Something that can be compressed a little so the pressure can build up, and to be very hard to compress... so find something with those two qualities
1:2
If it is a commercially produced water/air rocket, follow the instructions exactly. They will tell you how much you need.If you made it yourself from a plastic bottle, you should perform a series of experiments to see how much water gives the best altitude.BTW, there is no fuel in these rockets. Water is the propellant that "pushes" on the rocket as it leaves the nozzle. Air pressure pushes the water out the nozzle.
the best psi for launching a bottle rocket high is 65 psi because usually you want it to go high without a big chance of exploding in you face
THe ideal amount is 50% air pressure and 50% water. SHoot from 4-6 bar. I got mine to 92 meters
The 'operator' fills the bottle with water, then pumps air into it - creating pressure. When the stopper is released, the air pressure forces the water out of the narrow opening in the bottle's neck - creating a stream powerful enough to launch the rocket into the air.
it pushes the rocket into the air to make it go higher.
Water is non-compressible. The air gap is what allows additional air to be pumped into the rocket.
A popular rocket toy name is the "Stomp Rocket." It is a toy rocket launcher that uses air pressure to propel foam rockets into the air when stomped on.
it will become an air powered rocket
Air pressure does not come into play with most model rockets.
The rocket might explode from the added pressure when the combustion reaction hits the air.
make it hold as much pressure as possible and experiment with the amount of fluid/air.