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#1 to #150 (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.

Clark Cable-Tie Launcher
 

Introduction

The Clark cable-tie launcher is probably the most common type of launcher used by water rocketeers around the world today. It is inexpensive, easy to construct and reliable. The launcher is designed to launch rockets with full bore nozzles so you don't need a special nozzle for your rocket. The launcher also typically includes a launch tube which can improve the rocket's performance.

Ian Clark from Australia developed this launcher back in 1998 and since then a great number of variations on his design have been built. See the reference section for lots of examples. The basic design is based on a piece of PVC pipe with a ring of cable ties whose heads hook on to the flange of the bottle. A collar made from a larger PVC pipe surrounds the cable ties preventing them from spreading and hence holding the rocket on the launcher. When it's time to launch, the collar is pulled down allowing the cable tie heads to separate and release the rocket.

Launcher Components

When building your own variation of the launcher there are a number of important components to consider:
 

The Seal - The seal between the bottle and launcher is one of the more critical components. There have been numerous ways of creating a seal and here are some of the common ones:

  1. Bulging the PVC pipe - This technique requires you to make a bulge in the PVC pipe by heating it and pushing the two ends towards each other. Because there is no rubber you rely on plastic-on-plastic to form the seal. This can suffer from leaks because the rocket will tend to move up the launch tube under pressure partially sliding off the bulge.
  2. Tape - Tape is wrapped around the launch tube until its wide enough to form a seal. This will work for emergencies when you don't have anything else and is good enough perhaps for several launches, The tape usually needs to be replaced frequently. It will not work well at higher pressures.
  3. Flat seal. Some launchers suggest you can get a seal by pressing the bottle end into a rubber washer but this rarely works well as you pressurise the rocket the bottle has a tendency to move up the launch tube stretching the cable ties and breaking the seal.
  4. O-ring - This is the best option for sealing the bottle. It allows the bottle to move as the cable ties stretch. The o-ring, however, requires a correct size groove to sit in.

Collar - The collar or 'trigger ring' holds the cable ties together preventing the rocket from being released. Sliding the collar down allows the cable ties to spring open releasing the rocket. The internal diameter must be large enough to fit over the bottle flange and cable ties, but small enough not to allow the cable ties to separate too much.

Cable ties and their retention - The cable ties hold down the rocket by the flange on the bottle. Because of the forces involved it is important that all the cable tie heads are aligned with the flange so that some cable ties don't take more strain than the others, The number of cable ties can vary depending on their size and how much load they will need to hold which is usually dependant on the fill pressure. Ian Clark originally used only 3 cable ties.

The cable tie retention is also very important because ultimately the retention mechanism needs to support the entire force the rocket exerts. The most common form is a tightened hose clamp.

Spring - A spring is sometimes used to keep the collar in the locked position before you start pressurising the rocket. Once you start pressurising the rocket the force of the cable ties against the collar keep it in place. Typically this spring is made from a section of PET bottle.

Launch tube - The launcher is typically fitted with a launch tube whose length is determined by the size of the rockets being launched.

Bottle stop - A stop at the bottom of the launch tube is sometimes used to prevent the rocket sliding down too far under it's own weight. This helps ensure the seal is in the correct position relative to the bottle neck. 

Air supply connection - The kind of connection you use on the launcher will depend on the type of connector you have on your air supply hose. Typically this is a Schrader valve used on bicycle tyres. In this tutorial we use the Gardena quick connector.

Construction Tutorial

Below is yet another Clark cable-tie tutorial that shows our own variation of this classic launcher based on our needs. We wanted to eliminate all metal from the design and also be able to use it as a drop-in release head for our Gardena launchers. This allows us to easily use restricted nozzles as well as full bore nozzles with our existing launchers. The swap capability also lets us make up several launchers with different launch tube lengths. This Gardena quick connector approach is an adaptation of Mat Gardner's launcher.

NOTE: This tutorial assumes you already have a Gardena launcher. If you don't then you can replace the Gardena connector with more PVC pipe and any other air hose adaptor.

The main metal component we wanted to remove from the launcher was the hose clamp. This poses a potential safety issue if the launcher should fail. The hose clamp is usually used to prevent the cable ties from slipping. There have already been a few variations of this without the use of a hose clamp. See the reference section for details. In this design we use the heads from other cable ties to stop the main cables ties from slipping.

