PLEASE, do not aim the Bazooka at people, animals, your neighbour's prize-winning tomato plants, or other objects that may be damaged. Only launch outside. Don't do anything stupid, like lighting the bottle on fire before launching it. Use only water as a propellant. Do not over-pressurize the bottle. Use under adult supervision. Keep small children away (regardless of my video!) Use the thinking part of your brain.
MOST IMPORTANTLY, do try to retrieve every bottle you launch. Let's not litter, okay? I'd also suggest using "reclaimed" water as propellant, such as rainwater, snow melt, or the like.
Step 1: Materials and Tools
3-4 feet of 3" diameter ABS pipe
One 3" to 1.5" ABS reduction adapter
One 1.5" to 0.5" ABS threaded pipe adapter
One 1" inside diameter metal washer
About 6" of scrap 2x4
About 6" of 3/4" diameter plastic pipe (any kind)
One #4 rubber stopper, solid core (cork won't work, it has too many holes!)
A bicycle brake lever
Bicycle brake cable housing (cut to fit)
Bicycle brake cable (cut to fit)
A brake cable clamp or similar
Any sort of glue that will adhere plastic to plastic, and wood to plastic
Water bottles - 500mL size, or anything that will fit in the launch tube
A long Presta valve - a used bike tire tube is a good place to get one!
And here are some tools that you'll probably need:
Something to cut metal - a Dremel or metal-cutting bandsaw would work best
Something to cut plastic - a handsaw or miter saw or something similar
Something to cut wood - really, any sort of saw would work.
A drill press (or a hand drill, in a pinch)
A sharp utility knife or X-acto knife
Assorted measuring and marking implements
Step 2: Cut the Washer
With a permanent marker and a ruler, draw a line starting at one edge of the hole outwards. Draw a second line parallel to the first, on the other side of the hole. These are your cut lines.
I used a Dremel tool with a cut-off wheel to cut the washer, but you may use whatever you've got that will cut hardened steel. Work carefully, and try to get the cuts as smooth as you can. If you're using a dremel, you may need to flip the washer over and cut from the other side as well.
When the section is cut out, you may want to smooth it out with a grinding bit. The modified washer should slide easily behind the lip under the threaded potion of the bottle's nozzle. It should not slip past the lip even if it's push all the way to one side.
Next you may drill a hole in the washer, through which the brake cable will eventually be secured. Use a drill bit meant for cutting steel (a nice cobalt bit works great), that matches the diameter of the brake cable clamp. Alternatively, if you don't have such a clamp, you can drill and tap the hole and just use a regular screw.
When drilling the hole, run the drill at a slow rpm, and keep the work cool using a bit of oil. Doing this will prevent the bit from getting dull.
Step 3: Make the Stopper
I suggest using a Presta valve from an old bicycle tire tube. These are ideal for this application because you can inflate the bottle rocket inside the launch tube, then disconnect from the bike pump without losing pressure. The Presta valve is also long enough to go all the way through the stopper, where a Schraeder valve may not be. Simply cut it off the tube using scissors, leaving about half a centimeter of rubber around the base of the valve.
Select a drill bit that has a diameter slightly smaller than the Presta valve. Install it in the drill press, but DON'T try to drill a hole through the middle of the stopper! Instead, just use the drill press to push the bit through the stopper. Then, remove the bit from the drill press and use the press to push the Presta valve through the stopper where you previously made a hole.
Later on, you may find that the stopper sticks out too far from the neck of the bottle. You'll need to make the stopper a bit narrower. I found that the best way to do this was with a sanding bit on a Dremel (cutting the soft rubber with a knife is almost impossible). Simply use the Dremel to remove thin layers of rubber, until the stopper can be forced far enough into the neck of the bottle.
Step 4: Cut the Reduction Coupler
Wrap a piece of masking tape around the narrow end of the coupler. This will serve as a nice high-contrast surface to mark cut lines. With a marker, draw a line across the masking tape - it doesn't matter where. Then, on the exact opposite side of the coupler mark another line.
When you look inside the coupler from the narrow end, you will see a lip. Normally the smaller pipe will hit this lip, preventing it from going too far in. In this case, the washer will be resting on it. With a ruler, measure how far down this lip is on the coupler. In my case, it was about 1.7 cm. Make a mark on each of the lines that is 1.7 cm from the end of the coupler. The slot has to be the same width as the washer. Place another mark on each side of the coupler that is about equal to the width of the washer. Now, scribe a line between each of the pairs of lines. You may want to use a guide of some sort.
Using a saw, cut out the slot as carefully as you can. The washer should slide in easily, but should not wobble around. When you have completed the cut with a saw, you may need to widen the slot just a bit with a sharp knife. BE CAREFUL when you do this - ABS plastic doesn't really like being cut in this way and your blade may slip. Cut away from yourself, and make sure that either both hands are on the knife or keep one hand away from the work piece.
When you're done, the washer should slide in about halfway into the coupler. Slide in a bottle and make sure that it cannot be pulled out while the washer is in place.
Step 5: Hollow Out the Threaded Coupler
In order for everything to fit together, most of the threaded section must be removed. Unfortunately, it's recessed inside the coupler making it impossible to cut out with a saw. Instead, I used a drill press to cut out the threaded section.
Using a drill press it's easy to set the depth gauge so that you don't end up drilling all the way through the end of the coupler. Set the gauge so that the drill bit will stop at the same level as the inside edge of the coupler. Drill down though the thickness of the threaded section with a 1/2" drill bit, working your way all the way around. Clean up any plastic cuttings.
