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This is a step by step instructable of how to build a tennis ball rocket.

Step 1: Ingredients!

Three 2 Liter bottle
One ball of yarn/ string
One roll of tape
One plastic dry cleaners bag
One pair of scissors
One hot plate (if available)
One drill (if available)
One ruler
Three manilla folder
One bowl of water
One tennis ball
One small plastic bag (big enough to put a tennis ball in)

Step 2: Find a Bottle! ...Or Three.

The bottles will be used as the body of the rocket and the place where the parachute will be stored. Before doing anything to the bottles, label them one, two, and three, so you do not mix them up and this should be a little less confusing! Keep one of the three bottles (bottles #1 and #2) uncut and intact, while the other (bottle #3) should be cut. The last (bottle #3) should be cut so there is only a cylinder, no bottom or top (shown in the image). Like I said it is very important that you DO NOT cut the other bottles, you will at a later time, though. Thanks :)

Step 3: Heat It Up!

You will now use the hot plate and drill to form a dome shaped end to bottle number two (one of the intact bottles). Here's how to do it. Take the drill and drill a hole into the cap of bottle number two, keep it attached to the drill. Hold the drill so it is upright and on a flat surface, and the bottle can be held over the hot plate (without actually touching it). Turn the hot plate on and turn the bottle with the drill (like you would roast a marshmallow). The heat will make the bottle expand and form the bottom of the bottle into a dome shape. Once that the shape has taken (it may take a while, be patient) turn the bottle in water to cool it down, considering it will be hot plastic... Remove bottle from drill and cut off the end of the bottle with the spout.

Step 4: Back to One... With Fins!

Take bottle one (the intact bottle) and mark three vertical lines on the bottle even widths apart. These will be the place-markings where your air dynamic fins go.

For the fins: Take one manilla folder and make a right angle along the folded edge (as diagramed below) and add a rectangle to the top of the triangle (opposite side of the folded edge). Do the same thing on the other two folders.
The measurements of the triangle should be Folded Edge 7" single lined side (side above the scissors in the picture) should be 5" (4" for the side and 1" for the folded edge) . The last side should be about 5.5".
Attach the fins over the lines, using the folded lines as supports and tape down the folded tabs.

Step 5: Cut and Tape!

Once you cut the spout side of bottle number two tape the cylinder of bottle number three to the cut side of bottle number two as an attachment to make bottle number two longer so you have more space to fold the parachute into.

***Do not worry, with some effort you will be able to fit the bottle number two and thee combo (we shall call it bottle two now) over bottle number one. DO NOT tape it, if you do, how will the parachute get out to deploy?***

Step 6: Chute Me...

The Following steps may be used to make the parachute:
1. Take your dry cleaners bag (large, thin plastic bags or painters tarp may be used) and cut the edge with the whole for the hanger off so it now resembles a cylinder.
2. Cut along the side of the cylinder so when you open it, it's a triangle.
3. Fold three pieces of tape along each of the smaller edges, so there is one in the middle of the two sides' edge's and one on each corner. Then put three more, evenly spread, on the longer sides. (Make sure the tape is big enough to punch a hole though with a hole puncher.
4. Hole punch each piece of tape and string though 23" of string from each hole to the center of the bag.

Step 7: Tennis Ball, Anyone?

Take the tennis ball and put it in the plastic bag and secure it so you may recycle the tennis ball and play a game later.
Punch four holes (which are reinforce with tape, like the parachute) in the extra pieces of the bag so they are on each side (north, south, east, and west)
With each side corresponding to an edge on the mat, tie the strings on the south side to the hole on the south side of the bag with the ball in it, and so on. (north to north, east to east, west to west.)

Step 8: Fold Me!

Fold up the parachute so it resembles a narrow sliver of pie and then roll it into bottle two. Make sure the ball is near the opening of the bottle, but inside the bottle. Place bottle two on-top of bottle one (bottle one should be upside down) so the spout side is free to fill with water.

Step 9: Fill It Up...

Fill the bottle half way to two thirds with water and place on the pump. Pump away, pull the string and let fly. If the rocket does not separate from the storage capsule (bottle 2) try not to put the bottles together so tight.

Have Fun!
oh right,never had to do that one at school.sorry for ignorance :s<br />
this is really confusing, I don't have a clue what the tennis ball is all about. How do you attach a pump? Maybe a diagram would help
This seems to be a physics assignment. This is the fourth water rocket I've seen here that carries a tennis ball. According to the first one I read, the rocket's mission was to keep the ball in the air for 30 seconds. It must have also been in the assignment to write an instructable about it.

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More by M.J:Tennis Ball Bottle Rocket by Mike Grey and Jeanine Costa 
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