 

Materials

- 350mm length of 1/2" PVC pipe
- Gardena male-male adaptor
- 21 Cable ties
- 40mm length of 1-1/4" PVC pipe
- BS115 O-ring
- 2 x 1/2" PVC couplers
- 65x180mm section of PET bottle
- Electrical Tape

Tools

- Epoxy
- PVC cement
- Scissors
- Ruler
- Saw
- Sandpaper
- Pliers

 

Procedure

1. Cut a 1/2" PVC pipe into the following 3 lengths: 30mm, 34mm and 250mm. Try to cut them as square as possible. Two ends of these will form the o-ring groove so you want to have nice square edges.

TIP: If you are having trouble cutting square ends for the o-ring groove, you can often use the two square ends of the PVC pipe as it came from the shop.

2. Remove one o-ring from the Gardena double male adaptor as you won't need it. Keep it as a spare for future projects.

Rough up the inside of one of the PVC couplings with sand paper. Use a fairly coarse sandpaper something like 120 grit.

3. Mix up some epoxy and apply it around one side of the Gardena adaptor, and also coat the inside of the coupling.

Use a generous amount of epoxy as you want it to fill the gap. It's best to use the 24-hour epoxy rather than the weaker 5-minute epoxy.

4. Stand the adaptor on its end and put the coupling over the top. Gravity will cause the glue to settle at the bottom making a seal and joining the two components together. Let it cure overnight.

You can use the other coupling at this stage to help it stand up.

5. While waiting for the Gardena adaptor to cure, you can make the o-ring groove on the launch tube.

A simple way to make the groove in the PVC pipe is to insert a smaller diameter pipe into it. While this may sound easy, there aren't many plastic pipes readily available that snugly fit inside the PVC pipe. Resist the temptation to hammer oversized tubes into the PVC pipe which can lead to fine cracks in the PVC causing it to fail under pressure. If you have access to a lathe then you can machine down a bar of plastic to the right size.

6.

If you can't get your hands on the correct size pipe or a lathe then there is a simple alternative you can use and you already have it at home. This technique is shown in this tutorial.

Cut out a flat strip from a PET bottle. You want the strip to be about 65 mm (2.5 inches) wide and 180mm (7 inches) long.

Sand both sides of the PET strip to give the glue a better surface to adhere to. Use the same coarse sandpaper as before.

7. Roll up the strip tightly until it fits inside the tube. You want to make sure you have at least 4-5 wraps. If you have more, then that's even better. When you release it, the strip will expand to tightly fit inside the tube. If you are happy that the strip will fit, take it out again.
8.

Rough up the inside of the launch tube and the 30mm section of PVC pipe with sandpaper.

9.

For the next few steps you will want to use rubber gloves as it can get pretty messy.

Apply a coat of the 24-hour epoxy to the inside of both pieces of pipe.

 

10. As you roll up the strip coat the inside of it with epoxy. You don't need a lot. Spread the glue all the way to the edges. When you roll it up, the glue will spread.
11. Roll it up the rest of the way and coat one end of the roll that will go into the launch tube. Again you do not need a lot of glue.

Insert the strip about half way into the launch tube.

12. Now coat the other end of the strip roll with epoxy and insert it into the 30mm section of PVC pipe.

Slide the two together until you have the correct size gap for the o-ring. This should be 3.5 - 4mm. See notes below for details of the correct o-ring groove size.

As you bring the sections together some glue will be squeezed into the groove. Just wipe off all the excess with a tissue both inside the groove and any that may be on the PVC pipe. Image on the right shows the glue wiped clean.

13. Stand the launch tube on it's end and let it cure overnight.
14. Using PVC cement, glue the launch tube into the top coupling, and glue the 34mm section of PVC tube between the two couplings. Make sure that the gap between the two couplings is wide enough to accept the head of the cable ties you are using.

Let the glue cure.

15. Put a bottle on the launcher and measure the distance between the flange and the edge of the gap between the two couplings. This will help you position the tape in the next few steps
16.

To get even spacing between the cable ties we use a piece of cardboard with cut outs bent over. If you are happy with just eyeballing the spacing you can skip this step.

