Subject: Bottle rocket design features

Hello!
For science club, we have to come up with a bottle rocket design that will stay 
up in the air as long as possible. The rubrics are that we must use 1 one-liter 
bottle and 1 two-liter bottle, no metals parts on the pressurized rocket body, 
no pyrotechnics, and can't be heavier than 300 grams. The only ideas that we 
have considered are connecting the one-liter bottle to the two-liter bottle and 
putting helium in the one-liter bottle, adding a parachute, using the force of 
lift somehow, using gyro-forces, and the idea of aerodynamics.

Are there any designs, features, or forces that we may use to increase the time 
the bottle rocket is up in the air and how may we process toward it? 
Please, sent me infomation toward my subject. Thanking for help. Bye

Re: Bottle rocket design features