Saturday, July 25, 2009

Water Rocket Bottle Coupler Construction Tutorial

This week we will be presenting an article detailing how to build connectors to join together multiple bottles by the threaded necks. This type of bottle connector is generally used when making large volume or extended length water rockets like some of the rockets you may have seen on the TDFwaterrockets blog.

The connectors we will be building today are an advantage in this type of rocket because they allow for a modular approach to be applied to your rocket design, which simplifies construction and repair of a damaged rocket. There are commercial products that one can purchase called "tornado tubes" which some water rockets are constructed from, but these are not designed for significant pressure and cost more when compared to building them yourself. Doing things yourself is a lot more satisfying, and is a great lesson for children in managing money by doing for themselves rather than buying something overpriced off the shelf.
Getting back to the advantages of a modular rocket, construction effort is reduced because a module rocket can be built to sections screwed together with these connectors as opposed to one contiguous rocket made from multiple spliced bottle sections which all have to be perfect. In a module design, each module can be built and tested individually and if one happens to be defective you have not wasted the entire rocket because you simply replace the faulty module. This holds true if the rocket ever becomes stressed or damaged as well, where bad modules are replaced instead of scrapping the entire rocket for one leak.

The downside of a module rocket is that the modular sections and the connections between them add extra weight and are less aerodynamic than a standard spliced rocket, so they are great for experiments, but not really appropriate for setting any type of record. But for the casual water rocket enthusiast, these make life so much easier that we are devoting this update to showing how to build your own bottle couplers.

Let's begin the tutorial, shall we?

The first thing you will need is a tube of the "Low Surface Energy" glue we featured in our June 6, 2009 Update. If you want to learn more about this amazing new glue we discovered, please go back and check the update. We are finding quite a few uses for this glue and we think you will too!
You will also need to collect up some extra bottle caps. They come in all sorts of colors, so be creative and pick colors you think will look good on your rocket. Be sure to find caps that come from carbonated soft drinks and not those from water or juice bottles. The carbonated bottle caps have a built-in seal made of a soft rubbery material that will allow the construction of this coupler design without needing o-rings, which is a cost savings for you!
The  last ingredient in our recipe is a short piece of T-8 Fluorescent Tube Cover (FTC). Be sure and get T-8 tubes and not T-12 tubes, because the narrower T-8 tube fits snugly over the bottle caps. it almost looks as if whoever designed the T-8 tube had water rockets in mind when they picked the diameter! This is a lucky coincidence!
These parts, with a little effort and some simple hand tools are all you need to create these modular rocket couplers.

The first step in construction is quite simple. We need to use some sandpaper to roughen up the surface of our bottle caps to increase the bonding strength of the glue joints. Be sure to roughen up the entire outside surface of your bottle caps, including the sides. You will also want to scuff up the inside of your T-8 FTC a bit as we will be gluing the inside of the FTC as well.
Here are some of our sanded and un-sanded bottle caps side by side, so you can see how nice and rough they are. Before you go any further you should wash the caps in a strong detergent to make sure any soda and oils from your fingers are removed. You can also wipe them down with alcohol after you wash them to remove any other residue that the detergent couldn't remove such are sticky glues from labels or price stickers.
The glue we are using is specifically designed to work on Polypropylene and Polyethylene plastic, which is the type of plastic which the bottle caps are made from. If you refer back to our initial report on this glue, you will see how to identify these kinds of plastics. The glue we discovered is a two part glue which consists of a primer activator and a CA glue that is applied after the primer has evaporated. We used the Loctite brand glue for our tutorial here because the primer comes in a felt-tip pen applicator, which can be closed and reused multiple times.

The super-glue brand adhesive come with 2 vials of primer activator and these cannot be closed once opened, plus the adhesive part comes in a gel form which is great for some applications but we did not think it would flow as well as the less viscous loctite brand, so we did not use the gel.
The next step is to cut a piece of T-8 FTC that is twice as long as a cap is tall. A simple way to do this is to stuff a pair of caps inside the tube together and trim off the overhanging ends of the FTC tube and then push the caps out of the resulting cylinder of tube.
After the primer has fully evaporated, we apply the CA adhesive to one of our caps, coating the sides and top of the cap with a thin layer of adhesive. We then slide the cut section of FTC over the cap and push the cap all the way flush with the end of the FTC. Make sure the threaded side of the cap is facing outwards before the glue starts to set because you will have a few seconds to correct the mistake and that's not very long. The other cap is quickly coated with glue and slid in the opposite end of the FTC with the threads facing the opposite direction. Quickly clamp or press the caps tightly together and leave the glue to cure to full strength.
Here we have pushed one cap in the FTC already and the other one is ready to go in. We discovered that screwing a cap onto a scrap bottle before applying the glue to the outside allows the bottle to act as a "handle" so you can pick up the caps and push them in without touching the glue or getting it on your fingers or tools. Feel free to use this method as it will save you a ot of headaches.
Once the glue is cured, you will take a drill or a hole saw and cut a hole through the center of the bottle caps to allow air to pass through the connector. We used a hole saw for our holes because the center drill in the saw was very easy to center on the end of the bottle caps and get a perfectly centered hole through the middle without wandering off center. Since we use the built-in seal on the bottle caps in place of an o-ring, we wanted to be sure our holes were centered.
We noticed that the cured glue turns a whitish color which doesn't look spectacular cosmetically, except on the couplers we made using white bottle caps, which turned out fantastic looking. It's not a big deal and in any case the FTC can be painted to any color you want if you wish.
Notice how nicely the plain white couplers turned out.
The final weight of our couplers were measured at 6.2 grams, which is really quite good.
We waited 48 hours for the plastic glue to fully cure and then decided to give the couplers a good pressure test, just to make sure that they would hold up. We built a little test rig to pressurize a pair of bottles coupled with one of our custom built bottle couplers. The rig consisted of a spliced bottle with two female ends and a cap with a schraeder valve on one end. The opposite end connected to a second bottle with one of our couplers and the whole system was filled with water as a safety precaution before pressure testing.

By filling the setup with water, there is little air space inside the test rig and if something fails there will be less energy contained inside to cause harm in the event of a failure. Only compressed air inside the rig can store energy, so the incompressible water will take up room and reduce the available kinetic energy storage space, making the test much safer.

We pressurized the rig to 130PSI and left it there for 30 minutes. No water leaked out of the coupler and the pressure remained at 130 the entire time, so we concluded that the couplers passed the test with flying colors.
We decided to test a coupler to find it's failure point so we ramped up the pressure to find the point at which the coupler breaks but the test rig exploded at 145PSI before the coupler failed. We concluded that the couplers would be much stronger than any bottles we could splice, so we were pleased with these results.
That is all the time we have this week. Thanks for joining us, and we will be back next week with another update. We have quite a few new developments to report on so check with us next time for more surprises!

See you next week!


  1. Thanks for the fascinating ideas you guys. This is just the kind of instructions that I enjoy reading. You should post these builds on "instructables" to get them out to more readers!

  2. We will certainly look into posting on that site. Thanks for the suggestion, Tim.