Only issue is the 9v battery life. With 3 batteries in series from my test it only lasted for a day (4 hours). That can get pretty expensive considering. Drill a hole in a the 1.25 inch PVC cap for the button. The size needed will be different for whichever button you use. Low volume launchers are much more compact but require higher pressure to achieve the same altitude as the high volume launchers.

Never allow anyone to put their face near the launch tube. Air expelled from the tube, if forced into someone's nose or mouth, is powerful enough to cause the lungs to rupture. This is very serious. Tell your students about this and they will be frightened enough to never get near the tip of the launch tube. This is an optional step, but adding it prevents users from over-pressurizing the system to a point of failure. Thanks to aaron.linker for suggesting it! If the pressure goes over the max set on the relief valve, it will "pop" to relieve the pressure and then close again.

But please do try out different configurations! I’d love to know the best way to use the launchers. Along with the list of materials in step 2, are steps 3, 4, and 5 that include not exploded diagrams, but exploded pictures of how and where everything fits together. And it it is true that I don't say how to fit every part into every other part, I do specifically use words in section 4 to telly you how to prime and glue parts (but you shouldn't do that anymore because Schedule 80 and Galvanized steel use nipples). I don't call out each part, just that general technique that is applied to all of the parts as laid out in the pictures. Likewise step 5 covers how to tape the threads for all the threaded parts (which is all that should be used anymore). But again I do not specially say how to screw in a threaded connection (lefty loosey, righty tighty), just how to prep it and then I leave it to the pictures to do the rest. If you could perhaps tell me what was the most confusing or where specific instruction may have been useful I could consider adding that. But if that happens to turn out to be something like "prime and glue the 1 inch by 3 inch PVC pipe into the 1 inch PVC elbow" followed by "Then glue the 1 inch by 12 inch PVC pipe into the previous 1 inch PVC elbow with the 1 inch by 3 inch PVC pipe attached"... I'll pass. I think it's best that you create a new and improved instructable. But thinking of safety I would recommend against gluing anything - use threaded pipe. In the end all the designs require about the same number of pumps to achieve the same altitude. The amount of Work this system can do is after all the simple combination of Pressure times Volume ( W = pV ) with a little Volumetric Flow thrown in (which gives an advantage to the big high volume launchers). Lightly screw the nipple into the T. No need to tighten it all the way because you will use the sprinkler valve to tighten it the rest of the way.

This is not needed, but it does make it easier to transport the launcher, and helps reinforce the sprinkler valve. If you have easy access to a 3D printer, then this is a great way to add to the build. So, be careful. Wrap the device in a heavy blanket or a shield that will contain any potential shrapnel. I really don't think duct tape is sufficient; it can fail too. This is especially important if the device is a few years old. It was the pipe cap that failed, not the glue, and I think the point of failure was where the inflating nozzle entered the cap; i.e., the hole that was drilled weakened the integrity of the PVC pipe cap. But that's for new well care for pipe, not something that is getting bumped around constantly, pressurized and depressurized, and worst of all - left out in the sun to deteriorate the PVC. Sadly, as with all things, the integrity of your launcher will eventually deteriorate until it fails. So we take steps to lengthen the time to failure and reduce the hazard when that failure occurs. I modified the input side to connect a pop-off and the valve stem. Doing it this way allows the rest of the design to stay the same, but it does make the design more expensive and larger. Our old tubes had many usability problems (picture attached) including a leader had to hold them while another leader had to fill them with an air compressor, they were big and heavy (27in of 3in pipe, outletting to a 3/4in outlet/launch pipe), the launch was the leader holding it opening a ball valve, they were not maintainable, they were hard to store, etc, etc. Your design solves all of these problems!I'll probably just right a new instructable with updates and put a note at the top of this one pointing there instead of correcting here. The only launchers I make these days are galvanized steel with threaded ends. Good sprinkler valves are hard to come as well (3 x 9V batteries can overcome that in many cases - worst case scenario you can always twist the solenoid to launch). Our Scout group had one of these lying around, which I very happily inherited and started playing with immediately. Of course, I tried to break it, which I did.