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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. A model 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. The first and simplest type of rocket that a student encounters is the compressed air, or stomp rocket. The air rocket system consists of two main parts, the launcher and the rocket.
On the figure we show a generic launcher, although launchers come in a wide variety of shapes and sizes. The launcher has a base to support the rocket during launch. A hollow launch tube is mounted perpendicular to the base and is inserted into the base of the rocket before launch. The launch tube is connected to an air pump by a hollow feeder line. The pump is used to pressurize the inside of the body tube to provide thrust for the rocket. We have attached a pressure gage to the feeder line to display the change in pressure in the system. For many air rockets, the pump is simply a cylinder which can be collapsed by striking with your hand or foot, which is where the "stomp" rocket got its name. In our simulation, we are going to pump up the system, and then launch the rocket, to better control and explain how the system works.
The other part of the compressed air rocket system is the rocket itself. The rocket has a hollow body tube which is opened on one end and closed at the other end by the nose cone. The body tube is only slightly larger than the launch tube. When the rocket is placed on the launch tube, the body tube becomes a closed pressure vessel. The pressure inside the body tube equals the pressure produced by the air pump. Fins are attached to the bottom of the body tube to provide stability during the flight.
The flight of a compressed air rocket is similar to the flight of a ballistic shell or a bullet fired from a gun. Unlike a model rocket, bottle rocket, or full scale rocket for which the thrust force is applied to the rocket for a large portion of the flight, the thrust of a stomp rocket is completely expended in the first instance of flight. During the entire flight, only the weight and aerodynamic forces act on the rocket.
The launch of a compressed air rocket proceeds in three stages; the pressurization of the body tube, the initial acceleration along the launch tube, and the expulsion of the compressed air from the rear of the rocket. Here is a computer animation of the launch of a compressed air rocket: