Water rockets

When I was still at college I built my own water rockets and had a little website where I showed my designs. The website is long gone but there are still a couple of sites linking to my old site. That's why I copied all information into this article for people to have as reference.

My launcher

How it works

Before I came up with this launcher design I did quit a lot of research before actual building it.
My goal was to make a launcher that was both very easy to build and had a very good performance.

If you want a high performance launcher you need a launch tube. This is nothing more than a piece of PVC pipe sticking inside the bottle. This acts as a kind of piston at the start of a launch and gives a massive amount of thrust.

One of the biggest problems when making a launcher is to make it completly air and water tight. The easiest way of doing this is by using an o-ring. (click on the image on the right to see an enlarged version) The o-ring lies in a groove formed by two pieces of PVC. This way it won't get loose and you get the optimal sealing performance.

The rocket is pressurized using a pump of some sort with a pressure gauge. In my case air is pumped in using a small diameter(6mm) 10 meter long pvc tube with a car valve at the end.

Holding your rocket down

To prevent your rocket from prematurely leaving the launchpad it needs to be held down by some sort of mechanism. The rocket is almost always held down by the large ring just below the threads. I use a spring bend from 3mm thick welding wire which fits in the slotted release. Before you pump the rocket the spring is closed and held together by a small plate with two holes drilled in them. There is a piece of string attached so you can release your rocket from a safe distance. The wire prevents the neck from coming out of the release when the rocket is pumped with air. At launch the spring is released by pulling the launch cord. The wires jump away and the rocket is free to leave the launchpad. Look at the animation on the right and the picture on the left.

Installing the launcher

The launcher is installed ‘tent-style'. There are three guy-wires attached to the launcher arranged in a circle. These are hooked on three tent-pegs. It's a very simple, light and extremely stable way of installing the launcher. After this the air-hose is installed by inserting the tube in the launcher. You're now ready to launch some rockets. Shove the rocket over the launcher and install the release spring. If you haven't already done so fill your rocket with water start pumping and pull the launch cord.

Below is a complete description on how to make my launcher.

What do you need?

On the right is a drawing of all the parts you need to make my launcher. There are lots of different size PVC tubes around depending on the country you live in. I used 3/4″ and 5/8″ diameter PVC pipes. But if you live in the US it's best to use schedule 1/2″ PVC and the smaller 1/2″ CPVC pipes. The 1/2″ PVC fits exactly inside a bottleneck and gives the best performance.

  1. Launch tube - 3/4″ diameter PVC (length depends on size of rocket)
  2. Base inner - 5/8″ diameter PVC (length depends on the height of your launcher mine is 90 cm (35″) long but at least 50 cm (20″)for optimal stability)
  3. Rocket
  4. O-ring stop - 1″ of 3/4″ diameter PVC
  5. O-ring - about 7/8 outer diameter, 5/8 inner diameter
  6. Rocket release top - 40 mm inner diameter drainage pipe end stop
  7. Pipe protectors - 2 1″ pieces of garden hose
  8. Release locks - 3 M6x20 bolts (preferably aluminium.)
  9. Rocket release bottom - 4″ of 40mm diameter PVC drainage pipe
  10. Wire hooks - 3 Threaded M6 hooks (preferably aluminium)
  11. Base outer - 5/8″ diameter PVC (Length depends on the height of your launcher it should be about 15 cm (6″) shorter than part a.)
  12. End-cap connect - 1″ of 5/8″ diameter PVC pipe
  13. Copper end-cap - 15 mm Inner Diameter copper end-cap (If you can get them use a CPVC end-cap)
  14. Air tube - 10 meters of 6 mm (1/4″) diameter flexible tube
  15. Valve - Car valve (rubber removed)
  16. Tube clamp - 6mm (1/4″) tube clamp (not drawn)
  17. Release spring - 20cm (8″) 3mm welding wire (not drawn)
  18. Guy-wires - about 4m of small nylon cord (not drawn).

First where going to make the end-cap. If you can get CPVC end-caps use these because it's much easier to install them. The 1″ PVC pipe (l) has to be glued (using epoxy) inside the copper end-cap.
The pvc pipe doesn't fit exactly so you may need to sand the end a little. Leave it to cure completly.

You now need to make a decision on the size of your launcher. If your rockets aren't very tall (single bottle) you can make the launcher about 1 m high. This way the rocket has a convenient hight to fill and hook-up the release.
If you're rockets are very tall they can become pretty heavy when filled with water. Since PVC is pretty flexible it may bend a little to much for comfort when it's installed. Reducing the length will result in a much more rigid launcher. As a guidline keep the Outer Base Pipe(k) 15-20 cm(6″-8″) shorter than the Inner Base Pipe(b).

Insert the End Cap (l/m) and Inner Base(b) into the Outer Base(k). Make a mark on the Inner Base where it comes out of the Outer Base. Remove all parts. Make sure to sand the edges of the pipes and solvent weld the Inner base into the Outer Base. Make sure you glue both top and bottom. After this solvent weld the end cap into the bottom.

Now solvent weld the O-ring stop (d) in place, making sure you leave a large enough gap for the O-ring to fit in.
Drill a 6mm hole in the side of the launcher. Make sure you don't drill this hole to big, if the hole is the right size you won't need to use adhesive to make it air tight.
This completes the basic launcher now we need to ad the release mechanism.

