this is an instructable working with rockets. water rockets in particular . this launcher can send 2 liter bottles over 500 ft. in the air .use at your own risk. it is from http://sciencetoymaker.org/waterRocket/index.htm so i give all the credit to them

Step 1: Materials

1 10 foot piece of 1/2 inch, schedule 40, PVC (plastic) pipe. You might be able to find this in the plumbing section of a building center/hardware store, or in the form of conduit in the electrical area. Usually the conduit pipe is gray in color instead of white.The pipe size is printed on the pipe. If you actually measured the pipe, you would find the outside is about 21mm (almost 7/8"), and even the inside is 15mm (9/16"). Choose by what is printed on the pipe, not by actual measurements. The "schedule 40" refers to the thickness of the pipe, and rest assured that schedule 40 is the most common, easiest to find kind. See picture below.

1 piece of PVC 1 1/2" (one and one half inch), schedule 40 pipe. You only need 2 inches of it, but some building centers only sell it in 10 foot lengths (even a 10' long piece is only a few dollars). You might be able to find a sympathetic plumber or home handyman who can give you a couple of inches. 2 PVC 90 degree elbows for the 1/2" pipe.
1 end cap for the 1/2" pipe above.
1 small can of PVC pipe cement (or borrow some).
1 package of 8" (or close to it) general purpose ties (also called cable ties). These will be in the electrical wiring area of the building center.
1 hose clamp. The size is important. I get hose clamps with a range from 11mm to 23 mm (a little less than 1/2" to a little less than an inch). The hose clamp must be a little bigger than the pipe because that's what holds the all purpose ties securely to the pipe. Better to have the hose clamp a little too big than a little too small.
1 tire stem. This is the one thing you are not likely to find at a building center or hardware store. However, it is easy to find at any automotive parts store. Tire repair shops often throw them away. The inner core of the tire stem does not have to work, or even be there. If you tell the person at the tire repair shop they might give you some free.
A half inch (1/2", 13mm) diameter drill bit, a quarter inch (6 or 7mm) diameter drill bit, and drill. There are only two holes to drill, so if you don't have a drill find a neighbor who does. You will only take a minute of their time. A drill press is even better.

Step 2: Step 2

STEP 2: cut the long piece of pipe

First use the hacksaw (or just the inexpensive disposable hacksaw blade) to cut the 10' piece of 1/2" PVC pipe into two pieces: one 2' long and one 8' long. The pipe is plastic, easy to cut.

Step 3: Step 3

Step 3: determine where to put the bulge and create it. Read the whole step before you start.

Fit the bottle on an end of the long piece of 1/2" PVC pipe so the pipe end is about 25mm (1") from the bottom of the bottle. Use a pencil, pen or marker to mark where the pipe goes into the bottle. Or just make your mark about 280mm (11") from the end of the pipe.
In this step you will create a slight bulge which will function as a seal to keep the bottle from leaking. You will do it with heat. It is easiest if you do it somewhere that is not very drafty or shelter the candle from drafts. Light a candle and hold the pipe where it is marked about 50mm or 75mm (2 or 3 inches) above the tip of the flame. KEEP ROTATING THE PIPE SO IT HEATS EVENLY ALL AROUND. You might smell the heated plastic, but if it discolors the pipe is too close. After about two minutes the pipe will become quite soft and flexible where you are heating it.Before the pipe cools, grab on either side of the hot spot, push inward and you should see a slight bulge form. While still squishing the pipe, cool it under cold water or a wet towel (it helps if there are two people). Keep the pipe as straight as possible as it is cooling. If it is really important to you to have the pipe perfectly straight, you could put a snug-fitting dowel into the pipe to keep it from bending. I never get it perfectly straight.
Notice in the picture above that the bulge is not big. It only has to be big enough to keep the bottle opening from sliding past. Test it by pushing a bottle onto the pipe. It should get stuck at the bulge (I used a green bottle this time).

Step 4: Step 4

Step 4: Drill the hole for the tire valve and insert it

Drill a 1/2" (13mm) hole from the middle of the inside of the pipe end cap. It's a good idea to hold the end cap with a pair of pliers and back it up with scrap wood to the drill bit does not ruin the table when it comes through. Pull the tire valve stem through the hole in the end cap, from inside the cap to outside.Grab the stem with pliers, but do not grab on the threads. Pull hard and wiggle the stem through until the groove in the stem gets seated on the pipe. You will know when you have it seated because it will be impossible to back the stem back out anymore.

Step 5: Step 5

Step 5:Glue/solvent weld the pipes

At the very least you have to glue the end cap onto the long pipe. However, for a few more pennies you can end up with a "J" shape which has two benefits. First, it tends to keep the end of the valve stem from scraping on the ground. Secondly, if the check (one-way) valve in your air pump is leaking a little, the vertical rise should trap some air, so air will leak back into your pump rather than water.

This is how it will go together. Notice that the bulge end of the long pipe (and the bottle that it seals on, green in the picture below) is not glued into an elbow. The other end will be.Slather some cement on the inside walls of the end cap. Make sure you do not apply so much that it blocks the opening to the valve.
Slather cement around the end of one of the 30cm (1') piece of pipe. With a twisting motion, push the pipe into the end cap. You can wipe off the little bit of cement that squeezes out.

Repeat with the elbows. Make sure the end of the long pipe that DOES NOT HAVE THE BULGE is glued into the elbow. Set the pipe aside so the glue can dry.

Step 6: Step 6

Step 6: Start the tie/tape part of the triggering mechanism--position the ties.

