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The design of a model rocket is a fairly advanced topic and should be reserved for well qualified rocketeers. Model rocket kits are already designed and tested by the manufacturer for stability and safety, making them a great choice for model rocketry.
The basic steps for designing a model rocket are:
- Choose a body tube size (for example BT-50)
- Add a nose cone
- Add a recovery system
- Design and attach the rocket fins
- Test for stability, with the model rocket engine installed
- Test launch with a minimum size engine
Choose the Body Tube and Nose Cone Size
|Small size rockets||BT-20 or BT-50|
|Medium size rockets||BT-50 or BT-55|
|Large size rockets||BT-60 or BT-80|
The length of the body tube has the most influence on stability (see the Test for Stability section below).
The recovery system choice really depends upon the size of the rocket.
|Small size rockets||tumble recovery or streamer|
|Medium size rockets||parachute or streamer.|
|Large size rockets||parachute|
Design the Rocket Fins
The rocket fins are very important to the stability of the model rocket. Fins should always be placed as close to the bottom (the end with the rocket engine) of the rocket as possible. The critical dimension for the fins is how far they stick out from the rocket body tube, not how much area they take up. Looking at pictures of model rocket kits should give you an approximation for how to size the fins based of the size and length of body tube that was chosen.
Since the rocket fin design is so critical, a model rocket design simulator (like RocketSim) can be of great help, especially if designing a complex multi-state model rocket.
Test for Stability
There are two factors in the stability of a model rocket, the center of balance and the center of pressure. For a rocket to be stable, the center of balance must be ahead (closer to the nose cone) than the center of pressure, else the rocket will not fly straight, it will tumble in the air.
To determine if the center of balance is ahead of the center of pressure, the swing test may be used. Remember to do this with the largest engine you intend to us installed, as this will be the heaviest engine and have the most effect of the center of balance. First, find the center of balance of the model rocket by balancing it on your finger horizontally. At this point on the rocket, tie a five to six foot string around the rocket and secure it in place with a piece of tape. Now swing the rocket around your head, and the rocket should fly straight. If it does not fly straight, if it tumbles in the air, then the rocket is unstable. To fix this, either add weight to the front/top of the rocket to move the center of balance forward, or use larger fins to move the center of pressure backwards, then re-test.
- A Test Launch should be done with extra safely precautions and as few people present as possible.