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.

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

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.

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

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.

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

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.

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.

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

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.

Insert the strip about half way into the launch tube.

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.

Let the glue cure.

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.)

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.)

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

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.

You can now trim all the cable ties.

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.