Bottle Rocket Launcher: the Cheap, Doesn't Get You Wet Edition

Whether you're a teacher, a kid, or just someone that wants to have fun with some water, bottle rockets are a good way to explore aerodynamics and basic rocketry without breaking the bank. (Plus, it's a good way to reuse old water bottles. Think green!)

Step 1: Parts and Tools

Parts:
1   - 10' length of1/2" PVC pipe (not CPVC).............$1.68  
3   - 1/2" caps.............................................................0.27
2   - 1/2" elbows..........................................................0.28
1   - 1/2" cross............................................................0.98
1   - 1/2" to 1 1/4" hose clamp....................................1.05
1   - tire valve stem...................................................~2.99
10 - 8" Zip Ties (pack of 20).......................................1.95
1   - 2L bottle...............................................................free
$10.02
Tools:
PVC primer and cement ($7.51)
Drill or something with which to drill
Hack saw or other pipe cutter
Candles and ignition source
About 10' of string
Measuring tape
Sand paper
1/2" drill bit
Duct tape
Scissors
Marker
Pliers

Step 2: Measure and Cut the Pipe

Measure:
You're going to want to make the following pieces:
2 - 2"
2 - 15"
1 - 25"
1 - 61"

Since the exact lengths aren't too important, just get the measuring tape out and put a mark at the following inch marks:
2, 4,19, 34, 59

Cut:
You want to make the cuts as straight as possible and perpendicular to the pipe. To do this, I used a bit of duct tape (sticky side out) to make prefect circles as cutting guides. I wouldn't advise using you're bed to cut on like I did. (I needed a uniform backdrop for this instructable.) Bits of plastic get everywhere, so do it outside or in a shop.

Sand:
You can deburr the pipe any way you want, but I found the following system to work the fastest:
1. Cut a 1/2" strip from your sand paper.
2. With the bigger piece, swipe the outside edge while rotating the pipe.
3. Clear inside by inserting scissors, opening them, and rotating the pipe.
4. Using the strip, finish off the inside edge.
5. Clean up the remnants with the big piece flat against the cut.
Repeat if necessary.

*!!!!!*
NOTE: Do not try and fit the pieces together now, they can hold together very tightly without cement.
*!!!!!*

Step 3: Make the Pump Cap

The Pump Cap:
1. Make hole as dead center as you can on the cap using pliers and preferably wear something to protect your hand if the drill happens to slip. Do this in some dirt or other place where you aren't worried about your work surface. (You wouldn't like a hole in you desk would you?)
2. Deburr the hole as well as you can. (This seal is the most likely to leak.)
3. Pull the tire valve stem through the cap as hard as you can. Remember to put the cap on so you don't mess up the threads.There's a slot on the stem that you want to the cap to sit in. If your cap it too thick like mine, it wont fit in the slot. Don't worry, these things are designed to make a stronger seal the higher the pressure.

Step 4: Make a Bump Seal

*!!!!!*
NOTE: You will want to try this on the 15" leg pieces first so you know what you're doing.
*!!!!!*

The Bump Seal:
1. Put the pipe into the bottle about an inch from the bottom.
2. Mark where the bottle top meets the pipe.
3. Setup and light candle. (Make something to catch the wax.)
4. Continually rotate the pipe about 1/2"-1" above flame.
5. Do this for a minute or two, you should feel the pipe becoming malleable.
6. Push both ends together until the bulge adds more than an 1/8" to the radius.
7. Still squeezing tightly, submerge in water to set shape. (It wont cool down fast, so make the pipe as straight and even as possible.)

Step 5: Make the Zip Tie Latch

The Latch:
1. Align the zip ties flat side down on some duct tape. Make everything as square and even as possible, and only leave about 1/4" sliver of tape on one side of the zip ties.
2. Press the bottle down firmly onto the bump. It should hold itself there. If not, manually hold it for the next three steps.
3. Lay the first zip tie down on the lip of the bottle such that it 'hooks' and make sure it's parallel to the pipe.
4. Press down the sliver of duct tape to hold the everything in place during assembly.
5. Wrap the rest of the zip ties. Remember, the zip ties need to line up, not your duct tape.
6. Without messing up the assembly, put the hose clamp on and tighten around the middle of the zip ties.

Step 6: Make the Release

This one might look like a doozy, but it has a lot of wiggle room when it comes to design tolerances. Since it's all just duct tape and a bottle, don't worry if you mess up.

The Release:
1. Cut the top and bottom of the bottle. If possible, make it a bottle that doesn't have ridges in the middle area.
2. Cut a 1.5" strip in the smooth part of the bottle.
3. Put a 15" (or more) strip of duct tape on the 'outside' of the bottle strip, and a 2" strip of duct tape on the 'inside' of the bottle strip. The duct tape widths shown are half the normal width.
4. Wrap the bottle strip around the zip ties, using the 2" bit of duct tape to hold it together.
5. Wrap the 15" strip of duct tape around until it starts to overlap and can hold itself together.
6. Cut 10ish feet of string. (This number minus one foot will be how far you are from the rocket.)
7. Tie it around the bottle and duct tape strips as shown in the picture.
8. With the string going up, wrap the duct tape around once.
9. With the string going down, wrap the rest of the duct tape.
10. Slip the release off and make a crude outline of a hole smaller than the release is wide.
11. Cut the hole. You may need to start the cut with a knife.
12. Cut a same size hole on the opposite side. I found a spiral maneuver is the easiest way to cut.
13. Slide everything together and make sure it holds.

Step 7: Fitting It All Together

This last part is pretty straightforward. Just put all of the pieces together according to the picture and all identifying tags. Follow the instructions on your primer and cement cans, as they could be different from mine.

Some ProTips to remember:
- Do this outside.
- Try not to breathe in the primer or cement too much.
- Apply the primer and cement on both the inside of the fittings and the outside of the pipes.
- The purple primer stains most everything so wear gloves and cheap clothes.
- Acetone in the primer can eat through latex gloves. (and some other types of rubber)
- You're supposed to apply a 1/4 turn to all the fittings after you press them in, so stick them in 1/4 turn the opposite way.
- Put the 25" piece on right way up.
- Don't try and launch anything right away. Let the cement dry.

Step 8: T-Minus 5,4,3,2,1 LAUNCH!

ProTips: (Things to consider)
- fuel (water & air)
- weight (water)
- pressure(air)

The more water you have means more "burn time," or time that water is being shot out of the bottle causing it to rocket forward. However, this adds weight and takes away space that you could have for air, meaning you have to put more air into that smaller space. Let's say you fill the bottle 3/4 with water. Since you want every last bit of water to be pushed out, you need to put as much air in that 1/4 of the bottle as if it would fill the whole bottle. Therefore, you need the air inside to be more than 4 times the pressure outside. Since the air we live in is about 14 psi, you need the pressure inside to be greater than 56 psi. (At 56 psi, the last bit of water wont be pushed out so much as fall out, so multiply this number by 1.25 giving you 70 psi.) So, you're going to need a lot of pressure and still have a lot of weight. If you get up around 100 psi, the bottle is likely to deform, meaning you lose pressure to expansion.

Have fun with it. Experiment. Strap some fins and a nose cone to it for stability. Put a parachute on it. Try and have an egg payload and not break it. There's lots to do with this fun, cheap idea.