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Robinson Coupling

Splicing Bottles #1

Splicing Bottles AS#5

Reinforcing Bottles

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Flight Log Updates

#190 - Polaron G3

#189 - Casual Flights

#188 - Skittles Part #2

#187 - Skittles Part #1

#186 - Level 1 HPR

#185 - Liquids in Zero-G

#184 - More Axion G6

#183 - Axion G6

#182 - Casual Flights

#181 - Acoustic Apogee 2

#180 - Light Shadow

#179 - Stratologger

#178 - Acoustic Apogee 1

#177 - Reefing Chutes

#176 - 10 Years

#175 - NSWRA Events

#174 - Mullaley Launch

#173 - Oobleck Rocket

#172 - Coming Soon

#171 - Measuring Altitude

#170 - How Much Water?

#169 - Windy

#168 - Casual Flights 2

#167 - Casual Flights

#166 - Dark Shadow II

#165 - Liquid Density 2

#164 - Liquid Density 1

#163 - Channel 7 News

#162 - Axion and Polaron

#161 - Fog and Boom

#1 to #160 (Updates)

 

WATER ROCKET CONSTRUCTION - ADVANCED TUTORIALS

These tutorials show you how to build some of the components we use on our rockets. While it may not always be possible to reproduce these components exactly, many of the designs can be customized based on the materials you have available.

For a full list of all construction tutorials go to the Construction Index.

Robinson Coupling
Robinson couplings.
Diagram of how the coupling fits together.
   

A Robinson coupling typically consists of a hollow threaded tube that joins two bottles together allowing air or water under pressure to pass from one to the other. This increases the volume of the pressure vessel. Often a number of bottles can be cascaded this way to gain even more volume.

Robinson couplings can be used to join bottles neck to neck, base to neck or base to base depending on the rocket design.

The main advantages of Robinson couplings are that they relatively easy to make and are reusable. The main disadvantage is that they typically only have a small hole and hence restrict the internal flow somewhat. Robinson couplings are best used with restricted nozzles.

We present the construction and assembly of the Robinson couplings we use as a video tutorial.


Making a water rocket bottle coupling

(If the video does not play, try the latest Flash player from Macromedia)

There are a number of other ways people have built Robinson couplings. Searching for "Robinson Coupling" on the internet will yield other designs.

Here is an example of a Robinson coupling how they are typically made from a threaded lamp rod: http://dogrocket.home.mindspring.com/WaterRockets/coupling.html

My recommendation is to replace the O-rings in the above description with hard rubber washers as they will get a better seal and will not deform the same way o-rings will.

Here is a good tutorial by Todd from HHWRSA on how to make Robinson couplings using threaded lamp rods:  http://wrocket.hampson.net.au/?p=1081

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Splicing Bottles
Spliced Pairs of bottles shown on left. The video tutorial explains how to make these.
Splicing Tip #1 Photos
Example #1 showing where the food colouring has penetrated all the way through the splice to the edge.
Another older splice repeatedly filled.
New splice with red food colouring. You can see where it entered the little void on the left.
Sikaflex sealed splice undergoing pressure test.
Sealed splice shows no penetration of food colouring.

Splicing bottles together is another technique often employed by water rocketeers to increase the volume of their rockets. Splicing involves gluing sections of bottles together to make a longer pressure vessel.

Because of the forces involved inside a typical rocket, and because PET plastic is very hard to glue, there are only a few existing glues that are suitable for the job. The most commonly used is PL Premium construction adhesive, but VISE and a small number of others can also be used.

Splicing is not as easy as joining bottles using a Robinson coupling, is permanent and is less predictable at which pressure it will fail, but it has the advantages of  virtually unrestricted internal flow and potentially long pressure bodies can be made this way.

The technique presented here is based on previous work done by other rocketeers:

http://wrockets.trib-design.com/index.php?project=nick&page=splicing

In the following video tutorial we present a technique called 'symmetrical splicing' for joining two bottles. The same technique can be used for making much longer bodies. The join is just repeated for each section.

(If the video does not play, try the latest Flash player from Macromedia)

Asymmetric Splice #5 Tutorial

For splicing bottles of the same size see the Asymmetric Splice #5 tutorial.

Splicing Tip #1 - Sealing

Ever since we started using food colouring in our rockets we have noticed that bottles spliced with PL premium can leak ever so slightly. We didn't notice this leakage before with just clear water. It appears that PL can be quite porous.

The photos on the left show examples where the food colouring has penetrated the splice. We have noticed this with pretty much all of the spliced pairs of bottles we have made. Some do it more than others.

The bottles seem to hold the pressure, but if you have a rocket body with lots of splices then the leakage can add up. If you hold the rocket pressurised on the pad for an extended period, the pressure inside it can drop. There is also some risk in pressurised air entering the splice walls and then with a sudden release of pressure during launch the air may not have enough time to escape through the tiny holes and potentially cause the splice to delaminate.

