The following instructions describe how to build a launcher and a simple single stage water rocket capable of flights to altitudes of around 60 – 80m. (~180 – 250 feet). For simplicity the rocket does not use a parachute, and is built to withstand a high speed return to earth.

Go to the more advanced construction techniques section if you want to try to get more performance out of your rocket.

Warning

When cutting PET bottles with a knife, it is easy to slip and do some damage to yourself, or the work area. Always reach for scissors first as these are easier to control and much less dangerous. A rubber chicken is not sharp enough to cut through plastic bottles.

Tools

You will need at least the following tools when building the rocket and launcher but a well equipped workshop is always useful:

• Drill and drill bits.
• Round or half round file
• Scissors
• Stanley knife
• Fine sandpaper
• Hacksaw or tin snips
• Tape

Bottle Preparation

1. Get a 1.25L bottle and wash it out with dishwashing detergent to get the sticky residue out. The shape of the bottle can play a role in the aerodynamics, water flow within it and its center of gravity. For this reason a bottle with straight walls, no ornate protrusions and a smoothly tapered neck is a good choice. (Shaped Coke bottles are an example of an unsuitable bottle)
2. Remove the label.
3. If the label leaves a sticky residue you can easily remove it by using a little mineral turpentine on a cloth. You should then wash the turpentine off with a soap and water.
4. Inspect the bottle for any kinks or scratches. The bottle may burst at these places when pressurised to a higher pressures.
5. Measure the capacity of the bottle, don’t necessarily believe the label. Knowing the capacity will help you determine how much water should be put in.

That’s the end of the bottle preparation.

Storing the contents of the bottle in a plugged up sink with a note “will drink later” is probably less than ideal. Make sure you buy bottles with contents you will drink. Because a bottle looks aerodynamic in the store does not mean you will want to drink some cheap imitation lemonade. While making rockets you will need plenty of bottles to make different components. The best way get bottles is from your friends, that way they feel they have contributed to the race for the lower atmosphere.

We prefer to make ring fins for our small rockets. This is because they are easy to construct, are easily aligned, are very aerodynamic, and fit in the launcher nicely.

It is very important to align the fins as accurately as possible so that the rocket flies straight. Before we make the fins we will create a jig that will make the whole process easier.

Fin Alignment Jig

1. Get an empty washed-out bottle that is the same diameter as the bottle prepared earlier and with scissors cut out a ring of material. You can experiment with the width of the fin ring, but around 50mm is a good compromise. Making the ring narrower, reduces the weight of the whole rocket, but also is less effective. 
2. Find another bottle with straight sides or preferably a can of food whose diameter is about 2mm less than the ring you have just cut out. It does not matter if it is canned carrots or soup.
3. Now get a manila folder and place the can on top of it aligning the can’s edge with the edge of the folder. Put a mark on the can and the folder for alignment.
4. Roll the can along the edge and place a second mark on the folder where the original mark on the can was.
5. Now divide this distance into thirds, and accurately make 3 parallel lines on the folder that are perpendicular to the bottom. These lines will be used for aligning the fin struts.
6. Now roll the manila folder tightly around the can making sure that the lines are visible on the outside and that the bottom edge of the folder is flush with the can and sits upright when standing. Use tape to hold the folder on the can. Leave the can in the folder as it ensures that proper shape is maintained and provides a stable weighted base when assembling the fin.
7. The bottle ring should fit snugly on the jig but not too tight.

The fin jig is now finished.

Before dinner go down to the grocery store and replace the can of food you used for the fin jig!

Fin struts

You will need to make some struts to hold the ring in place. These struts should be as stiff and as light as possible. You will need three of them.

Some suggestions:

• Slim aluminium Venetian blinds: These are lightweight and relatively sturdy, they are very easily attached with a stapler, however, they do have a tendency to buckle on impact. If you use these, you should consider reinforcing them with something lightweight like bamboo skewers.
• Anti-static IC packaging: These are very strong, light weight and relatively indestructible. Use the ones for DIP (Dual In Line) packages. You should be able to pick some up from a local electronics store, perhaps for free if it is waste.
• Extruded plastic moulding: - This tends to not be as stiff as the IC packaging and also tends to be a bit heavier. If the moulding is relatively wide, you may be able to just use 2 instead of 3.
• Various plastic tubing or thin fibreglass rods are also suitable alternatives.
• Bamboo skewers ( see the D.Y. or Clifford rockets )

In this example we will use the Venetian blinds as they are quite common, light weight and provide extra surface area for the fins.