Now, you can go back and use the drill press as a sort of milling machine (of course, if you have a milling machine, use that!). Plunge the bit all the way down to the stop, and pass it over the drilled out parts to eliminate any peaks that remain. Do this until the inside of the coupler is relatively flat.
Step 6: Make the Release Assembly
Grab a piece of scrap 2x4. It can be less than 6" long. Using the finished reduction coupler as a guide, draw a sort of L-shape on the wood. The entire L-shape should be about as long as the entire coupler. The base of the L-shape should be about as long as just the wider section of the coupler. Make sure that everything is sized thick so that it stays strong.
When the pattern has been drawn on the wood, you can cut it out. I used a scroll saw, but a band saw or even a table saw would work. Cut just the main shape of the L-shape first; don't cut out the corner that makes it an "L" just yet.
Hold the semi-complete assembly in place on the coupler, and estimate where the hole for the cable will need to go. It will be in the center of the wood, a centimeter or two from the end. Mark this with a pencil.
Start with a bit just large enough for the brake cable. Drill a hole down far enough that it passes all the way through to where the corner will eventually be cut off. Now, select a different bit that is just large enough for the brake cable housing. Using the depth gauge, drill a hole about halfway to 2/3 into the top of the assembly, but NOT all the way to where the corner will be cut out. You must leave a bit of wood there for the cable housing to rest against.
With both holes drilled you can cut out the corner of the release assembly.
There is just one thing left to do. Using the edge of the coupler as a guide, draw an arc onto the release assembly where it will eventually mate with the coupler. Use this mark as a guide, and cut out the curve on a saw (this is where a scroll saw really comes in handy). When cut, the assembly should sit flush against the large part of the coupler.
Step 7: Attach the Handles and Trigger
First I cut the toilet plunger handle into two equal lengths. They are about 8" long, but you can make them however long you wish.
If the plastic you're using is black, as my handles were, wrap a piece of masking tape around the handle, about 3" down its length. Using the 3" ABS pipe as a reference, draw an arc at the end of each handle. The arc should be relatively long, so that there is a large amount of contact area between the barrel of the bazooka and the handles for gluing.
With a scroll saw, carefully cut along the line, being careful not to rotate the handle. Clean up the cut using an X-acto knife or a sander (depending on what the handle is made of). Dry-fit often, to ensure a snug fit against the edge of the pipe.
When the handles are finished, try a few positions for them on the pipe. You may want them closer or further from the end, depending on the arm length of the user. You may also change the angle for better ergonomics. Glue one handle using hot glue, then re-fit the second handle and glue it in place also.
The last step is to attach the brake lever. In my case, I had to add a shim to get the lever to clamp down securely. A piece of bike tire tube (the same one I stole the presta valve from) fit perfectly. Position the brake lever on either the left or right handle, and secure it by tightening the screw.
Step 8: Assemble the Release Assembly
Slide the reduction coupler onto the end of the 3" pipe and push it down as far as it will go. The L-shape should be on the top of the launch tube. It probably won't be necessary to glue it in place, since the fit will be pretty tight. But, you may go ahead and use glue if you like.
Insert the stopper in an empty bottle as far as it will go. Load it into the launch tube, then slide in the washer to lock the bottle in place. Now, take the threaded coupler and insert it into the narrow end of the reduction coupler. The threaded coupler should push right up against the stopper - remember, when you're filling the bottle with air the stopper will want to pop out. The threaded coupler is there to stop that from happening. Dry-fit the coupler and test the release lever a few times. Everything should be precisely positioned so that the bottle can be securely locked by sliding the washer in the slot. Glue the threaded coupler in place when you have found the optimum position.
Step 9: Run the Release Cable
Run a brake cable through the housing. The cable should be about six inches longer than the housing. Start at the brake lever, threading the knobby end of the cable into the catch on the lever. Slide the housing in place on the brake.
On the release assembly, thread the cable through the hole. Grab the cable clamp and slide it through the washer. Slide the cable through the hole in the cable clamp. You may add a small spring between the release arm and the washer if you like. Holding the washer in the "locked" position, pull the cable tight, and tighten the nut on the cable clamp. Test the release assembly by squeezing the brake lever - the washer should pull out of the slot just enough to release the bottle.
Step 10: Operation
Grab a water bottle and fill it about one-third to half full with water*. Stuff the stopper in the end as far as it will go. The stopper should stay in place even if the bottle is inverted. Note that you can tighten the little nut on the end of the Presta valve to prevent water from coming out during loading.
Set the bazooka on a flat surface, and slide in the filled bottle stopper-end first. When loaded, the presta valve should poke through the hole in the threaded coupler. Slide the washer into the locked position - it should stay in place even if the bazooka is moved or turned upside-down.
Loosen the nut on the end of the presta valve, and attach a bicycle pump with the appropriate presta valve head. Inflate the bottle to between 50-80 PSI. Note that the pressure to which you can inflate the bottle greatly depends on the strength of the bottle. In general, the thicker the plastic, the higher the pressure. Listen for fizzing noises coming from the stopper - hopefully there will be none at full pressure.
Now, simply hoist the bazooka onto one shoulder, and point it at least 5 degrees from horizontal (so that water is covering the bottle's nozzle). Hold onto the handles, and squeeze the brake lever to fire!
*according to Mythbusters the optimum ratio is 1:3 water to air, but that's for vertical flight. Since we're launching on an angle less than vertical, it may be necessary to add more water to cover the nozzle.