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I had been experimenting with model rockets for a few weeks when I decided to build my own custom rocket and launcher. A significant amount of rocket energy is expended at launch in order to accelerate the rocket to a minimum aerodynamically stable velocity. Since the rocket engine produces copious amounts of hot gasses I wanted to take advantage of this. By sliding the rocket into a tube and closing the bottom, the gasses produced by the rocket build up pressure to help force it out of the tube with greater force than the rocket motor alone could provide. Because the rocket has to fit into a tube, it can no longer have wide fins to stabilize it in flight. Instead, I designed collapsable fins that fold forward into the body of the rocket. A washer-type ring with outer diameter equal to the inner diameter of the launch tube, and inner diameter equal to the outer diameter of the rocket motor places around the rocket motor acts as a sabot, sealing the combustion chamber while allowing the rocket motor to build pressure in the chamber. In addition to the exhaust gasses in the chamber, flammable gas is injected into the combustion chamber prior to launch. When the rocket motor ignites, it ignites these flammable gasses as well, boosting the pressure in the chamber. The rocket is packed with a standard parachute to facilitate recovery.
Below is a section view of the tube showing the rocket's position in the tube and how the ignition of the motor ignites the gasses, building pressure, forcing the rocket out of the tube.
Below is a section through the diameter of the rocket in the tube. Two fin supports are mounted to the cardboard rocket body with enough space between to fit the aluminum fin. A pin through the fin and supports makes the hinge. In order to push the fins past their center of gravity a thin plastic ring of a diameter slightly less than the rocket body acts as a spring to continuously press the fins outward. Once the rocket leaves the tube the fins are free to fold out. As soon as the fin's center of gravity is pushed past the pin, the rocket's acceleration provides the remaining torque to complete the deployment.
Here is a clip from a video of the launch. You can see the exhaust gases leaking out of the injector port in the side of the chamber, as well as past the sabot out of the end of the tube (it was a loose seal to prevent jams). As soon as the rocket leaves the tube, the fins are seen deploying. The rocket left the tube with much greater speed, and proceeded to much greater altitude than when it was launched with the end cap removed from the chamber, where it relied upon rocket alone thrust to accelerate out of the tube.
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