Procedure

1. Cut three 300mm lengths of the strut material.
2. If you are using other materials you may want to consider tapering the leading edge of the strut with a knife or scissors for better aerodynamics.
3. Place the ring on the jig and hold in place with a small piece of tape.
4. Place one strut along the line drawn on the manila folder and hold it at the top with a clothes peg or a piece of tape.
5. Now tape the bottom of the strut to the ring. This is only temporary while everything is aligned.
6. Repeat the steps above for the remaining two struts.
7. Now remove the fin assembly from the jig. This now gives you a well aligned ring fin with parallel struts.
   
   Design Note: We found that tape was insufficient to survive more that a couple of impacts when using IC packaging fin strut material so we wired the struts to the rings. An easy way to make small holes in the plastic is to heat a needle over a flame such as the stove and then simply push the needle through where you want the hole. Use a pair of pliers to hold the needle … well for obvious reasons. Also do not try to use a sewing machine to make things go faster as getting the flame to sewing machine needle it is a little tricky.
   
   Burning the hole through as opposed to drilling it, makes sure that the material does not develop tiny cracks around the edge of the hole that could make it fail on impact.
   
   After you have made all the holes, use some wire to hold it together. This gives a very strong bond. You can now remove the tape that was used when aligning everything.
8. Now staple or tape the struts to the ring for a secure bond.
9. You can reinforce each strut with a bamboo skewer simply by taping it on. We were surprised at how well this works without adding too much weight. We have never had a buckled strut since.
10. Place the ring assembly back on the fin jig but leave the ends of the struts showing over the top.
11. Put the rocket body (bottle) upside down into the jig. This allows you to align and hold the bottle in place while you are attaching the struts.
12. Tape the struts to the body of the rocket and then simply slide the rocket off.

And you are done with the fins.

1. Acceleration is lower and therefore less drag on the rocket. There is also less stress on rocket components during takeoff.
2. Burn time is increased so it looks better.
3. The nozzle being a standard garden hose fitting makes it easy to construct the launcher.

The nozzle diameter is ~9mm. This type of nozzle is generally referred to as a "Gardena" nozzle.

Procedure

1. Get a standard garden hose tap fitting as shown in the pictures on left.
2. Remove the threaded section with a hack saw or as I prefer to use a pair of tin snips. These work great on the soft plastic. Be careful not to damage the inside of the fitting where the bottle will sit. Trim just enough plastic so that the nozzle shoulder fits snugly in the cap.
3. File or sand the edges of the shoulder so it is smooth and no pieces of plastic are left hanging.
4. Put one sheet of very fine sand paper on a flat board and lightly sand the top of the nozzle until it is completely flat. Depending on the hose fitting manufacturer there sometimes is a lip that needs to be sanded off.
5. Remove the cap from the bottle and wash it out with some detergent so it is not sticky from the contents.
6. Remove the seal from the inside of the cap and set it aside, you will need it in a minute.
7. Now drill a hole in the cap, and widen it with a round or half-round file. When filing the hole, it is a good idea to hold it in a vice, but in order not to damage the cap in the vice, you can cut the neck off a PET bottle and hold it firmly in the vice. Screw the cap onto neck and you can now work on it.
8. Keep enlarging the hole until the nozzle snugly fits in.
9. Remove any plastic burrs with fine sand paper or Stanley knife.
10. Place the nozzle inside the cap.
11. With a Stanley knife, cut a hole in the seal that you had earlier removed from the cap.
12. Place the seal on top of the nozzle. It should hold in place by it self.
13. Make sure that the o-ring on the nozzle is not split and is clean from dirt.
14. Put the new nozzle on a bottle and tighten it and make sure it sits square.
15. You should be able to blow into the bottle through the nozzle to make sure it is sealed properly. If the seal is not sitting properly, you may hear air escaping around the nozzle. If it is leaking, check for debris under the seal. You should also be able to see if the top of the bottle is contacting the seal properly by cleaning the seal and then screwing the nozzle onto the bottle. Removing the nozzle again should leave an imprint in the seal. Check to make sure that the imprint goes all the way around.

The nozzle is now finished.

Now go down to the hardware store and buy a new garden hose tap nozzle to replace the one you just pulled off the garden hose tap before mum finds out!

The nosecone is perhaps the trickiest to get right so that it survives multiple landings. You can make it fairly lightweight and smooth, but it needs to be strong enough to withstand impact. If it is too rigid it may damage the bottle or the person it lands on. Be aware that the nosecone should not be too light as this will cause the center of gravity to move back and possibly make the rocket unstable in the air.

Experiment with using different density foams, and the cut off sections from the tops of PET bottles.

Some high density foams are easy to cut with a Stanley knife, but can be hard to get smooth. You can use coarse sandpaper to make them more streamlined. A disk sander also works well for shaping the nosecone.

Part of the fun of making water rockets is experimenting with different configurations and different materials you may have at hand.

