Build an Overhead Water Rocket Launcher Part 4 seal bump

there are some rings you can just roll up your pipe. If it fits on the pipe and I can still move the bottle on the pipe, will that be watertight?

O rings don't work that way ("just roll onto a pipe") under pressure. I know you're not a fan of the melted bump, but by the time you work out what O rings involve, it might not seem so bad to look at the video and try the bump method again.
Slater

I'll give it another try. If I find out any other ways to make it tight ill post em here.
Thanks for your answers.

Okay.
This Rocket is very easy to make, the difficult part is the bump. We did alot of time doing that. If someone can't do that using a candle, you might do the other things first. When you did that, you can hold your hand very tight on the end (where the soda bottle is placed) to make it airtight. if you connect the pump to the pipe and you heaten the place where the bump is using a burner (DON'T HEAT IT TOO MUCH!) you can make the bump without pushing.

How can i make it tight with O-rings?

O rings invove somehow machining a grove into the PVC to hold it in place and recess it. But that weakens the pipe, so you neet to reinforce it with a smaller pipe inside, possible 1/2" CPVC. Check other sites for ideas.
Slater

how could I prevent the bottle losing pressure? Even though it has a little bump at the end of the pipe, it is leaking water which means its losing pressure?

It's hard to tell without seeing, but you might have to loosen the hose clamp and move it down a little to hold the bottle tighter onto the bump. If it's just leaking a few drops of water, that does not really matter.
Slater