections with a marker pen to help you align them during gluing.

Step 14:

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Glue the two sections together with Sikaflex 11FC.

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.

Step 15:

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Apply a generous amount of the glue to both surfaces. With a little practice you will know how much is enough.

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.

Step 16:

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Spread the glue evenly using a skewer stick. Make sure that the glue completely covers all the sections to be glued.

Step 17:

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Step 18:

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Let the splice cure for a day. Polyurethane glues like Sikaflex and PL premium need atmospheric moisture to cure, so if you live in a dry climate you may need to wait a little longer.

Step 19:

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When the glue is cured, remove any excess glue and remove the electrical tape. The Sikaflex doesn't adhere well to the tape so it is easy to pull off.

This should leave a nice clean edge.

Step 20:

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Cut out a 5cm wide sleeve from the bottom section of bottle #3. (Save the top section)

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.)

Step 21:

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Lightly sand the inside of the sleeve and patch.

Step 22:

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Apply more electrical tape along the outside edges of the sleeve. This will make for a neater looking splice.

Step 23:

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Place the sleeve on the bottles and center it on the seam between the bottles. Use a piece of tape to hold it in place.

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.

Step 24:

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Remove the sleeve and lightly sand the outside of the glued bottles where the sleeve will fit.

Step 25:

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Spread a coat of PL premium on the bottles as well as the inside of the sleeve.

We like to stretch out and tape the sleeve to the table to make it easier to apply the glue.

Step 26:

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Wrap the sleeve on the bottles making sure it is centered over the join. There will be a small 3-5mm gap in the sleeve when applied due to the thickness of 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.

Step 27:

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When cured, remove the tape from the sleeve and lightly sand around the edges of the cut in the sleeve. The area to sand should be the size of the patch.

Step 28:

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Apply PL Premium glue to both the bottle and the patch.

Step 29:

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Use the skewer stick to spread the glue evenly on both the bottle and the patch.

Step 30:

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Put the patch on the bottle and center it over the seam. Work out any bubbles that might be trapped under the patch.

Step 31:

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Use tape to hold it in place while the glue cures. Let the whole splice cure for at least 5 days before pressurising. This allows the PL premium to reach maximum strength.

The splice is safe to handle usually within 24 - 48 hours.

Step 32:

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Remove the tape and any excess glue from the bottles and patch.

Step 33:

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At this point the splice is complete and can be used in your rockets. However, beyond about 120psi (8.3bar), the necks of the 2L bottles will start getting stressed and become permanently distorted. The next section covers how you can reinforce the spliced bottles so that ~140psi (10 bar) is a usable operational pressure. The reinforced spliced bottles will fail at around 190psi (13 bar).


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.

Step 34:

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Cut the necks off bottles #3 and #4 so that the hole left is just big enough to slip over the necks of the spliced bottles.

Take care when doing this step with a knife.

Step 35:

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Cut three equidistant slits in these sleeves all the way up just past the transition from cylindrical to the curved part of the bottle. This will enable it to fit over the ends of the spliced bottles.

Step 36:

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Slide both reinforcing sleeves over the ends of the bottles as far as they will go. The splits will open perhaps 1 or 2mm.

Step 37:

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You can either glue them in place, but we prefer to just tightly wrap them with glass strapping tape. You will need to go around 2 or 3 times. Add two or more wraps to each reinforcing sleeve. There will be a small section of bottle next to one side of the splice that is not reinforced. Add an extra wrap of of the glass tape there.

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.

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These spliced pairs of bottles can be connected to others using tornado tubes to make longer and higher capacity rockets.

You can insert a fairing between the bottles to give the rocket a streamlined shape.

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Here are a couple of examples where these splices have been used

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


wizodd0 (author)2016-08-16


I wonder though, about shrinking both bottles used in the main construction, still so that one will fit snugly into the other, but enough so that the larger will fit inside an unshrunk one, using unshrunk ones to reinforce the necks.

This would permit the reinforcing band to be a single unbroken piece and eliminate the need deal with the gap created using a cut band.

The only issue I can see would be getting the shrinkage right on both bottles.

It would be possible to double wall the entire double bottle if this could be done, maybe even eliminate the separate reinforcing band. (By cutting one outer sheath bottle long enough to go past the join, and the other to meet it.)

It would probably be heavier, but might also stand higher pressures.

I wonder about the pressure transfer tube looping outside the separater assembly...

Perhaps this could be internalized by using smaller diameter tubing, since it carries only air, diameter shouldn't be a problem. (I have some 1mm & smaller PTF tubing in my parts box.)

I was looking for a two-stage connector because I have a design for a rocket glider and wanted to try a ducted rocket nozzle, but that assembly would destroy my design's aerodynamics and center of gravity shifting mechanism....

Putting the ducted nozzle on a first stage booster would make it much easier...I had though about ejecting the nozzle at the end of the boost stage. but that would require an ejection mechanism.

A lot of work in the design, thanks for sharing it!

EzekielD (author)2015-12-04

How far it went?

EzekielD (author)EzekielD2016-03-15

Never mind

marapellys (author)2015-11-22

sir i want to full construction details

marapellys (author)2015-11-22

sir i want to full construction details

SirMac27 (author)2014-05-27

Amazing Acts of Science brought me to this page! I saw it on the show and I have to do this now before the Summer ends! Thanks for posting.

hg341 (author)2011-02-25

about time that you got on here

About This Instructable




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