These four water rockets (eight 2 liter bottles total) have been joined together by simply screwing double sided bottle caps together. Each water rocket is actually two each 2 liter bottles joined together with a short threaded lamp rod, so BOTH bottles in each rocket assembly HOLD PRESSURE.
In April 1997, I attended Heather's (pictured at left) Discovery Middle School carnival. I brought 11 different styles of rockets, and had 75 great launches within a 2 hour period. BUT ONE ROCKET (with only two each 2 liter bottles) PERFORMED EXCEPTIONALLY, and here's the detailed story how I constructed it.
A special praise to Bill Robinson of California (USA) for submitting the original idea of using threaded lamp rod. We now dubb this the "Robinson Coupling". I've experimented and improved to make the process stronger and air tight. The final result is a rocket that blows the socks off all the other rockets I've made.
Basically, this rocket performs better because it holds larger volume of air, and half of the air is DELAYED because of the reducer, which is the threaded lamp rod. This rocket is also noiser, making a swishing sound. Once it gets it's initial blast off and speed, the delayed air pressure (from the second chamber) carries the rocket further.
The process in this example starts with 2 each 2 liter bottles, yet most any size soda bottle will work.
To draw perfectly straight locating lines for the fins, I use a 2"x4" wood piece, a felt tip marker, and a bottle. 
Measure three spots located 4.5 inches apart around the circumference.
Draw lines from top to bottom of bottles, using the 2"x4" wood as an elevated straight edge.
Using sandpaper, rough up the surface of the bottom of the bottle. Be sure to sand between the "feet" on the bottom.
Wrap the circumference of the bottles with filament pachage tape. In this example I wrapped the bottles three times each. I like to wrap the girth of the bottle 1.5 times, just to have enough overlap to secure the tape on the bottle. Remember, these bottles expand 5/16 inches when pressurized. This filament tape prevent the walls of the bottle from rupture. The bottle that was marked with three lines will be the "inverted bottle". The other bottle will be the upright bottle for the nose of the rocket. It is mostly filled with water for weight for the gluing process.
A home made rocket stand is helpful for assembling rockets. To make one go to HANDYSTAND. Make sure you make the handystand with only slip joint connectors. This will allow you to use the homemade PVC socket wrenches mentioned below. Many days, I use three of these stands for holding three rockets while glue is curing.
Invert the "bottom" bottle on the stand.
I've been very happy with PL Premium adhesive for many water rocket applications. Read more about this adhesive on another page. This is a perfect application for choosing this adhesive above others. It has great surface tension (attaches to surface of PETE bottles) and is the perfect consistancy when cured. It's semi-ridgid, not too soft & not too hard.
Apply the PL Adhesive to the bottom of the inverted bottle. Mound it up in the middle, between the five feet, and on top of the five feet. Do, the same to the upright bottle.
Put the upright bottle ontop of the inverted bottle. See how the five feet interlock between the two bottles. This also centers the bottles. It couldn't have been designed any better, even if this is a lucky coincidence. (Did the bottling manufactures knowingly design these bottles primarily as water rocket first, and then for soda containers secondary???)
Use the tip of the glue cartridge to fill any voids, and smooth surface of glue.
This needs to cure at least overnight, and perferably 24 hours or more.
(I have well water, and it turns yellow when left in a container overnight, typical of Florida well water, yuk.)
The PL Premium will expand while curing. A "weighted" bottle on top prevents the bottles from being separated during the curing stage.
Drain water from bottle. Now, it's time to drill the hole between the two bottles. I use a 3/8" electric hand drill, with a 12" long extention, and a 3/8" drill bit.
In this picture you will also see a white pipe. This is a 16" long section of Schedule 40 PVC pipe. This will be used as a "drill guide". It will protect the inside walls of the neck from getting scratched & will aid to center the drill bit when drilling the hole.
This is a close up view of the 3/8" drill bit which is connected to the 12" long extension. This drill bit has a reduced shank which is only 1/4" in diameter to fit inside the hole of the extension.
The "drill guide" (the 16" long section of Schedule 40 PVC pipe) is first inserted into the bottle, followed with the drill bit.