The way you attach your nosecone will depend on the design that you use, but typically you can use a ring of plastic from a bottle to make the transition from the bottle to the nosecone. We recommend using only tape to attach it as it will have a bit of give when the rocket impacts, and makes it easy to repair if needed.

Pictures on the left show a variety of nosecone designs we have used. Some worked better than others.

The launcher consists of a stable base with a vertically mounted release mechanism. The launcher also provides a guide for the rocket during take off. This helps to steer it in the right direction before the fins can take effect.

Feel free to modify this design as you see fit.

Procedure

Base

1. First we start with the base. Cut a square or rectangular piece of wood as shown in the photo. Don't worry about exact dimensions as it does not matter. If it is too small the launch pad will tip over, but if it is too big it becomes cumbersome to carry and transport. Make sure the wood is not chipboard or MDF as it will get soaked quite a bit. Marine plywood is usually the best choice. The wood should be fairly thick to provide enough support for the guides.
2. Attach an aluminium frame to the base. This helps keep the launcher from tipping over. Again exact dimensions are not important.
3. Find the center of the board and draw two concentric circles, one at 95mm and one at 115mm. These will represent the two most common bottle diameters 1.25L and 2L respectively. If you are using different size bottles measure the diameter and add ~5mm for clearance.
4. Divide each circle into thirds and mark the locations on the board. This is where the vertical guides will be placed.
5. Find wooden dowel or aluminium tube of around 10-12mm in diameter and cut into 3 equal lengths. These will become the rocket guides. The length of these will depend on the release mechanism height. The guides need to be long enough to guide the rocket during the first part of the launch. Exact length is again up to you.
6. Drill vertical holes at the markings on the board such that the guides do not overlap the circle. Make sure that these holes are a snug fit when you insert the guide. This is best done with a drill press rather than a hand drill.
7. Repeat the drilling for the other circle(s) spacing the holes around the circumference.

This guide design was chosen because it allows for both ring fins, and the more conventional flat fins.

Release mechanism

1. The release mechanism is consists of a garden hose fitting attached to an air hose. We have used a brass fitting because it was of reasonable quality and had a smooth pullback action. Be careful when using cheaper plastic fittings as these may not seal as well and may prematurely release the rocket at higher pressures.
2. Attach an angle bracket vertically to the center of the base. This should be long enough to enable a rocket to sit on top and still have enough clearance for the fins.
3. Attach a length of garden hose to the garden hose fitting, and then attach the air hose directly to the garden hose. Use of hose clamps and adapters is encouraged. The air hose attachment method will depend on what you have available and what your air supply has as its output. Try to keep the number of transitions to a minium in order to minimise potential sources of air leaks. We used a quick release adapter at the base to make it easier to pack up and transport.
4. It is a good idea to put a non-return valve in line with the air hose to prevent water from going back into the air supply. We built a custom non-return valve into the hose fitting, but others have used bicycle tyre valves or other forms of valves. If you cannot get your hands on a valve, it should be possible to just create a loop in the air hose whose apex is above the the rocket's fill waterline. Kind of the S-bend principle of a toilet (where the air is pumped in from the sewer end). I highly suggest you don't use a toilet as your non-return valve.
5. Attach the hose and/or the base of the hose fitting securely to the vertical angle bracket using hose clamps, or cable ties. Ensure that the centerline of the hose fitting is inline with the centerline of the concentric circles on the base.
6. Using another hose clamp attach either side of a piece of string to the movable part of the hose fitting. To this string attach a long piece of string that will be your launch string. Create a hole in the bottom of the vertical angle bracket to feed the release string through. If the edges are too rough you could set up a pulley system as we have done, to stop the string being cut. The idea is to turn the horizontal pulling action into a vertical one to pull down the hose fitting.
7. You can hammer in a couple of nails in the base board that will allow you to wrap the string around for tangle free transportation.
8. We painted the board with water proof paint to help stop the wood from absorbing too much ... you guessed it ...water.

Air supply

Your choice of air supply will depend on what you have available and are comfortable using. A bicycle pump is cheap and easy to use, but requires a lot of work. A car foot pump is easier but still requires a bit of work.

We prefer to use a small 12V compressor intended for filling car tyres. Ours goes up to around 120psi. We use a small 12V 7.5Ah sealed lead-acid (SLA) battery to power the compressor. Both are easy to transport and a battery charge will last you all day. The typical fill time for a small 2L rocket is around 40seconds - 1 min.

For higher pressures you can use more expensive compressors, but for really high pressures you can use a SCUBA tank with an adjustable regulator. We prefer to use a SCUBA tank on the larger rockets, but it is a little cumbersome to carry around.

Many different designs for launchers are available on the internet so make use of your favourite search engine.