Making and launching a water rocket can be a fun learning experience. A water rocket is a type of model rocket using water as its reaction mass. The water is forced out by a pressurized gas, typically compressed air. It demonstrates Newton's third law of motion and the second law of thermodynamics. This Instructable will cover the launcher and the base. Feel free to make your own bottle as pretty and rocket-like as you wish! We went for a more minimalistic approach.

Supplies

You will need:
Materials

1m 3/4" PVC
2 3/4" PVC 90 degree elbows
Bicycle inner tube tire with air valve
Sealing cap for 3/4" PVC
2 wooden boards (to be used as base and backpiece)
2 wooden blocks of equal dimensions
Nails
Tie wraps
Duct tape
2 20-32mm hose clamp
O-ring with 3/4" inside diameter
Plastic 1.5l Coca Cola bottle (~1" diameter)
PVC collar to fit over the bottle head (~1.5" diameter; ~2.5" length)

Tools

Hot glue gun
Hammer
Electric drill
Scissors
PVC cutter/saw

Step 1: Cutting Pipe Components

Using your PVC cutter or saw, cut off both bigger parts from one of the elbow joints, as illustrated with a dotted line (see image). You'll be using these bigger parts, so sand them down to remove jagged edges. The bent section of this elbow joint will not be used, but you should still have an intact joint, which will be used later.

From your long 3/4" pipe, you will need to cut two sections, again using the appropriate tools. Stick the pipe completely inside the bottle you will use as the projectile. When completely in, mark the pipe where it exits the bottle. Remove the pipe again and make a full cut where you marked it, resulting in one section that just fits into the bottle. The remaining part you need to measure and cut again, this time to obtain a 20cm section. Sand down the cuts on the three separate sections of 3/4" PVC you now have (the section that fits in the bottle, the 20cm section, and what remains of the original tube).

Step 2: Fitting and Assembling

Now, two sections of 3/4" PVC (the 20cm one and the one that fits into the bottle) will be connected using the parts you cut off an elbow joint. This will keep the O-ring in place exactly where you want it.

Fit a cut-off piece from the elbow joint onto the PVC pipe that fits into the bottle. It should fit on snugly, but take care not to slide it on all the way, as this thicker piece will be used to connect both 3/4" sections.

Using pliers, slide the second cut-off elbow joint piece entirely onto the 20cm PVC section, continuing to slide it a bit further. Put the O-ring over the protruding side of the 20cm section. This side should now be the right length to fit into the cut-off elbow joint piece that you initially mounted. If this is not the case, slide the cut-off elbow piece with O-ring on the 20cm section to obtain the desired result.

Once both 3/4" sections are connected by a cut-off elbow joint piece and the O-ring is in place (see image), the PVC components can be glued into place with a hot glue gun or PVC cement.

Next, the second, intact elbow joint will be used with what's left of your initial 3/4" PVC tube. Insert the leftover piece of the 3/4" PVC into the elbow joint and glue them together. In your final assembly, the 20cm section will be placed in the other end of the elbow joint, but this end should not be glued.

Step 3: Adding the Cable Ties

Next, you'll be making the mechanism that secures the bottle as you prepare for takeoff. This will allow you to control at what pressure you let the bottle shoot up. The mechanism we used incorporated cable ties, which reach past the head of the bottle (the part the cap screws onto) to keep it in place.

Start by placing cable ties around the pipe, in such a way that their 'heads' are facing inward (see image). The cable ties should be placed such that they reach just over the head of the bottle (the ends of the cable ties pointing down). Place the bottle on the pipe to ensure their placement is correct; the O-ring should just enter the bottle when you do this. Position cable ties around the entire pipe; you can keep them in place with an elastic band during this step. Once you are satisfied, duct tape the cable ties in place. Add the garden hose clamps over the duct tape for extra security.

To complete the holding mechanism, fit the PVC collar over the cable ties. When the bottle is in place, the collar should be tight around the cable ties to keep the bottle from shooting off. Note: the holes in the side of the collar that can be seen in the image are not required.

Step 4: Closing Off the Pipe

Take the inner bicycle tube and cut the air valve off with scissors, leaving a small edge of rubber around the base. Take the end cap for your 3/4" pipe and drill a hole just big enough to fit the air valve. Insert the valve from the inside, pointing out, and use the hot glue gun to glue the rubber from the valve onto the inside of the end cap. Be careful not to let any glue fall into the valve itself, as this would shut off the valve.

When the glue has dried and the valve is secure, place the endcap over the 3/4" PVC that is attached to the elbow joint. Completely fill the space between the end cap and the pipe using the hot glue gun to make sure everything is airtight. Let dry for an additional 30 minutes. Once the glue has dried, test whether the piece is airtight: no air should escape from the sides when you blow on the valve.

Step 5: Assemble the Base

Take the two wooden blocks and place them onto the wooden board. They should be placed a distance apart that's just big enough for the 3/4" PVC pipe with the elbow joint to fit in. The pipes should fit snugly so that they don't require any fastening. Nail the wooden blocks to a back piece, with this assembly, in turn, being nailed to the wooden board you will use as base.

Your structure that keeps the pipes in place is now complete.

Step 6: Blast Off!

You have now assembled all pieces you need to launch your bottle rocket. Now it's time to launch it.

Place the 3/4" PVC pipe with elbow joint and valve between the blocks. The elbow joint should touch the backboard, with its open side facing up.

Fill the bottle for a third with water, this acts as the propellant. Take your straight PVC pipe assembly and fit it into the bottle, placing the cable ties over the head of the bottle. The bottle should cover the O-ring, so no water or air can escape. Move the collar over the cable ties to firmly keep the bottle in place.

Turn the bottle upside down. No water should be spilling out at this point. Place the free end of the PVC construction (the other end being inserted into the bottle) into the elbow joint you placed between the blocks.

Using a bicycle pump on the valve, pump air into the bottle until you start feeling resistance from the bottle's pressure. Carefully remove the collar. You might have to give the bottle a gentle nudge before it shoots off

Congratulations! You have now launched a bottle rocket!