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April 26, 2022

How to make a Water Rocket Launcher

Introduction

When my children were at primary school, we were invited into let the kids have a go at making and launching water rockets. For the uninitiated this is where you take a 1.5 - 2L plastic soft drink bottle(commonly referred to as PET bottles) and launch it 50+ metres into the air using nothing more than water and compressed air.

Essentially, it’s the best fun you'll ever have with your recycling, and kids absolutely love it.

The rig shown on the drawings attached was originally based on one described in an article from The Shed magazine. Ours has a modified release system, and a supporting frame. This was because we soon realised that excited kids are a lot stronger than they look!

Water rockets can be as simple or as complex as you want to make them. A simple one for young children may just be a bottle covered in stickers. To go higher you need to start looking at the aerodynamics involved by adding stabilising fins, shifting the centre of gravity, and lengthening the body.

Materials

Apart from the high-pressure plastic piping, the materials and dimensions given in the attached drawing are all nominal. You should utilise and adapt to what you can find in your own garage or shed, adjusting the given design to suit. In fact, a quick search on the internet will present you with a multitude of alternative options.

In addition to the water rocket launcher, you will need the following:

Soft drink bottle (obviously). Initially you can start just with a plain bottle then progress to more aerodynamic designs (a simple example of one is included with the drawings).

Hand pump It is recommended you use a floorpump type to minimise your pumping time. Whatever pump you use it must include a pressure gauge, so you know when you have got your rocket up to pressure.

Water Simple tap water is all you should need; however, you may like to experiment with more exotic fuel mixes such as food colouring, salt water, and dishwashing liquid to name a few.

Pipework

The piping used for the launcher is a half inch (15NB) PVC pressure pipe. This pipe size has an OD of around 21.4mm, which makes for a good sliding fit on the opening of a soft drink bottle.

A union coupling provides a shoulder where an O-ring can be fitted, providing a seal to the bottle opening. As this is not an ideal seal face for an O-ring you will likely find you need to build up the contact area by wrapping some Teflon tape (PTFE, also known as plumbing thread tape) over the O-ring.

PVC pressure pipe is commonly referred to as high pressure pipe and is readily available from most hardware or plumbing merchants. High pressure pipes and fittings should be labelled as conforming to AS/NZS 1477 and have a minimum pressure rating of 15Bar (look for PN15 on the labelling). If you're in doubt, ask.

Also note that pressure piping has specific gluing requirements, with solvent cements particular to that material type. Again, look for reference to AS/NZS 1477, and ask if you're in doubt.

Frame

The frame performs several functions:

  • It provides protection and support to the plastic pipework and connections.
  • Provides a mounting point for the trigger mechanism, along with guides and limits on the pull rope.
  • Has peg holes allowing the launcher to be pegged to the ground.

In our frame we have used offcuts of timber decking and plywood. The stand is held in place with a coach bolt and wing nut, allowing it to be broken down to fit more easily into the boot of car, or for storage.

Trigger Mechanism

The trigger mechanism sits either side of the bottle. One half has two angle fingers that slide over the neck of the bottle, and  the other half clamps down on top of it. This provides a balanced arrangement to the force coming from the centrally mounted bottle. Only a little force is necessary to release.

Figure 1. Trigger Mechanism Closed

Figure 2. Trigger Mechanism Open

Both sides have double nuts allowing the trigger to be adjusted to the right clamping pressure. Once adjusted you should find this arrangement will work without further adjustment for hundreds of rocket launches.

The pull rope ties to one end of the trigger and then runs through a guide hole in the frame. Knots tied either side of the guide hole will keep the rope captive and prevent over pulling of the trigger.

 There are lots of other examples of trigger mechanisms on the internet. If you look at alternatives try to find one that won’t interfere with any fins that extend down from the rocket.

Launching

So on to the fun part. Half fill your bottle with water, slide over the end of the open pipe of your launcher onto the O-ring seal, and then clamp it in place with the trigger mechanism. Making sure everyone is standing well clear, pump air into the bottle until the pressure reaches 60 –80 PSI (400 – 550 kPa). Then after a suitable countdown sequence pull the rope attached to the trigger.

Safety

A water rocket launcher will and can inflict damage if misused. The following safety steps are recommended:

  • Don’t let young children use without adult supervision.
  • Always use in an open space, such as a park or beach. And position yourself well away from other people.
  • Maintain a clear area of at least 5 m around the launcher. Use soccer cones to mark out the space.
  • Extend the hose on your pump (you can use a vacuum hose to do this) and use a long enough pull rope to allow you to position yourself away from the launcher.
  • If you use a compressor instead of a hand pump it must be fitted with a regulator or relief valve.

 I prefer to use a hand pump rather than compressor as most kids don’t have the strength or the body weight to get the pressure much above 60 PSI (400 kPa).

Soft drink bottles can reliably sustain pressures of over 120 PSI (830 kPa) and won’t tend to fail until the pressure exceeds 140 PSI (965 kPa). Most modern bottles are one piece with a formed base, which is where they tend to fail due to the stress raiser of the formed shape. So, bottles that fail tend to burst at the end, provided you’re not standing over the top of it, you shouldn’t come to harm.

If you have any doubts with a bottle, it’s probably best to get rid of it. Otherwise, you can hydraulically test them (and the launcher at the same time). To do this fill the bottle and rig completely with water, then pressurise it to 140 PSI (965 kPa) in the launch position to test it.

Don’t store your rocket launcher outside. PVC is susceptible to UV and will degrade rapidly in New Zealand’s harsh summer sun. Keep your launcher somewhere that it will not be exposed to direct sunlight.

Design Principals

The design principals of a water rocket can be every bit as technical as a real rocket. Newton's Third Law states that for every action there is an equal and opposite reaction. In the case of our water rocket this means that the water forcibly ejected from the bottle creates an equal and opposite force on it, causing it to move in the opposite direction.

So, what can you change to achieve more height? You need to consider the following with your rocket design:

Opening(orifice) size. The rate that the water leaves the bottle is governed by the opening. Adjusting this opening (orifice) size will influence the height you achieve. Some alternative rocket designs use Gardena style hose fittings to provide a smaller opening.

Water volume, density, and viscosity. The more water you fill your rocket with the more energy it will take to launch it, as the rocket becomes heavier. Increasing the density (by dissolving salt) of the water will increase its mass without increasing its volume. Changing the viscosity (by using dishwashing liquid or jelly crystals) has a similar effect to changing the opening size, as it also affects the rate that water leaves the bottle.

Aerodynamics. For water rockets to fly stably the centre of gravity (COG) needs to be in front of the centre of pressure (COP). This can be achieved in several ways (refer to our advanced rocket example below)

  • Add fins to the bottom of your rocket. This will create some drag shifting the COP back towards the fins.
  • Add mass to the front of your rocket. This pulls the COG forward.
  • Increase the length of the rocket. Increasing the body length will increase stability by creating better separation of the COG and COP.

Enough theory, go and experiment!

We'd love to hear how you got on with designing your water rocket launcher. Share with us here along with any pictures.

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