What can you do to a design of a rocket to make it go higher? (soda bottle rocket)
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You want to do two things - stabilise it's flight, and reduce drag, to make it go higher. To that effect, it's good to add fins, as these will keep it flying straight rather than tumbling, and a nose cone. You'll also want to try out different water/air mixtures - you'll find a perfect mixture where the power/weight ratio is maximum.