To stop the leakage with this kind of splice:

  1. Mark a line on the inside of splice where the two bottles will meet.
  2. Run a healthy bead of Sikaflex glue along the line on the inside of the sleeve. Sikaflex does not bubble during the curing process.
  3. Use PL premium on the rest of the splice as normal, but don't put any PL on the curled edges of the bottles.
  4. When you bring the splice together the Sikaflex fills the little gap between the curled edges and the sleeve and also squeezes a little bit inside the bottles. This provides an air and water tight seal, but with the strength of the PL holding the splice together.

We've tested the splice under pressure using coloured water and there was no visible leakage. If you can't get Sikaflex you can probably find other equivalent non-porous flexible glue or sealant. These don't need to have the strength of PL. Sikaflex is a pretty good glue in itself for holding on to PET so I would recommend it. If you are doing all Sikaflex splices then you can disregard this technique.

Splicing Tip #2 - Clean Splice Edges

Splicing bottles can be very messy and you can end up with unsightly edges. An easy way to clean up the excess glue is to do this:

  1. Wrap one piece of masking or electrical tape all the way around the edge of the outside of the sleeve.
  2. Slide the sleeve onto the bottle you will be gluing it to and position it in its final place.
  3. Wrap another piece of tape around the bottle so that it sits right up against the sleeve.
  4. Do this to both ends of the sleeve and both bottles.
  5. Splice as normal.
  6. When the glue cures you can remove the tape for a clean edge.

    Note: you should probably leave the tape until the glue cures as some may come out of the splice as it expands. We use electrical tape as it is strong when removing it with the hardened glue.

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Reinforcing Bottles
Single Bottle
The three bottles required to make one reinforced bottle.
Creating the sleeves.
Fitting sleeves to main bottle
Heat shrinking
Robinson Coupled
The three bottles required to make one reinforced bottle.
Creating the sleeves.
Fitting sleeves to main bottle
Heat shrinking
Reinforced bottles. This combination was tested to 200psi with no leaks or visible stress marks.
This view shows the upper bottle with PL premium sandwiched between the main bottle and outer sleeve.
This view shows the strapping tape used to retain the upper sleeve.
A close up showing where the sleeves overlap each other.
   

 

Bottle reinforcing is a technique often employed by water rocketeers to allow them to safely increase the pressure inside the rocket in order to gain more altitude. The procedures outlined below are based on a technique described by Richard Wayman here:
http://wrockets.trib-design.com/index.php?project=RICHARD&page=hp.

CAUTION: Because reinforced bottles can hold significant pressure, all safety procedures should be followed when testing or when launching rockets based on these bottles at higher pressures. Always inspect these bottles before every launch for signs of stress.

Reinforcing a single bottle

Materials

  • 1 x 1.5L bottle with straight sides (P&N in Australia)
  • 2 x 1.25L bottles with straight sides
  • PL Premium or equivalent glue.
  • Strapping tape (optional)

Procedure

  1. Remove the caps, cap retaining rings and labels from the bottles.
  2. Clean the glue from the bottles using mineral turpentine and rinse then dry the bottles.
  3. Cut the top off the 1.5L bottle so that the sleeve is as long as possible so that the 1.25L bottle can slide in. This is the lower sleeve. These 1.5L bottles have a diameter a few mm bigger than the 1.25L bottles.
  4. Make small 1mm holes in each of the lobes of the lower sleeve. These allow air pressure to escape when the sleeve is being shrunk.
  5. To make the upper sleeve, cut the bottom off a 1.25L bottle so that you end up with about 2-3cm past the top lip.
  6. Now cut the neck off the upper sleeve so that the hole is just big enough to fit over the thread of the main bottle.
  7. From the Main bottle trim the neck stop so that it is as narrow as the thread. This makes it possible for the upper sleeve to have as small a hole as possible. This is pretty tough to trim with a Stanley knife, but we found that using wire cutters is a lot easier to trim it away.
  8. Now since the upper sleeve is the same size as the main bottle you cannot fit it over the top. You need to cut at least 3 slits to about 1cm past the sleeve's lip so that it can stretch over the main bottle.
  9. Now slide the upper sleeve over the main bottle. Once past the main bottle lip the sleeve's lip will sit properly around the main bottle. The slits will be slightly open which is okay.
  10. You can put some strapping tape just under the lip to help hold the upper sleeve from sliding off under pressure. This step may not be necessary as the lower sleeve will overlap this section and hold it anyway.
  11. Now fill the the main bottle with cold water so that there is no air inside it and cap it. Dry off the bottle before proceeding to the next step.
  12. Now put some glue on the base lobes of the Main bottle. PL is good for this since it will not run and binds well to PET. The glue helps prevent the lower sleeve from sliding off the main bottle under pressure.
  13. Slide the lower sleeve over the main bottle and push it up against the base so that the glue fills the gaps in the base. Wipe up the glue that leaks out of the holes.
  14. Now we shrink the sleeves. There are a number of ways you can achieve this for example using a heat gun, hot water or as we prefer is doing it over a gas kitchen stove. Hot water would be the last option as it is dangerous and also tends to give less ideal results. It is always important to continue to rotate the bottle over the heat source so that you get an even rate of shrinking.
  15. Start by shrinking the lower sleeve where it overlaps the upper sleeve. This helps lock the lower sleeve in place preventing it from sliding off as you shrink the rest of it. Rotate the bottle and work your way towards the base. And then return to the upper sleeve and shrink that. The upper sleeve won't need much shrinking since it is a tight fit already.
  16. Make sure that the sleeves are shrunk as evenly as possible and that the gap between the two bottles at the base is either completely eliminated by shrinking or the glue fills the gaps that cannot be shrunk. More glue will come out of the holes in the lobes during the shrinking and glue curing process, so wipe it off to prevent a mess.
  17. Let the glue cure for a few days before pressure testing.

