You really need to read the article before commenting. No PVC pipes or steel ones are flying. The PVC is a launch tube and steel would be safer because it would not have a brittle fracture problem like PVC.
The "rocket" itself weights 60 grams and is mostly paper and foam shock absorbing material. Although I try to avoid being hit by it, I don't think it would do much damage to me. I fly my rockets on my property and I'm the only one that they could land on.
I suppose the absolute answer is more a question of how much acceleration your payload can withstand. These are projectiles, not rot really rockets, so the speed and eventual maximum altitude are limited by the barrel length and the acceleration.
200g acceleration over 20 inches will result in 146.5 ft/sec, 400g will get you 207 ft/sec.
For a launcher built from PVC pipe such as I'm using, I limit the maximum pressure to 50 PSI. This is well below the pressure rating for the PVC, but PVC under pressure fails with a brittle fracture that is very dangerous with compressed air. If you want to go with higher pressure you should encase the PVC in steel that can contain any potenetial explosion of the PVC. Better yet use a metalic pipe rated for the pressure that will fail gracefully if it does fail.
My analysis shows that I should be able to get a 100 gram to 120 gram total mass up to 250 to 300 feet with the PVC type technology I'm using. To do this I'll double the length of the 2 inch air cylinder and increase the rocket body diameter to something around 1.5 inches and lengthen the launch tube slightly.
To go larger and higher you'd probably want to go with a higher pressure system, or a real rocket.
October Sky - Now thats a GREAT movie, Charles! Very inspiring. I remember watching those boys in their rocket house blowing up rockets until they got it right. I need to go rent it again. We did a birthday party a few years ago and bought a 12 pack of easy to build rockets for one of our son's birthday parties. We took them to the park by our house and all the kids got to shoot off their rockets. So much fun is to be had with model rocketry!
I'm still using the original camera. it has survived quite a few flights. Even if I use a parachute for a gentle descent, the camera still has to withstand around a 200 g launch acceleration. It's a good thing the camera is inexpensive, I never would have tried this with a $100 camera.
As far as more construction details you should try the build instruction link. That will let you download what should be sufficient information to build one of these.
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