The drill will be going through the center of the first bottle, then through a layer of cured PL Premium adhesive, and finally through the next bottle. Make sure you spin the drill bit in the hole to clear out a 3/8" hole.
This is a very simple depth gauge that is use to determine how long of a threaded lamp rod will be needed. It's just a pencil, two rubber bands, and another long rod. First align the tips of the pencil & the rod so the ends are flush. Next, insert this device into the neck of the bottle. When you arrive at the new hole, push the long rod through the hole, leaving the pencil to stay against the inside wall of the bottle. When the rod becomes visible on the other side, STOP. As you pull out this device, you will have a measurement of the thickness of the hole.
To determine the cut length of threaded lamp rod, simply add
(1) determined depth from depth gauge above.
(2) thickness of two washers.
(3) thickness of two 1/8 IP locknuts.
(4) optional - add width of 1 or 2 threads if you want a little extra.
This is threadlock adhesive I use to fasten one of the 1/8 IP locknuts to one end of the threaded lamp rod.
It is desireable to have one locknut secured on one end of the lamp rod, so if you don't have the red threadlock adhesive, you might try substuting a super glue, PL Premium (with 24 hour cure), or jamming two locknuts (but allow longer lamp rod for extra nut), mechanical staking, roll pins, or whatever.
It's really hard to beat the threadlock adhesive because of it's strong gripping power and speed of setting up.
Just a drop of red threadlock is needed.
Spin the locknut all the way to the edge of the lamp rod. As the locknut covers the threadlock adhesive, the "lack of air" causes the adhesive to cure. Allow about 5 minutes for adhesive to set up hard. If you happen to accidentially get a sweet taste of this adhesive, it's because of the sweetness of the poison called "strychnine", so keep you hands out of your mouth if you get it on your hands. Now that I've scared you, don't worry, there's probably not enough strychnine in a bottle to hurt anyone. But still, be cautious, don't let the students make milk shakes with it.
Slip one nylon washer on. The will be the subassembly which will be inserted into the bottle first.
Now you need to either make two LONG socket wrenches below or use two commercially available socket wrenches with long extensions. Making your own is very cheap and fast.
Here's the tools needed:
(1) two sections of 1/2" Schedule 80 PVC pipe cut 16" long.
(2) one 1/8 IP locknut for threaded lamprod.
(3) propane torch
(4) non-combustable surface, like a chunk of iron.
The process:
Place the locknut on the non-combustable surface. Heat the nut to a temperature that will melt PVC. Immediatley push one end of the 1/2" schedule 80 pipe, on center, over the locknut. After the locknut embeds into the pipe, rush to quench it in the water. Repeat the process for the second PVC pipe. As a means of identifing which end of the pipe has the socket, I wrap masking tape around the opposite end of the pipe, about 1 inch from the end.
See the locknut embedded in the PVC pipe. If you have a remove the locknut, just screw some lamprod into the locknut and pull it out.
Insert long wrench and subassembly into bottle. When lamp rod meets with the hole, you will need to rotate the wrench clockwise, spinning the threaded lamp rod into the hole. Stop turning when the washer & locknut meet with bottom of bottle.
If you encountered a slight problem of thread locking adhesive not holding, don't worry, I have a solution at this point. Unscrew the locknut back to it's original position, (near the end of lamp rod). Ok, remove the long wrench, and replace it with the 3/8" drill bit & extension, used before for drilling the hole. Center the drill bit on the lamp rod, and SLOWLY spin the drill bit. The drill bit will bite into the lamp rod and spin it into position for you.....cool solution huh.
If you have a "handy stand" as mentioned above, now is the time to place it on your table. Insert the other long wrench into the slip connector (of the handy stand) with the socketed end on top. Embed the locknut in the socket, and then place a washer, on center, on top of the locknut.
Now, carefully slide the opposite bottle over this wrench. Align the exposed lamp rod through the washer and onto the locknut and screw the bottle clockwise. Once the threads are ingaged, you are almost home free.
Grab opposite wrenches and twist in opposite dirrections to tighen the whole assembly.