Things2Make: Coke Bottle Rocket
by Bob Shaw
Large coke bottle 1-2L, a cork from a wine bottle, an old bicycle inner tube (Preferably with the thinner type of valve), a drill, and a bicycle pump.
Cut out the valve from a bicycle inner tube, leaving a small circle of rubber around the bottom. (Keep the rest of the inner tube to make a catapult on another day). Check the length of your valve against the cork to make sure the valve pokes through enough so the pump can be attached. If not cut the cork down with a sharp kitchen vegetable knife. Using a drill that has a diameter equal to your valve carefully and slowly drill through the centre of the cork. Insert the valve into the cork (A smear of vaseline or cooking oil may help). Fill a coke bottle with 1/3 water and firmly insert your cork-valve assembly. Make a launching sled for the rocket. We dug a couple of bits of wood into the mud and propped them up with stones in a "V" shape to support the bottle. Stiff cardboard would also suffice. Finally check there are no aircraft above and attach your pump. Get pumping until it takes off. The bottle will definitely go over the fence into your neighbours garden or on a roof so make sure you have a spare available.
Attach wings to the bottle to make the space shuttle challenger! Slightly adjust the water level for optimum performance.
HOW IT WORKS:
Firstly the cycle inner tube valve is a 'One way' valve that lets air into the bottle but not out. When you pump, the bottle becomes pressurised, this is your energy being stored. Eventually the outward force of the pressure will overcome the retaining friction of the jammed cork and the bottle will be released. The water then regulates the release of the pressure and drives the bottle forward. To find out exactly why the bottle moves forward we refer to English scientist Sir Isaac Newton (1687). Newton’s third law of motion states, “For every action there is an equal and opposite reaction.” In the case of the rocket, the expulsion of the water from the bottle is the action, and the forward movement of the rocket is the reaction. Simple eh!
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