This is the second activity in our "Build Your Own Model" category.  It is a very cool, very fun, end of year activity for the kids.  They will make a water rocket launcher that shoots pop bottles into the air.  There is a lot of interest, a lot of water and plenty of screaming, applause, and cheering.  You can make an entire week meaningful and memorable if you work it right.

The Experiment
This activity is going to give you an opportunity to build a launchpad using sprinkler pipe and then launch 1 and 2-liter pop bottles 50-100 feet into the air.  As you work to achieve perfection in attaining these stratospheric heights, you will have to experiment with and determine the proper ratio of water to air to find the best propellant (the push to get the rocket off the launchpad).  Likewise, you will have to address issues of vehicle stability and determine the most efficient pattern of fins and cones to aid your rocket in its journey into the galaxies and back down on top of the neighbor’s doghouse. So, as they say in Houston, “Godspeed.”

Materials & Procedure
1.  The first thing that you will need to do is put together your rocket launcher with all the PVC parts listed below and a little glue.  All of these items can be found at the local hardware or sprinkler supply company.  Using the pattern we have included as a guide, assemble all of the pieces.  You will need to drill a half-inch-diameter hole in one of the end caps, you may want to enlist one of your folks or the neighbor with the cool drill press and too much extra time for assistance. 
Remember that perfection rarely comes the first time out of the chute, so be patient. When you are done, you will have something that resembles a giant, slightly mutated, capital T.

1   2-liter Pop bottle, preferably clean and empty
1   Bicycle pump, air tank, or air compressor
1   Bucket of water
1   Large, plastic, drinking cup to refill bottle
1   Bottle of PVC glue
1   Pair of PVC scissors
1   Drill with half-inch bit
1   Roll of masking tape
1   Tire valve
1   Pile of cardboard or poster paper

These PVC pieces are half-inch in diameter.
4   12" Pieces of pipe
1   8" Piece of pipe
1   4" Piece of pipe
1   3” Nipple
1   T joint (slip, slip, slip)
1   T joint (slip, slip, screw)
1   Coupling ( slip, slip)
1   Elbow
3   End caps

Procedure
1.  Once you have your launcher assembled, it is time to experiment with shooting the bottle into the air.  To start with, fill the bottle half full with water and wiggle it on to the rocket launcher. If you look carefully at the illustration, there is a single wrap of masking tape around the nipple.   The tape is there to create a tight seal, so that none of the water leaks out while you pump air in.  If the bottle is a little loose as you slide it over the nipple, you may need to add a second wrap to make the seal a little tighter.  We are going to let you decide for yourself as you work through the different launches.

2.  When you get the leaky water thing worked out, attach the pump and start to cram air into the bottle.  You will be able to see the air gurgle up into your soon-to-be rocket.  As the pressure inside the bottle increases, the rocket will be more and more susceptible to launch.  Keep pumping and it will eventually shoot off the stand and up into the air.  You don’t have to do a thing except to keep pumping air into the bottle until the pressure gets too great to hold it on the stand anymore.
If you find that you pump and pump and simply cannot get the bottle to shoot, carefully walk up to the rocket and, laying on your stomach so you don’t get a bottle up your nose, wiggle it upward.  After the rocket launches, remove one of the wraps of tape so the seal isn’t quite so tight.

3.  Once you have launched your rocket a couple of times, you may notice that your rocket tumbled through the air, didn’t quite produce a smooth and uniform flight path, and generally looked like a football tossed into a crosswind by a heavily sedated gorilla.  To get the wobble out, you need to streamline your rocket—make it move through the air more smoothly.  To do this you may want to add a nose cone and fins.  No, actually, you will definitely want to add a nose cone and fins. That is what the cardboard or poster paper are for. 
By now you  have a pretty good idea what it takes  to get the rocket off the launchpad.  Now you have to improve the design to get it to go even higher.  Try the following ideas  and see what works for your rocket.

    a.  Design a nose cone to go on top of your rocket. Vary the steepness of the cone and change it from almost flat to very pointy.  This may or may not make a lot of difference.

    b.  Experiment with placing fins on the side of the rocket.  If you can figure out how to position the fins so that they create a spiral effect, science has shown that this adds stability and efficiency to the flight path . . . but how to get the rocket to do that?

4.  Once you get the wobble out, it is time to experiment with different ratios of air to water to get the rocket to shoot higher.  This will ultimately allow you to figure out the right proportion of water-to-air mixture to give your rocket the optimum boost.