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Construction - Basic

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#147 - Descent Rates

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#140 - Dual Thrust 2

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#137 - Axion G4

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#134 - 7 years

#133 - LaRF

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#131 -Materials Challenge

#1 to #130 (Updates)

 

WATER ROCKET CONSTRUCTION - BASIC
So you want to build one...

The following instructions describe how to build a simple single stage water rocket capable of flights to altitudes of around 60 – 80m. (~180 – 250 feet). For simplicity this rocket does not use a parachute.

For more advanced construction tutorials go to the Construction Index.

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
Body
A typical 1.25 L bottle... about to be transported faster than ever before.
Drink the contents of the bottle and wash it out. Remove the label, cap and the plastic ring.
A complete bottle will be the fuel tank that will also hold the pressurised gas.

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.

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Ring Fins
Cut a plastic ring from a bottle.
A can with an appropriate diameter and a manila folder.
Mark the can and paper.
Roll the can along the edge and then divide the length into thirds.
Roll the manila folder around the can tightly, and tape when finished.
Finished fin jig. Make sure that the can and folder sit square on the table.
Slip the ring over the fin jig.
Choose an appropriate strut material. We will use slimline Venetian blinds cut to length.
Align the strut along the lines on the jig, and tape the strut to the ring. Repeat for all struts.
Slide the fin assembly off the jig.
Staple and or tape the struts securely to the ring.
Reinforcing of the struts is recommended if you are using Venetian blinds.
Place the ring assembly back on the fin jig and leave an overlap at the top.
Place the main bottle body into the jig. This helps align the body with the fins.
Tape the struts to the body and then just slide the whole thing off.

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.
  • Rohan sent in a good suggestion as an alternative to Venetian blinds and that is to use old tape measures. He reinforces them with skewer sticks.
  • 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.

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Flat Fins
A cardboard fin template
Corriflute cut out to shape
Small holes drilled along the edge to be joined.
Trim the left over plastic after drilling.
Lightly sand the area to be glued.
Lightly sand the bottle where the fins are to be attached.
Fin alignment jig sitting on top of the rocket.
Run a bead of PL Premium along the edge making sure it penetrates up to the holes.
Place the fin on the bottle.
Run a bead of glue on the outside to cover the holes and create fillet.
Completed fin set that can be fitted to different rockets.
For larger rockets, ring fins are not always practical and so the more conventional flat fins should be used.

Here we are making the fins from Corriflute (Correx) corrugated plastic that signs are commonly made from. These are durable, lightweight, water proof, nice and flat and above all inexpensive.

One of the most common problems with Corriflute is that it is made from polypropylene and so is very difficult to glue with any great amount of strength. Most glues will not stick to it well . The procedure described below will ensure a good strong bond to the bottle. In this procedure we attach the fins to a section of a bottle that can slide over the top of another bottle which allows it to be reused on other rockets, but the fins can be glued directly to the rocket body. Fins need to be attached well because large G-forces and drag could brake them off or if the rocket lands on them during descent.

Procedure

  1. Make a cardboard template of the fin shape you would like to use. Make sure that it contours the bottle as closely as possible. As far as the fin shape goes that is up to you. The main thing to remember about fins is that they need to be stiff, as far back as possible and lightweight. Always remember that fins are there to add rocket stability and so ultimately the surface area of the fins is important.
  2. Trace this template onto the Corriflute material so that the channels run as close to perpendicular to the bottle as possible. On curved surfaces an  approximation to perpendicular is good enough.
  3. Cut the fins out with a sharp craft knife.
  4. Now using a 2mm drill bit, drill a hole all the way through the fin in each channel. The holes should be 3 to 5 mm from the joining edge. If you are using thinner Corriflute, adjust the spacing accordingly. (See photo)
  5. Clean up the holes with a sharp knife, cutting off any bits left over after the drilling.
  6. Clean the surfaces around the holes and lightly sand them.
  7. Now lightly sand the bottle where you are going to glue the fins. The number of fins you use is again up to you but anything above 4 is of little benefit. 3 or 4 fins are the most common. A good way to measure the distances around the bottle is to wrap it with a piece of paper and then use a ruler to divide it up into thirds or quarters and mark the positions. Put the paper back on the bottle and transfer the marks to the bottle.
  8. For the next step we use a simple alignment jig to keep the fins pointing in the correct direction. You can use a length of plastic, wood or aluminium angle and simply rest it on top of the rocket. It will naturally align itself with the axis of the rocket. We then attach a pair of smaller angle brackets to it in order to support the fin from either side. This also allows the aligning jig to keep clear of the glue fillets. You can tape the jig to the bottle to secure it further. We wedge the rocket between stacks of heavy books to keep the rocket steady on the table.
  9. Run a bead of PL premium into each of channels so that it penetrates all the way up to the holes.
    NOTE: Anytime you use PL premium you should use gloves and always work in a well ventilated area.
  10. Place the fin onto the bottle and align using the jig.
  11. Now run a bead of PL on the outside between the bottle and the fin. You can use a gloved finger to make a nice fillet between the bottle and fin but make sure a healthy thickness of glue remains over the holes. Do this from both sides.
  12. Let the glue cure overnight before doing the next fin.

You now have a strong bond between the fin and the bottle. The holes allow the glue on the inside and outside to merge creating little fingers that keep the Corriflute in place mechanically even if the adhesion fails in parts.

 

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Gardena Nozzle
A typical tap garden hose attachment.
Make sure it is of reasonable quality with a good o-ring.
Remove the thread leaving a shoulder.
Remove the plastic seal from the cap.
Cut a hole in the cap and the seal.
The garden hose attachment should fit snugly in the cap.
Place the seal against the hose fitting.
A completed nozzle. Notice the tight fit between the hose fitting and cap.
Nozzle fitted to the bottle.
Now for the business end ... the nozzle. We have chosen to use a restricted nozzle for three reasons:
  1. Acceleration is lower and the overall top speed is also lower and hence less drag on the rocket. There is also less stress on rocket components during takeoff.
  2. Burn time is increased which makes for nicer video.
  3. The nozzle being a standard garden hose fitting makes it easy to construct the launcher.

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

If you want to use a different type of nozzle on your rocket see the full Nozzle article on the subject.

The following video tutorials show you how to make the nozzle. Some manufacturers have recently stopped making bottle caps with removable seals, and the video shows an alternate source for caps suitable for nozzles. The second video shows how to solve a common problem where the nozzle does not lock properly into the launcher.

Video tutorial: Making a water rocket nozzle

Video Tutorial: How to fix a Gardena launcher where the nozzle does not lock in properly.

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. 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
  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!

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Nosecone
Nosecone made from medium density foam. Shaped with a knife and smoothed with sand paper.
Same material as above. Attached to the bottle using a ring of plastic material.
Nosecone made from the top of another bottle. This one is not designed for impact.
Same as above but a different shape. Both are designed to hold a parachute.
A kids ball with whistles.
Cutting off the front makes for a very good strong nosecone.

For rockets without a parachute, 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. Make sure that the nosecone is not too light as in order to maintain rocket stability.

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.

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.

Parachutes:

A full tutorial on how to make a side deployment parachute mechanism is now available.

For lots more information and details on how to add a various recovery system such as a parachute go to the:
Recovery Guide.

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