I designed this rocket and fin collar for a great air rocket launcher I got from the Maker Shed (http://www.makershed.com/products/compressed-air-rocket-launcher-v22). I wanted my second grade students to be able to see the rocket in flight without fins, and then to be able to design and easily attach their own cardboard fins for testing. The rocket has a small ridge and fits in a groove on the inside of the collar to prevent spinning during flight. The air rocket is quite powerful, so we wear eye protection, and the kids stand well back. The difference in flight is obvious, and the students enjoyed the whole experience very much.
The inner rim of the rocket body and the collar both need a couple of shaving passes with an exacto knife and should then fit perfectly.
The rocket file here has no text on it, and the collar does not have the notch for the text.
The collars stay on the rocket body during flight and return, but the little arms that secure the fins can sometimes break if it lands wrong. I'd suggest printing several collars.
The fins stay put during launch if they have 1/2 to 3/4 of an inch of cardboard sticking out of the tail end (as opposed to the fin end).
The launcher requires assembly, so allow time for that (have older students do it?).
The launcher valve can be tricky. I found this tip from Air Rocket Works very helpful:
"If the Quick Exhaust Valve Leaks: Sometimes, when using a bike pump, the QEV diaphragm will fail to seal the outlet, and the air will leak out the launch tube with each pump. There is one super-easy trick that almost always works if the QEV diaphragm fails to seal. First, with the slide valve in the "Launch" position, complete one stroke on the bike pump. This will build up pressure in the hose of the bike pump. Next, slide the valve to the "Pressurize" position. The sudden inrush of pressure built up in the pump hose will almost always seat the QEV diaphragm. In fact, the more you launch, the more the diaphragm gets broken in. Now, pump up the chamber normally."