In this step you will start building the part of the water rocket launcher that holds the bottle on the pipe despite the pressure, the releases it at the right time. First, you will carefully tape the all-purpose cable ties so they apply pressure evenly to hold the bottle on. Tape the ends of two rulers or a ruler and a hack saw blade (a paint-stirring stick would also work) so they are parallel to each other and the outermost edges are 100mm (4 inches) away from each other. Take care here that they are parallel by measuring on both ends (I know the rulers in the picture below don't appear to be perfectly parallel, but it is because of cameral distortion). Push 9 cable ties under so they are held down. It will take the next 3 pictures to make it clear exactly how this should be done. Notice that the ties are all parallel and the heads are together against each other and against the ruler.

Step 7: Step 7

Step 6: Continue the tie/tape part of the trigger mechanism--tape the ties.

This requires a bit of finger dexterity because in addition to the ties that the tape is supposed to stick to, it also sticks to your hand and the table. Cut off a piece of duct tape about 90mm (3 1/2") long. Lay the tape down beside and below the straight edge (a hacksaw blade in the picture below). Press the tape onto the ties so it really sticks to them. If you are using electrical tape, add more pieces until it is wide like duct tape, about 50mm (2").Now remove the straight edges that were holding the ties down and carefully (so the assembly does not fall apart) flip the whole thing over to the other side. Cut another piece of duct tape and carefully lay it where the first piece is (although it is on the other side this time). Again push the tape on. When the tape sticks to the ties and to the other piece of tape, the assembly is sturdy.Trim the duct tape on either side so that only about 5mm (3/16") sticks out. Wrap the tie/tape assembly around a pipe. If it is so wide that the ties on the end overlap each other (unlikely unless you have a brand of ties with very wide heads), then you might have to cut off a tie. If there is a bit of a gap--as in the picture below--that is OK. By the way, notice in the picture below that the this particular brand of ties bends near the end (the end opposite the heads). I am going to cut them off right where they bend because it is hard to slide the hose clamp past them in the next step.

Step 8: Step 8

Step 8: Finish the tie/tape part of the trigger mechanism--attach the ties/tape assembly to the pipe with a hose clamp.

Thread the hose clamp onto the long pipe on the end that you made a bulge, and slide the hose clamp past the bulge. Then push a 2 liter bottle onto the same end of the pipe until it stops on the bulge. Then lay the tie/tape assembly underneath as shown in the picture below. The smooth side of the tie heads should face down, the protruding side up. If you do not know what I mean, I will elaborate in the next paragraph.Wrap the tie/tape assembly around the pipe. The tie heads should be positioned so that they can grab onto the handle of the 2-liter bottle. IMPORTANT: The protruding heads of the ties must face the bottle. If you do not understand that, go back up and look at the second picture from the top of this page (of the a single green tie positioned against a bottle handle) .Slide the hose clamp onto the bottom of the tie/tape assembly as shown in the picture below. Tighten the hose clamp, but not too much because you will make a fine adjustment to it later.

Step 9: Step 9

Step 8: Cut and drill (or tape) the slide pipe/trigger. 1 1/2" diameter schedule 40 pipe is very common.

Cut about 50mm (2 inches) of 1 1/2" diameter, schedule 40 PVC pipe.
Take a moment to see how the slide trigger/pipe works. Take the bottle off. Slide the slide trigger/pipe on the end of the long pipe and over the heads of the ties. Put the bottle back on--pressed onto the bulge as shown in the picture below.
Slide the slide trigger/pipe back over the heads of the ties. Now you should not be able to pull the bottle off. When you tip the launcher vertically so the bottle points up, there should be enough friction between the short trigger pipe and the ties so the trigger pipe does not slide down just from the pull of gravity. If the trigger pipe is so loose that it slides down by itself, the water rocket will be launching before you are ready. To increase the friction so the trigger pipe does not slide down unexpectedly, put some electrical tape (duct tape would work too) on the inside of the trigger pipe as shown below. drill a 1/4" (7mm or so) hole near the end of the pipe. Thread the end of the rope through the hole and tie it so it does not come out.

Step 10: Step 10

Step 10: Adjust the launcher

Now that you have the slide trigger pipe done, it is easy to adjust the trigger mechanism so there is no leaking. Put the slide trigger pipe on the launcher. Push a two-liter bottle onto the long pipe and push it HARD onto the bulge. It helps to twist the bottle a little as you push it down.

Lock the bottle on by pushing the slide trigger pipe over the heads of the ties.

Loosen the hose clamp a little with a screwdriver. Pull the tape/ties assembly down, and while still pulling, tighten the hose clamp back up, quite tightly (this might work best with two people, or place the other end of the launcher against a wall).

Step 11: Launch It

Step 11: TEST IT! (do this outside--expect to get wet)
Fill a 2-liter bottle a third or half way with water. Push the bottle on to the bulge on the pipe (a little bit of water will spill--that's ok). Lock the bottle on with the cable ties and big pipe, as explained in step 8, above. If the big pipe does not stay on, line the inside with duct tape as explained at the end of step 8. Hook a bicycle pump to the tire valve. Once you start pressurizing the bottle, you must keep the launcher pointed toward the sky until the bottle has been released. Consider it to be like a loaded gun.Start pumping air. If the pump has a pressure gauge, pump up to 70 psi (5kg/cm) or whatever you can comfortably reach, but not higher. If it starts leaking (and assuming the bottle does not have a hole in it), then you might have to further adjust the trigger but If it is a slow leak, you might actually want to keep it that way because it disarms the launcher if some knucklehead leaves it around pressurized.

With the launcher pointing to the sky, pull the rope...HAVE FUN!