We chose to use 8 cable ties in this design as that is sufficient for the desired range of pressures. (See testing experiments at the end of the tutorial.)

17. Tape the cardboard template to the table and put the cable ties in the gaps. Tape a ruler along the heads to keep them aligned, We use the tabs on template to stop them moving down. If you are not using a cardboard template you can just use another ruler under the heads.

Apply a piece of tape to the cable ties so that they are held together. You want to make sure the outside of the tape isn't more than the distance you previously measured. (The tape should not cover the gap between the couplings.)

18. Take the cable ties out of the template and flip them over and apply a second layer of tape to the other side. Have a piece of tape extend past the last of the cable ties. This will help in attaching the cable ties to the launcher.
19. Wrap the cable ties around the upper coupling and secure with the end of the tape.

Make sure the heads sit snugly against the flange on the bottle and that the tape does not cover the gap between the couplings.

20. Wrap a single cable tie around all the cable ties and tighten it just enough to apply some pressure to the cable ties.

Thread a bottle onto the launch tube and rest it against the top of the coupling.

Now you can adjust the position of the individual cable ties so that they rest firmly against the flange. The tape will allow you to move them with a little effort. You may want to use a pair of pliers to help with this step. When you are happy with the fit for all the cable ties, tighten the wrapped cable tie.

21. Cut the heads off another set of cable ties.
22. Thread the heads individually onto the cable ties that are in place. Slide them all the way up until the heads sit in the groove. These will help grip the individual cable ties to stop them from sliding up when the launcher is pressurised.
23. Wrap a second cable tie on the other side of the heads and tighten it.

You can now trim all the cable ties.

24. Drill 4 holes equally spaced near the bottom of the collar. The holes should be big enough to pass a cable tie through.
25. Insert 2 cable ties as shown on opposite sides and tighten them so that the heads are on the outside. 
26. Bring the two ends together on the lower coupling and secure with a piece of tape.
27. Add another cable tie over the top and tighten it. These two cable ties act as springs to keep the collar up before the rocket is pressurised.
28. Slide the o-ring over the launch tube into the o-ring groove.

29. Tie a loop of string to the other two holes. These will be connected to your pull string. Use strong braided nylon for the string to make sure it does not break.

The launcher is now ready. Here the launcher is plugged into our regular Gardena launcher.

You should first pressure test your launcher from a safe distance to make sure all the glue and cable ties hold.

Notes

  • O-ring groove - The size of the o-ring groove is important. The groove needs to be wide enough to let the o-ring deform as you put the bottle on. The BS-115 o-ring is the correct size for this launcher. The PVC tube is 21.4mm in diameter and the internal diameter of the bottle neck is about 21.6mm. The o-ring at 22mm OD and 2.62mm cross section is the correct size so that it is squeezed a little when the bottle is on the launcher. Looking up the tables for the correct groove dimensions we get a depth of 0.078" = 2mm. The PVC wall happens to be 2mm so having the pipe/PET strip on the inside wall gives us the correct depth. The tables also tell us the groove width should be 0.137" = 3.5mm. This is the minimal size so if it is a little wider that's fine. Do not make the groove too narrow otherwise it will be difficult to put the rocket onto the launcher.

  • Pressure Testing - We have built 2 of these launchers and hydro-statically tested them both to 150psi (10.5bar) without problems.

  • Cable tie testing - We have tested the strength of the small cable ties, and in particular the cable tie heads in holding down the main cable tie. The head let go of the cable tie at 9.4Kg (20lbs) hanging off it. The little locking tab in the head failed. With 8 cable ties at 150psi there is approximately a 5kg (11lbs) load on each cable tie so there is a comfortable safety margin. If you expect to use higher pressures then you can use larger cable ties or more of them.

  • Adding a little silicone grease to the o-ring makes it much easier to put the bottle on the launcher.

Using the Launcher

  1. Unclip the launch string, and remove the Clark launcher from the Gardena quick connector.

  2. Fill your rocket up with water and while it is still upside down insert the Clark cable tie launcher into the rocket and lock the collar into place.

  3. Place the rocket back on the launcher and lock it into position.

  4. Connect up your launch string.

  5. The rocket is now ready to be pressurised and launched.

References

 

20/2/2013

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