Flying Model Rockets
Flying model rockets is a relatively safe and inexpensive way for students to learn the basics of forces and the response of a vehicle to external forces. Any rocket is subjected to four forces in flight: weight, thrust, and the aerodynamic forces, lift and drag. There are many different types of model rockets. An interesting variation of the two-liter bottle rocket is the whoosh rocket.
This version of the whoosh rocket was developed by Roger Storm of Fairview High School and Mark Skor of North Royalton High School; both high schools are located in suburbs of Cleveland, Ohio.
The standard bottle rocket uses a two-liter soda bottle as the frame of the rocket, and pressurized water as the propellant. The whoosh rocket replaces the heavy water with a much lighter, combustible, alcohol-air mixture. The pressurization of the bottle occurs during the combustion of the alcohol. Because the exhaust products are much lighter than water, the whoosh rocket does not generate as much thrust as a water rocket, and the flight trajectory more closely resembles a ballistic flight than a water rocket trajectory. Although the whoosh rocket does not normally fly higher than 50 feet, it is instructional for students because the thrust is generated by the combustion of a liquid fuel.
On the figure we show the launching of a whoosh rocket using a model rocket launch pad. A straw is attached to the side of a two-liter soda bottle to guide the rocket along the rail during ignition. The bottle cap is drilled to create a 3/8-inch hole which serves as the rocket nozzle. Two or three drops of rubbing alcohol are placed in the empty bottle and the bottle is shaken to mix the alcohol with the air in the bottle. The rocket is then slid unto the launch rail and an igniter is placed near the nozzle exit. As the flame from igniter rises up through the nozzle, the mixture is ignited. Inside the bottle a chemical reaction occurs which converts the alcohol and the oxygen into carbon dioxide, water, and heat as described by this chemical equation:
The reaction occurs very fast, and the heating of the exhaust gases produces high pressure in the bottle. The exhaust gas is pushed out the hole in the cap and this produces thrust as described by Newton’s third law. When the thrust is greater than the weight of the bottle, the rocket accelerates up the rail as described by Newton’s first law. The powered portion of the flight of a whoosh rocket is quite short because of the speed of the chemical reaction. The majority of the flight occurs with weight and drag being the only forces on the rocket.
WARNING – Extreme care must be exercised in flying a whoosh rocket and students must be supervised when using this type of rocket. Do not attempt to build and fly this rocket without the assistance of your teacher. Only use soda bottles for the frame. Soda bottles are designed to withstand the pressure associated with carbonated liquids. Water bottles are not strong enough to withstand the pressure of combustion and may explode. The cap of the bottle must be drilled to produce a nozzle for the rocket. Do not make the nozzle hole smaller than 3/8 inch because this can produce excessive pressure in the bottle during combustion resulting in explosion of the bottle. Use only rubbing alcohol (isopropyl alcohol) for the fuel. Other types of fuel may cause the bottle to explode. Because the fuel is highly flammable, be sure to have a fire extinguisher available, keep the fuel containers capped when not in use, use safety glasses, and only fire the rocket outdoors in an isolated location. The rocket may be hot to the touch when it lands, so exercise caution in retrieving the rocket.