First make two slots in the middle of the Release Top(f) about 5mm wide and 15mm deep. It's best to take several different bottles and insert them. Measure how deep each of them goes in and determin then where the exact position of the slots should be.
After this take the release bottom (i) and make marks where the holes for the bolts should go. Stay about 15mm from the edge of the pipe and make sure all the holes are nicly aligned. Now drill the holes with a 5mm drill-bit and making sure all of them are drilled straight in.
After that tap the holes using a M6-tap, or if you haven't got one just use the bolts and screw them in and out a couple of times (Watch out that they go straight in).
Now solvent weld the release top (f) onto the release bottom (i).
Take the Welding wire (q) and bend it around a piece of PVC until you get a v-shape with a loop in the middle.

It's now time to assemble the launcher. First take the two pieces of gardenhose(g) and put them in some boiling water. This makes them very flexible and much easier to slide over the PVC pipes. Slide the O-ring (e) in place.
After this screw the bolts(h) and hooks(j) in the release mechanism. Make sure the ends are aligned with the inside wall. Keep the release in place and turn each of the bolts one turn. Keep repeating this until the top is securly in place. By making single turns one at a time the release will self-center itself on the launcher. Do the same with the hooks. The pieces of gardenhose will protect the soft pvc pipes from the hard metal parts and will distribute the forces a little better.
Remove the rubber from the valve stem and insert it in the airhose. I used a small tube-clamp to keep it securly in place.
Make three guy-wires from the nylon cord and attach them to the hooks. I also attached the release-spring with a small piece of string to one of the hooks. This will prevent it from jumping away at launch. The spring can be held in place by a small metal plate or pvc tube with two holes drilled in them.
Attach a long piece of string to it so you can pull the lock of from a save distance.

Rockets

Below I will describe how I built my first rockets:

Bottles

The most important part of a water-rocket is off course the Soda-bottle. Here in the Netherlands we mainly have 1,5-liter bottles. These area pretty good to build water-rockets from. The frontal area is pretty small and since they are reused for up to 10-12 times the wall-thickness is much larger than other bottles. This means they can resist much higher pressures.

Some common style bottles in the Netherlands.

  1. The bottle I used so far is a 1,5-liter Coke bottle.
  2. One of the most common 1,5-liter bottles. This is used by a lot of different manufacturers.
  3. A 1,5-liter bottle with a glued foot at the bottom.
  4. A 1-liter bottle.

At the moment i've been using the Coke-bottles which work pretty good for me. It's pretty hard to pick a good bottle there are lot's of different sizes and manufacturers. You can best try some different sizes and see what works the best for you. Below is a picture of my ‘Lucifer' rocket made from the 1,5 liter Coke bottle.

Fins

The rockets I made so far are pretty simple in design but I still had a lot of trouble getting them stable at high speeds. For the first couple of flights I made fins from thin sheets of plastic. They looked pretty rigid but when I launched at higher pressures (higher launch speed) they just flexed and came partly lose. This was mostly due to the fact that they where much to big.

Since then i've switched to Balsa fins. Balsa fins are very easy to make, very light and rigid enough to withstand flexing. To get a fin that doesn't break when your rocket hits the ground you need to laminate two pieces of 1mm balsa. When you glue them together make sure the grain of the wood is perpendicular to each other, to give it uniform strength.
After this sand the leading edge into a nice round shape and give the trailing edge a nice taper (don't make it to thin or it will just break, or reinforce the trailing edge by soaking in some thin super-glue). I used heat-shrinking foil (used on model-aircrafts) to finish the fin, but you can use any type of paint.

To attach fins it's best to use some sort of flexible glue. This gives a very clean shape. I however have used packaging tape to attach them. Because i'm still experimenting with the sizes and the amount of sweep of the fins I can remove them easily and don't have to make a new rocket each time a fin design doesn't work. It's also possible to glue the fins to a separate sleeve to make easy changes possible.

Nose cone

I used a piece of foamrubber to make the nose cone. Cut it in a nice shape and attach it using some packaging tape. The foam rubber acts as impact resistance and prevents serious damage to things like cars and houses. It also protects the bottle from the impact. For these single bottle rockets it isn't really necessarry to ad a parachute. They are easy to make and easy to replace also the parachute and deployment device will ad a lot of unnecessary weight to such a small rocket.

Long-tail rocket

This is the first idea i'm going to try out. Long-tail rockets are very stable in comparisson to the normal delta fins i have been using so far. All the weight is in the front and the tail is a long way behind.

The way i want to do is by using a o-ring seal in the neck of the bottle. This will keep the bottle air and water tight. The tail is held in place by a bottle-cap and the tail-end can be switched between bottles. on the end of the tail a cut-off bottle-neck is glued. This is necessarry to hold the rocket down or switch from a internal launch-tube to a quick-connect launcher (screw on a quick-connect coupling).

My launcher design needs to be modified a little to use this type of rocket.

  1. Soda-bottle
  2. Bottle-cap
  3. Tail - 3/4″ PVC pipe
  4. Cut-off bottleneck
  5. O-ring stop - 1/4″ of 3/4″ PVC pipe
  6. O-ring former 2″ of 5/8″ PVC pipe

Categorieën: experiments water rockets