Reinforcing a bottle with a Robinson coupling in the base

Materials

  • 1 x 1.5L bottle with straight sides (P&N in Australia)
  • 2 x 1.25L bottles with straight sides
  • 1 x Robinson coupling
  • 1 x Thin soft washer with inner hole the size of the coupling. PL premium (optional)

Procedure

  1. Remove the caps, cap retaining rings and labels from the bottles.
  2. Clean the glue from the bottles using mineral turpentine and rinse then dry the bottles.
  3. Drill a hole for the coupling in the center of the base of the main bottle. Remove all burrs and clean out any shavings from the bottle.
  4. Drill a hole for the coupling in the center of the base of the 1.5L bottle. Remove all burrs and clean out any shavings from the bottle.
  5. Cut the top off the 1.5L bottle so that the sleeve is as long as possible so that the 1.25L bottle can slide in. This is the lower sleeve. These 1.5L bottles have a diameter a few mm bigger than the 1.25L bottles.
  6. Make small 1mm holes in each of the lobes of the lower sleeve. These allow air pressure to escape when the sleeve is being shrunk.
  7. To make the upper sleeve, cut the bottom off a 1.25L bottle so that you end up with about 2-3cm past the top lip.
  8. Now cut the neck off the upper sleeve so that the hole is just big enough to fit over the thread of the main bottle.
  9. From the Main bottle trim the neck stop so that it is as narrow as the thread. This makes it possible for the upper sleeve to have as small a hole as possible. This is pretty tough to trim with a Stanley knife, but we found that using wire cutters is a lot easier to trim it away.
  10. Now since the upper sleeve is the same size as the main bottle you cannot fit it over the top. You need to cut at least 3 slits to about 1cm past the sleeve's lip so that it can stretch over the main bottle.
  11. Now slide the upper sleeve over the main bottle. Once past the main bottle lip the sleeve's lip will sit properly around the main bottle. The slits will be slightly open which is okay.
  12. You can put some strapping tape just under the lip to help hold the upper sleeve from sliding off under pressure. This step may not be necessary as the lower sleeve will overlap this section and hold it anyway.
  13. Invert the main bottle and place the thin soft rubber washer over the hole.
  14. Now slide the lower sleeve over the main bottle, sandwiching the washer between the two bases.
  15. Fit the Robinson coupling through the two bottles with the appropriate washers inside and out (see video) and tighten the coupling. The glue is not really necessary here as the coupling holds the bases together. You can add the glue if you wish to help fill the gap between the bases.
  16. Screw in another bottle into the Robinson coupling and fill the two bottles with cold water so there is no air left in the bottle. Screwing in a small 600ml bottle makes it easier to handle during the shrinking process. This bottle is only temporary to hold the water in. Try not to get water between the sleeves before shrinking.
  17. Now we shrink the sleeves. There are a number of ways you can achieve this for example using a heat gun, hot water or as we prefer is doing it over a gas kitchen stove. Hot water would be the last option as it is dangerous and also tends to give less ideal results. It is always important to continue to rotate the bottle over the heat source so that you get an even rate of shrinking.
  18. Start by shrinking the lower sleeve where it overlaps the upper sleeve. This helps lock the lower sleeve in place preventing it from sliding off as you shrink the rest of it. Rotate the bottle and work your way towards the base. And then return to the upper sleeve and shrink that. The upper sleeve won't need much shrinking since it is a tight fit already.
  19. Make sure that the sleeves are shrunk as evenly as possible and that the gap between the two bottles at the base is either completely eliminated by shrinking or the glue fills the gaps that cannot be shrunk.
  20. If you used glue, then let the glue cure for a few days before pressure testing.

NOTE: The Robinson couplings we use for these reinforced bottles use a lid with a longer uninterrupted thread compared to the standard bottle lid. We also have a reinforcing ring over the top of that to prevent the cap from distorting when tightened. .

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