Flying model rockets is a relatively safe and inexpensive way for students to learn the basics of aerodynamic forces and the response of vehicles to external forces. Like an airplane, a model rocket is subjected to the forces of weight, thrust, and aerodynamics during its flight.
There are two main categories of rocket engines; liquid rockets and solid rockets. In a liquid rocket, the fuel and the source of oxygen (oxidizer) necessary for combustion are stored separately and pumped into the combustion chamber of the nozzle where burning occurs. In a solid rocket, the fuel and oxidizer are mixed together and packed into a solid cylinder. Under normal temperature conditions, the fuel and oxidizer will not burn; but they will burn when exposed to a source of heat. Some type of igniter is used to initiate the burning of a solid rocket motor at the end of the propellant facing the nozzle. Once the fuel starts to burn, hot exhaust gas is produced, which is used to propel the rocket, and a "flame front" is produced which moves into the propellant. Once the burning starts, it will proceed until all the propellant is burned. With a liquid rocket, you can stop the thrust by turning off the flow of fuel; but with a solid rocket, you would have to destroy the casing to stop the engine. Liquid rockets tend to be heavier and more complex because of the pumps, and you usually put the fuel in the rocket just before launch. A solid rocket is much easier to handle and can sit for years before firing.
The relative safety of building and flying model rockets is the result of the production and availability of pre-packaged solid model rocket engines. These engines are produced by several manufacturers and are available in a variety of sizes with a range of engine performance. The engines can be bought at most hobby stores and some toy stores for a modest price (average current price is 3 engines for $5). The engines are used once and discarded; a new engine is inserted into the rocket for the next flight. Before these engines became available, many young rocket builders lost limbs or life in the process of mixing rocket fuels. With these engines, you can still have the fun of building and flying rockets, learn the fundamentals, and then move on to the more dangerous and complex problems of propulsion.
On this slide we show a drawing of the parts of a model rocket engine so that you can learn how it works. We have laid the engine on its side, and "cut" the engine in half so that we can see what is inside. (Never disturb, cut, or modify a real model rocket engine. The propellant can ignite at any time if there is a source of heat.) The engine is installed in a rocket shown by the dashed lines on the figure. The engine casing is a cylinder made of heavy cardboard which contains the nozzle, propellants, and other explosive charges. At the right side of the engine is the nozzle, a relatively simple device used to accelerate hot gases and produce thrust. Model rocket nozzles are usually made of clays or ceramics because of the high temperature of the exhaust. The hot gases for a model rocket are produced by the solid propellant, shown in green. An electric igniter is used to launch a model rocket engine. As the flame burns through the propellant, the rocket experiences powered flight. When the flame front reaches the far left of the propellant, thrust goes to zero, and a delay charge, colored blue, begins to burn. During the delay, no thrust is produced and the rocket coasts up to its maximum altitude. The length of the delay varies between engines from 2 to 8 seconds and the amount of the delay is listed on the engine casing. When the delay charge is completely burned through, the ejection charge, shown in red, is ignited. This produces a small explosion which ejects hot gas out the front of the engine, through the engine mount, ejects the nose cone, and deploys the parachute for a safe recovery.
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